R-value, U-value... What Do They Mean?
Each of these values is a measure of heat transfer through a material.

The U-Value (or U-factor) is a measure of thermal conductivity — how well heat flows between warm side of a material and its cold side. The lower the U-value, the more slowly heat is transferred.

The R-value is a measure of thermal resistance to conductivity. The higher the R-value, the more resistance to heat transfer the material has.

The two rating systems are opposites. The more a material resists heat transfer (high R-value), the more slowly heat is transferred (low U-value). A material that does not resist heat transfer (low R-value), conducts heat very well (high U-value). In fact the U-value of a material is what mathematicians call a reciprocal of its R-value, and vice-versa.

Converting from U-value to R-Value, and Back Again
The conversion formula from R-value to U-value is: U-value=1/R-value. So if the material resistance to thermal transfer of R-2.2, its conductivity rating or U-value is 1 divided by 10 (1/10) or U-0.45. This a typical U-value for a double-pane thermal window.

U-value is typically used in rating windows. The U-value of a window is an average of measurements taken at several points on the window. Converting these into the more easily understood R-value is basically the same process as converting from R-value to U-value. R-value = 1/U-value. So a window glass rated at U-0.45 has an R-value of 1/.45 or R-2.2. Compare this to the R-13 required for house walls by the Nebraska Energy Code, and you can see that the window is a significant hole in your wall insulation.

American vs. European U-values
Just to make things even more confusing, here are actually two widely used U-value ratings, the English/American rating and the European or metric rating, also called the K-value or K-factor. When looking at U-values, you need to know whether it is the English/American U-value, or the European rating. Generally a U. S. rating will be written on window labels in the form "U-value (U.S./I-P)" which distinguishes it from the metric factor.

R-value is used primarily in the U.S. and Canada. The rest of the world uses the European U-value except Britain, which uses the English U-value. The European U-rating (based on meters and degrees Kelvin) is not a reciprocal of a material's American R-value (based on feet and degrees Fahrenheit). To arrive at the metric U-value of a material, divide 1 by its R-value then multiply the result by 5.682. To convert a metric U-value to an American U-value, multiply the R-value by 0.176 then divide 1 by the result.

Will the Real R-Value Please Stand Up?

To enormously complicate decisions about insulation materials, there is not just one R-value, but several. Each of these conveys useful information, but confusion can result unless you know what R-value is being reported.

Center of Cavity R-Value The reported R-value rating for an insulation material rates only the insulation material. A 4" batt rated at R-13 states only the resistance of the batt material itself. It does not rate the entire wall in which the batt is installed. This rating is commonly referred to as the "Center-of-Cavity" rating. When you see R-13 printed on the back of a fiberglass batt, this is its center of cavity rating, and is likely to be higher than its actual performance in your wall after it is installed. Manufacturers of insulation materials are required to calculate and conspicuously display this R-value on their materials by federal law.

Clear Wall R-value A more accurate way of measuring thermal loss is to install the material in a wall and then measure the thermal resistance of the wall including its necessary framing members (but not windows, corners or joints at roofs, foundation or floors). This is the "Clear-Wall" R-value and it is almost always lower than the center-of-cavity rating because it includes such things as wood framing members in the measurements, and wood framing members are usually not as insulating as dedicated insulation materials such as fiberglass or cellulose. (See the chart in the main article).

Whole Wall R-value In a recent study of wall insulation ratings, the Oak Ridge National Laboratory (ORNL) has developed a more accurate rating: the "Whole Wall" rating. According to the study, measures of "Clear-Wall" and "Center-of-Cavity" thermal resistance are misleading because they do not take into account all of the possible framing material "thermal shorts" or "bridges" through the insulation. A short or bridge is simply a place in the wall where the insulation is interrupted by other materials. A stud in a conventional wall is a short, as is the gap left for an electrical box.

Oak Ridge proposes an R-value rating for the entire opaque wall (not including windows and doors) to measure the thermal performance of not only the insulation and structural elements, but also the effects of their installation and typical wall interface details such as intersections with other walls, floors, foundations and windows. The standard also considers such previously ignored factors such as moisture resistance (the insulation value of some materials when wet may degrade considerably), thermal mass, and air transfer resistance (heat moves with air) of insulation materials.

The results were surprising and even scary. The Laboratory studies found large differences between the reported ratings of insulation and its actual thermal performance in a wall. Materials can lose up to half of their rated R-value when installed in a typical wall. The best performers were insulated concrete forms and structural insulated panels(SIP). A 4" SIP wall was found to be more effective at blocking heat transfer than a 6" conventional stud-framed wall and with 15 times less air infiltration. The worst performers were batt materials, especially fiberglass batts. Even very careful installation of these materials leaves small gaps and voids through which heat escapes, dramatically lowering the material's effective R-value.

To read a summary of the study report, go to the ORNL Building Envelope Research web site. To calculate the R-value of the insulation in your home, use the ORNL Whole Wall Thermal Performance Calculator. The results will probably surprise you.

Anyway, now you have been introduced to U-values and R-values and understand better why insulation contractors spend a significant portion of the year in therapy.

Insulating Your Old House
J. M. Edgar, CMC, CRC

Is your old house drafty in winter, swampy in summer? Almost impossible to heat and cool effectively? That's because when your house was built a half-century or more ago, no one thought much about insulation. Energy was abundant, and cheap. Half of the world's oil was produced in the U.S. Conserving energy was just not very important. Experts believed that the 4" of "dead" air space captured inside the stud cavities of your walls was enough of a heat barrier.

We now know they were wrong.

Air actually is a good insulator, if it can be kept from moving. Dry, absolutely still air has an R-value of 3.6 per inch of air — as good as most insulation materials. It will inhibit the conduction of heat from one wall surface to another as effectively as almost any insulation you can buy. So why pay for insulation when just plain air works almost as well? The answer is that the air inside your walls neither still nor dry. It's moving continuously, and with that movement creates a convection flow that results in significant heat transfer.

You will undoubtedly remember from high school physics that heat tends to move from hot to cold. Its objective is to reach a balance where all of the objects in the environment are the same temperature. Hot objects lose heat and cold objects gain heat until that balance is reached. If the air in your house is warmer than the air outside, the warm inside air wants to lose heat to the colder outside air. There is nothing we can do to keep this physical law from operating. All we can do is slow it down. We do this by creating a barrier between the hot and cold objects so that transfer takes longer. This barrier is insulation.

The Building Envelope

No matter the shape or size of your house, it is, from an environmental scientist's perspective, merely a box composed of a roof, floor and walls. This box separates us from the outside environment. It keeps the wind, rain, bugs and critters out. It also is our main line of defense against being too hot or too cold. Environmental engineers refer to the box as the "building envelope".

Most people are most comfortable when the temperature of the air around them is about 70� F and humidity is around 40%. To maintain this environment in our homes, we add heat (and sometimes humidity) to the box in winter and extract heat and humidity through air conditioning in summer.

When we do this we create a heat imbalance. Adding heat to our house in winter means the inside of the building envelope is hotter than the outside. Nature begins searching for ways to restore the balance. The heat inside tries to get outside where there is cold air to warm up. To get outside, it has to pass through the building envelope. This is where we try to block it.

Its a contest we cannot win. Heat always finds a way out — eventually. The best we can do is a guerrilla-like delaying action. We can make it so hard to get out that it takes a long time. And that's the objective of insulation and other weatherization measures — not keeping heat from moving though the building envelope, but making it take longer.

The longer we can hold heat inside the building envelope, the less often we have to add heat. The less often we have to add heat, the more money we save, and the less polution we generate. Without insulation, our homes can lose all their heat up to seven times each hour in winter. With adequate insulation and weatherization, we can reduce that to as little as once every three hours. This it is a very substantial difference, saving you a lot of money, and reducing your impact on g;obal warming.

How Heat Moves

Heat can transfer through your building envelope in three ways: convection, conduction and (to a much lesser extent in our area) radiation. Convection is the star player. It has a role in nearly all heat movement in and out of your house. Heat and cold piggy-back on the moving air. Hot air leaking out through gaps in your walls and around doors and windows carries heat out of your house, and cold air leaking into your house has to be heated up. In either case, you have to add more heat to your house to make up for the heat loss through this air movement through the building envelope. Air leaks can be a major cause of heat loss. The EPA estimates that 15% to a whopping 40% of heat loss is through air leaks.

Air Leaks and Heat Transfer

If you open your door in winter, hot air flows out at the top of the door, and cold air flows in at the bottom. A heat transfer has taken place — an infiltration of cold air, and an exfiltration of warm air. Heat and cold have piggy-backed on the flowing air. The same effect occurs where your walls have air leaks in them. Air moves through even very small voids in the wall coverings and through the gaps that may be left around window and doors or where unlike materials meet. Different materials expand and contract at different rates in response to changes in temperature and humidity. The joint where two different materials meet is always a weatherization problem. Even if the joint was well sealed to begin with, after a few years of expansion and contraction, a gap has probably opened. It may be a very small gap, but every small gap hurts. Heat moves with air flow through the gaps in your building envelope.

Some air movement through your building envelope is necessary. You need to exhaust stale interior air from your home, and bring in fresh outdoor air. At least eight complete air exchanges every 24 hours is the minimum recommended by the American Society of Heating, Refrigerating and Air-Conditioning Engineers. Dwellings that are so tightly sealed that less than that minimum exchange occurs need to use some form of mechanical ventilation system to augment passive air transfer.

As an old house owner, you don't have that particular problem. You have the opposite problem. You have too much air flow through your walls and roof. Old houses may allow three or more complete air exchanges every hour. Each air exchange means that all of the heated air in the house has leaked out and the new, cold exterior air has to be heated. As much as 40% of your heat loss is through air transfer. Controlling and minimizing this transfer is the job of "weatherization" — the process of sealing cracks, gaps and holes (especially around doors, windows, pipes and wiring) with caulk and weatherstripping and replacing drafty doors and windows or weatherproofing them.

But insulation does have a role. Certain types of insulation, notably the foams and cellulose are good at seeking out and sealing small voids and cracks. These types of insulation permit very little air flow, and while they do not replace a good program of weatherization, they do their part in helping it succeed.

Convection Inside Your Walls — The Heat Conveyor

Most of the heat transfer through any uninsulated wall is by convection, which creates, a conveyor belt of air inside your wall that is very effective at moving heat from the warm side of the wall to the cold side.

Let's say it's winter. You are pouring heat into your house to stay warm. It's a toasty 75� inside your house. The interior drywall or plaster of the stud cavity is, therefore, nice and warm. Outside, it's 35�. The exterior siding and sheathing enclosing the wall cavity is very cold. The air next to the interior wall draws a little heat from the warm interior drywall and, like all warm air, starts to rise. As it rises, it continues to draw heat from the warm side of the wall. When it gets to the top of the stud cavity, if can no longer rise. But there is more warm air below continuing to rise, pushing up on our little packet of air, crowding it against that frosty exterior side of the wall. As soon as it touches the exterior wall, it starts giving up heat, growing colder.

Cold air is heavier than warm air, so it starts to fall. As it falls it loses still more heat to the cold outside surface of the wall cavity, growing colder and colder. At the bottom of the stud cavity, it stops and would be delighted to stay there forever, but above it is a heavy column of cold air pressing down on it until it is eventually pushed against the warm interior side of the wall. It starts drawing in heat again, and rises once more. And the cycle starts over.

This is the heat conveyor. It occurs inside every uninsulated wall cavity. The greater the temperature difference between the warm side of the wall and the cold side, the faster the air circulates. This circulation is a heat exchanger — and, unfortunately, a very effective heat exchanger. It draws heat from the interior side of the wall and conveys it to the exterior side, which in turn conducts it to the outside air.

The conveyor is a continuous, year 'round. In the summer it merely reverses, transferring heat from the warm exterior side of the wall to the air-conditioned interior side. From 50% to 70% of the heat gain and loss in your walls is through this conveyor process.

Heat Conduction and Thermal Bridging

Heat can also be transferred through conduction — the movement of heat on a microscopic level from molecule to molecule within a material. When an atom is heated, it electrons move faster, which tends to excite the electrons of adjacent atoms so they move faster. These, in turn excite even more electrons, and the process spreads. This is how heat moves from one atom to the next. Some materials, like most metals, are good heat conductors. Heat one end of a metal bar with a propane torch, and very quickly the other end gets hot.

Most gases, including air, are poor conductors. The air in your wall cavity is a lousy conductor of heat. And when it is replaced by a suitable insulation material, so convection is slowed, the wall cavity is an effective barrier to heat transfer. But air is not the only material in your walls. There is also the wall's wood framework. The wood framing penetrates through the wall from outside to inside, creating what is called a "thermal bridge" along which heat can pass through conduction.

Wood (which is denser than air, and contains water — a very good heat conductor) conducts heat better than air, and much better than most insulation materials. The R-value of the pine, fir and spruce used in wall framing is about 1.25 per inch. Compare this to 3.6 per inch for dry, still air, 3.85 per inch for dense pack cellulose and 6.25 per inch for closed cell foam. Other materials are even less resistant to heat transfer. Steel studs, for example, are very good heat conductors. Fortunately they are almost never used in residential construction in exterior walls.

To minimize thermal bridging, you have to keep framing members to as few as possible. There is a limit to this, of course. If you reduce framing too far, your house might fall down. But there are many things that can be done. For example, spreading out studs from 16" on center to 24" on center provides walls that are just as strong, but contain fewer thermal bridges. Using less lumber in framing is also good for the environment since it requires the destruction of fewer trees. It also uses less labor, so it costs a bit less than traditional framing.

The framing techniques used to reduce lumber use have been developed by construction engineers over the past 20 years under the sponsorship of the Department of Energy and Housing and Urban Development (HUD). They are collectively called Optimum Value Engineering or OVE. OVE uses engineering principles to minimize material usage while meeting model building code structural performance requirements. (1) It reduces the number of framing members (2) eliminates pockets in framing, particularly at corners and wall intersections that cannot be effectively insulated and (3) reduces the number of framing members that penetrate completely through the wall.

In new construction, if we don't use structural insulated panel (SIP) construction (which we prefer), then we use OVE techniques in all of our wall and roof framing. In your old house, framed the traditional, lumber-intensive way, the techniques are of less use. But the studies that led to OVE standards have told us a lot about where insulation problems are likely to occur in traditional walls. For example, we pay special attention to corners and to the intersections of interior walls and exterior walls. These are special problem areas for installing effective insulation.

Radiation

Your Old Windows
The largest thermal break in your walls is not wood framing members. It is your windows.

From an insulation engineer's point of view, windows are holes in your wall through which a lot of heat gets out no matter how air tight and well-insulated the rest of the wall might be. Unfortunately, there is not much that can be done about it.

The culprit is glass. Glass is a terrible insualator. And, windows are mostly glass. A single pane of glass has an insulation value of a little less than R-1. Double-pane thermal glass improves this to R-2.2.

Triple pane thermal windows, using all the latest technologies including argon or krypton gas fill, low-emissivity (low-e) coatings and careful sealing can reach R-7.5. But many of these measures are just temporary. (Low-e coatings break down, gas fills leak away eventually.) And, R-7.5 is a far cry from the minimum R-13 that should be in you walls.

To learn more about insulation and your old windows, see Your Old Windows. Heat, like light, can move as electromagnetic waves. These are waves in the infrared spectrum. Like light, they need no material to move through. They easily move through vacuum, which is how we get heat and light from the sun. And like light rays, heat radiation can be blocked and reflected.

Radiation in winter is good. In well-designed passive solar systems it can add a lot of free heat to the house. But in summer, radiation can significantly contribute to your cooling load. The sun heats up the outside of your house walls and roof. Any hot material radiates heat. The hot wall radiates heat into your wall cavity where it is picked up by the convection and conduction processes and transferred to the interior side of your wall where the air-conditioning has to deal with it. The hotter your outside wall or roof gets, the more problem it causes. On your west wall, in summer, steel siding can easily get hot enough to cook on. Vinyl and cement siding materials stay a little cooler, and wood is the best performer.

How Insulation Works

Slowing the transfer of heat from the warm side to the cold side of the building envelope of your old house is the job of insulation. Now that we know how heat gets transferred we have a pretty good idea what we need to do to damp it down.

Slowing The Convection Conveyor Most of the heat loss through your walls is by convection. Studies vary in their estimates of heat loss through convection, but it is somewhere in the 50-70% range. The primary job of insulation is to dramatically slow the convection heat conveyor. If insulation does only that, it has handled most of your heat movement issues.

All insulation slows convection by dividing the one big air cavity in your wall into thousands of tiny cavities or cells. Each of these cells will convey heat from warm side to cold side through the air trapped inside the cavity, but thousands of cycles are required rather than just one big cycle. This greatly increases the amount of time it takes for heat to transfer all the way through your wall — as much as 20 times longer. Some materials are better convection slowers that others, but all insulation focuses primarily on slowing convection.

Calculating the R-Value of Your Exterior Walls, Ceiling and Windows

What is the R-value of your walls, and do your need to add insulation? Determining the R-value of your walls is not rocket science. Anyone can do it with fair accuracy using the right tool on a cold winter day.

The right tool is an infrared thermometer.

This is a device that measures the temperature of a surface using a laser beam. The temperature of the surface is displayed on a screen. The devices are not expensive. The one we use cost about $40.00.

The interior and exterior wall surface temperatures and outside air temperatures are measured. By comparing the difference between wall temperatures with the outside air temperature, you can get an estimate of the R-value of the wall using the Table below. This works on any exterior wall whether or not it contains insulation. Even uninsulated walls have some R-value just from the construction materials in the wall.

Infrared thermometers measure an area that gets larger the farther the thermometer is held from the surface. It does not measure the temperature of the exact spot where the laser beam hits, but an average temperature of the area around the beam. The farther away from the surface being measured, the larger the area averaged. If you are looking for an average temperature over a large area of wall, hold it about 3-4 feet from the wall. If you are looking for specific areas of air infiltration or voids in your insulation, you need to be about 1 foot away.

Avoid areas that may be affected by radiators, heating ducts or lights. Take these measurements in the evening or a couple of hours after dark to reduce the affect of solar radiation on the wall that may warm the wall several degrees and give a false result.

Test when the outside temperature is very cold. Below zero is best for the most accurate results.

Step 1: Outside Air Temperature.

Aim the thermometer at outside objects, tree trunks, for example or fences to determine the ambient outside temperature. Do not aim at an exterior house wall. Or just read it from your outside thermometer if you have one.

Step 2: Interior Wall Temperature.

Aim the thermometer at an interior wall to get the interior temperature. An interior wall is one that is heated on both sides and in the same room as the exterior wall to be tested. If you are testing a long wall, you may have to do this for each room along the length of the exterior wall to be tested.

Step 3: Exterior Wall Temperature.

Aim the thermometer at the inside of the exterior wall to be tested to get the exterior wall temperature. Measure the temperature on the inside of the wall.

Step 4: Temperature Difference.

Subtract the exterior wall temperature from the interior wall temperature. Use this result to determine the R-value of the wall from the table below.

Table: Pedersen & Hellevang

Click to Enlarge Table.

For example, if the interior wall is 70° and exterior wall is 66°, the difference is 4°. If the temperature outside is -20°, the estimated value of the insulation in the wall is just under R-15. (See table, above).

Ceiling and Windows:
Ceilings and windows can be measured the same way. For ceilings your exterior temperature measurement should be of a ceiling with the lights off and allowed to cool. For windows, take the temperature of the glass near the center.

Air Leaks:
To find air leaks, take the temperature of outlets, around windows and doors, along the base of the wall and other places where leaks are likely. If the temperature differs a few degrees from the overall wall temperature, you probably have an air leak or at least an area that is not well insulated.
Source: C. Pedersen and K. Hellevang, "Determining Insulation and Air Infiltration Levels Using an Infrared Thermometer", North Dakota State University Extension Service, March, 2010. (Download PDF)

Preventing Conduction If creating tiny air pockets was all that was needed from insulation, then any material would work. We could use something like aluminum or copper foam, for example. But aluminum and copper don't work as insulation because these metals are very good heat conductors. Heat simply flows around the air pockets moving from molecule to molecule through the metal.

To be effective, the insulation material itself must be a relatively poor heat conductor — that is, a good insulator. Almost all insulation materials are lousy heat conductors — which is why they are used as insulation. Some, however, are better insulators than others. The plastic in foams is probably the best insulator, followed by the paper that makes up most of cellulose insulation. Fiberglass and rock wool are the poor insulators. Fiberglass and rock wools insulate by trapping air pockets, and it's the trapped air that is the insulator. The tiny glass and rock fibers actually transmit heat fairly well, but they are so tiny that very little heat is actually transferred. Which is why, despite glass and rock being fairly good heat conductors, in the spun fiber form used in insulation, the material as a whole works pretty well — but not as well as form nor as well, dollar for dollar, as cellulose.

Reflecting RadiationHeat radiation out of your house in Winter, and into your house in Summer is bad. Either event moves heat in the wrong direction. Radiation into your house in Winter, and out of your house in Summer is good. It helps move heat where we want it. Unfortunately no one has yet come up with a unidirectional radiant barrier; one that blocks only radiation moving in the wrong direction. Luckily heat loss and gain through radiation is not a big problem in our area like it is in the desert Southwest and parts of the South. This is good for us because all radiation blocking materials now in common use have significant drawbacks.

Blocking Air Infiltration Insulation helps stop air transfer by plugging leaks. The EPA estimates that 15-40% of your heat loss is through air movement. So insulation that is good at plugging air leaks is better than insulation that is not. Some insulation is better at this than others. The foam materials are good leak blockers, as are cellulose and rock wool. Blown in fiberglass is not nearly as good. Event the most effective wall and attic insulation is not, however, going to take care of the majority of your air infiltration. Most infiltration occurs in the joints around windows and doors rather than through your walls or roof. Blocking it is the job of good weatherization, not insulation. And weatherization is the subject of another article.

Fire Resistance Insulation is not supposed to be a fire retardant. It is supposed to insulate. But there is a lot of nonsense floating around about the supposed fire suppression effects of certain insulation materials, so we need to deal with this issue briefly.

What we can reasonably expect from insulation is that it will not add to the risk or severity of fire by being flammable or otherwise harmful. All fire and safety codes contain requirements that make insulation, at worst, fire neutral. All commercial insulation materials, if installed according to code, are fire safe. But some materials are more fire resistant than others. Rock wool and fiberglass, for example, will not burn at all, and will not emit harmful gases when in contact with fire. The most they will do is melt, and if the fire in your house is hot enough to melt glass and rock (very, very unlikely), you have many more immediate worries than the status of your insulation.

Insulating cellulose products also will not burn, not because the paper used in cellulose is fire resistant, but because it is treated with fire retardant materials. Cellulose will also not give off toxic gases when in contact with flame. The foams won't burn, but when in contact with high heat give off toxic gases. This is why every building code requires them to be covered with a fire-resistant barrier.

So, after many independent studies over many years, we can be confident that none of the materials commonly used as insulation will worsen a fire, but do they have any fire-suppression effects? Will they retard a fire or help keep it from spreading?

Rock and slag wools appear to have some fire protection effect in wood stud walls, increasing the wall's resistance about 54% according to a National Research Council of Canada (NRCC) study.

There are stories floating around that cellulose can also help smother fires in stud wall cavities. The story goes that dense-pack cellulose helps keep oxygen from flowing through the wall to reach the fire, and this keeps the fire from spreading. Fiberglas, more loosely packed, does not have this effect. This conclusion is supposedly supported by an NRCC study of various insulation materials in walls. But this rumor, frequently repeated on the web-sites of cellulose dealers and installers, is completely untrue. There is no NRCC study that concluded that cellulose has a fire-retardant effect in walls. In fact, the study concluded that in non-bearing walls…

"…the installation of the cellulose fiber insulation (wet sprayed) in the wall cavity did not affect the fire resistance rating of the assembly compared to a non-insulated assembly." (Emphasis supplied).

(M. A. Sultan and G. D. Lougheed, Fire Resistance of Gypsum Board Wall Assemblies, IR-833, CNRC 2002.) Download and read the CNRC study. Requires Adobe Acrobat Reader. Download Adobe Acrobat Reader).

In ceilings, however, cellulose may be a more effective fire retardant. A 1995 study by the NRCC found that cellulose fiber insulation produced an increase in fire resistance of 104% — that is, more than double that of an uninsulated attic. But this result does not appear to have ever been replicated by any other study, which, in the scientific community makes it somewhat suspect.

So, here is what we know from actual fire research, as opposed to the rumors and fairy tales:

Rock or slag wool insulation, in certain circumstances, helps walls resist fire penetration. A fire starting on one side of a rock or slag wool insulated wall takes much longer to penetrate the wall to reach the other side. Cellulose and fiberglass do not add any fire protection to a wall, but cellulose may help protect your ceiling and roof in a fire.
Foams give off toxic gases when exposed to flame, and must be covered with a fire-resistant material like gypsum board in order to comply with fire codes. All other materials are at least fire-safe, and do not have to be protected from fire.

This is all we actually know. So don't buy insulation as a fire-retardant. It might be, or it might not be. Right now no one actually knows for sure. Buy insulation to insulate.

Insulating Your Old House

So now we know that to be effective, insulation must

Dramatically slow the convection heat conveyor by creating thousands of tiny air pockets,
Be made, preferably, of a material that is itself a very poor conductor or heat,
Do its part to seek out and seal tiny air leaks and
If possible, reflect radiated heat.

If you are building a new house, insulation is normally installed in the walls before the walls are sealed up. This is "open wall" insulation. Open walls give you many more choices of insulation materials and installation methods. The material used in most new construction is fiberglass batts. This is usually spun fiberglass attached to a paper backing. There are many others, and most of these others are actually better insulation than fiberglass. But since this is an article on insulating old houses, we will leave new house insulation for others to explain.

For old houses the choices are limited to materials that can be blown or pumped or sprayed into existing wall cavities without removing the wall covering — "closed wall" insulation. The usual options are:

Plastic foams which can be pumped into your walls as a liquid, then expand and cure as a rigid, seamless plastic foam similar to the familiar foam coffee cup,
Blown-in particles, which are commonly fiberglass chips, rock wool, and cellulose.

The Petro-Foams

Forms can be sprayed into closed wall cavities by trained and experienced applicators. Although DIY kits are sold on the internet, this is not a job for a novice. Considerable experience is needed to judge when there is just enough foam in the wall to completely fill the cavity, while stopping short of blowing the wall out — which is entirely possible with a product that expands to as much as 120 times its liquid volume, generating considerable pressure in the process. We have also noted that the DIY foam kits cost almost a much per cubic foot of insulation as having a certified professional apply the foam. There is very little cost savings, if any, doing it yourself.

The most common petro-foams are isocyanurate and polyurethane. Both can cure to an open cell or closed cell structure. Open cell structures allow water to penetrate the insulation. Closed cell materials do not, and in many localities closed cell polyurethane foams can be used as a vapor barrier. Closed cell structures are also better at blocking convection. The R-value of open cell foam is about 3.5 per inch — no better than the R-value of cellulose or blow-in fiberglass. Closed cell foam is a much better insulator. When newly installed it has an R-value of about 8 per inch. But over time the value drops to about R-6.25 per inch as the hydro fluoro compounds in the cells leak out and are replaced by air. In a typical 2"x4" stud wall, open cell foam provides a center-of-cavity thermal resistance of about R-13, the same as dense-pack cellulose or fiberglass. Closed cell about R-21.

Given that closed cell foams are so much more effective as insulation, why would anyone choose open cell foam? The answer is that closed cell formulations are quite a bit more expensive, up to four times the cost for just 50% more insulation. They do not expand nearly as much as open cell foams, so more material is required. But where the cavity is narrower than the standard stud wall depth of 3-1/2 to 3-7/8 inches, closed cell foam may be the only option that gets you the R-value you need.

The foams are stellar at finding and penetrating every nook and cranny in a wall cavity. No other material is quite as effective in completely filling the wall. Foams have disadvantages however.

Petro-foams are not at all environmentally friendly. They are made out of petroleum, and use hydro fluoro compounds (HFCs) as blowing agents. Although much more chemically friendly than the urea-formaldehyde foam formulations from the 1970's, these new foams still contain some nasty materials such as benzene and toluene. These unavoidably get into the air during application and by outgassing as the foam ages. According to the EPA, HFCs are 12,500 times more potent than carbon dioxide in producing global warming and some are targeted by the Kyoto Protocol for eventual elimination.
Foams give off some highly toxic gases when in contact with fire — gases that can kill you rather quickly. They are the only commonly used insulation material that is an actual danger in a fire. All building codes now require that foams be completely covered by a material that provides at least 15 minutes of fire protection — standard drywall is the usual choice.
Foams are expensive — up to five times the cost of cellulose.

The Bio-Foams

Some manufacturers have started a switch to bio-oils to replace some of the petro-chemicals in foams. Soybean oil is the most popular substitute at the moment. It does not wholly replace petroleum-based chemicals, but at least one manufacturer claims to replace about 96%, and, as a bonus, uses water as a blowing agent rather than HFCs. While much, much greener than the petro-foams, these products are also much more expensive, and less insulating than petro-foams. And blowing water into a closed wall is a risky proposition. If too much is used and it persists in liquid form, it can breed mold and mildew. Application by factory trained and experienced workers is an absolute requirement with these products. No manufacturer even offers a DIY kit, and most will sell only to their own trained and certified applicators.

Mineral Wools and Fiberglass

The insulation industry considers any insulation composed of mineral fibers the be mineral wool insulation. This includes the common fiberglass insulation products and rock or slag wool. They are made much the same way. Molten material (glass or rock and slag) is spun in a centrifuge to form fibers. Both products are virtually fire proof and will not support combustion. They will melt, but only at very high temperatures — 1800� F, for rock wool. All can be blown into closed walls.

We hear a lot of nonsense about mineral wools, especially fiberglass. The biggest myth is that they cause cancer. They don't. At one time, after the asbestos scares of the 1970s, rock wool and fiberglass insulation came under suspicion as a possible carcinogen — primarily because they are, like asbestos, mineral fibers. Subsequent studies failed to find any connection between either of these materials and any cancer, and the materials were removed from the list of suspected carcinogens in 2000. Still, neither of these products is good for the lungs, and care should be taken to reduce the amount of material that gets airborne during application. All applicators should wear a UL approved particulate filter mask.

The Igloo Factor

How much heat would you have to add to your house if the walls and roof were almost perfect insulators that allowed virtually no air transfer? Answer: About one candle's worth.

H alf a lifetime ago, while in the 11th Special Forces, during an era that worried about the Soviets overrunning Alaska from the Siberian Steppes, some genius in the Pentagon thought it a grand idea for my unit, most of which had just returned from sunny Southeast Asia, to be trained in winter warfare — just to round out our Army experience. So the Army sent us to Arctic Ranger School at frosty Ft. Greely, Alaska.

Ft. Greely was quite a change from Asian jungles. But, we learned a lot of new things: how to navigate unnavigable rivers; how to hide in a place where there is no place to hide; how to parachute in a white-out and live to brag about it over beer the next day; and how to survive an arctic blizzard.

The blizzard thing was not actually pzrt of the training, but since nature presented us with a rather impressive blizzard, the opportunity was just too good to pass up — or so our instructors told us. We trainees were somewhat less enthusiastic about the whole thing.

You need some deep unpacked snow, a U.S. Army M-1943 "Shovel, Folding w/ Cover, Khaki" (or any other shovel, for that matter), and a candle. After you dig out your cave, climb in and seal the doorway with loose snow. Poke a hole the diameter of your arm near the ceiling for ventilation. Light the candle. In a few minutes the heat from the candle will glaze the ceiling with a thin coat of ice, which prevents dripping as the snow cave warms.

Snow is an excellent insulator, containing millions of little air cavities. It allows very little air transfer, and virtually no heat conduction or convection. Very soon the cave warms to the point at which your heavy parka can be removed. A little while later the candle can be extinguished and your body heat alone is enough to stay nice and toasty for the duration of the storm.

This is a good time to catch up on all the sleep you missed since Basic Training. You might be there for a while. These storms can last for days.

We seldom use either product for blow-in application from inside the house. There is too much risk that small fibers will escape and end up in your carpets and furniture. We use cellulose. If cellulose fibers escape, it is usually of no particular consequence. They are just paper, after all.

Rock Wool

Modern rock wool is a manufactured product comprised of a mix of limestone, slag waste from steel blast furnaces, and basalt. The proportions of these minerals vary by manufacture, and it may even be composed of 100% waste slag, in which case the product is usually called "slag wool". The fibers that are typically white in color, but may also be gray or even brown.

Because its main component is waste from steel-making that would otherwise be dumped, rock wool is relatively environmentally benign. It does require a lot of energy to produce, but no more so than fiberglass. Because the fibers are non-combustible and have melting temperatures in excess of 1800� F, rock wool is used to prevent the spread of fire and is the primary material in fire-rated ceiling tile and sprayed-on fire proofing. It is also used as building insulation, pipe wrap, and thermal insulation in ships, mobile homes, and domestic cooking appliances. Tear apart your kitchen range, and you will probably find rock wool inside. Its use as residential insulation has dramatically declined in the U.S. since the advent of fiberglass. Fiberglass is lighter in weight and easier to handle, and generally less expensive than rock wool. But it is still widely used in Europe and Asia. And if you have any insulation in your old house, especially from the 1930s and 40s, it is very likely to be rock wool - now dirty, dusty and an ugly brown, but still relatively effective insulation.

Rock wool in loose blown-in form for use in attics has an R-value of about 2.5 per inch, equivalent to blown-in fiberglass or cellulose. Rock wool tends to settle due to its relatively great weight, so generally more than is needed is blown in to account for eventual settling.

The bottom line on the rock wools is that fiberglass is lighter, less expensive to buy and cheaper to install, and generally more a more effective insulator than rock wool, so, unless you have a special need for the particular properties of rock wool, fiberglass is a better choice.

Fiberglass

Loose fiberglass suitable for blowing in attics has an R-value of about 2.5 per inch. In the chopped-up form ("prime" fibers) used to blow in walls, fiberglass compares to cellulose at about R-3.5 per inch. Fiberglass can have a greater R-value, up to 4.0 per inch, if it is in the form of batts, especially when backed by Kraft paper or foil. But in these forms, fiberglass cannot be blown into a closed wall. You will get about R-13 blowing it into a standard 2"x4" stud wall.

Fiberglass insulates by trapping air in pockets. The glass fibers themselves are not insulating — in fact glass is a fairly good conductor of heat. Nor does fiberglass fill niches and crannies as well as foam or cellulose. Its relatively large fibers tend to clump and leave voids around pipes, electrical wires and electrical boxes in walls. The cavities formed by fiberglass are also relatively large compared to cellulose. When wet, fiberglass loses much of its insulation value, but regains it when it dries. It is glass, so not attractive to bugs as a food source. It is also absolutely fire resistant. Glass can melt, but it will not burn.

The material is about twice as expensive as cellulose, but it is much less expensive than any of the foams or rock wools.

Cellulose

Unlike fiberglass, the material in cellulose insulation — essentially shredded paper — is itself insulating. It does not rely only on trapped air to provide its insulating effect. It is made out of 80-85%% shredded post-consumer newsprint, and is generally considered the "greenest" of the common insulation materials. The Cellulose Insulation Manufacturers Association (CIMA) claims that insulating a 1500 square foot house with cellulose will recycle as much newspaper as an individual will consume in 40 years. We always take trade association claims with a large grain of salt — but in this case, after a few minutes work on the construction calculator, we figure the claim sounds about right.

The finely shredded paper fibers are chemically treated with non-toxic borate compounds to resist fire, insects, mildew and mold. Borates are a naturally occurring minerals. There are virtually no petrochemicals in cellulose — another "green" plus. Cellulose also uses less energy in its manufacture than fiberglass. In eco-speak, it has less "embedded energy" than any other insulation material. Fiberglass and rock wool manufacturing requires about five times more energy.

Properly treated cellulose is permanently fire resistant. Treated cellulose will not burn, in fact, it will barely char. Independent laboratory tests have repeatedly confirmed that cellulose is safe and is approved by all building codes for use in exposed applications — unlike the foams which must be covered with a fire-resistant material. All commercially available cellulose is UL rated for safety. We used to demonstrate the fire resistance of cellulose by holding a clump in one hand, placing a penny on top and melting the penny with a propane torch. The top 1/4" of the cellulose would be charred, but under the char the cellulose was pristine — in fact it was barely warm. Try this test with fiberglass only if your Blue Cross/Blue Shield is fully paid up. (In fact, don't try it at all, just take our word that it won't work).

But mistakes in manufacturing do occur and cases have been reported of cellulose igniting when exposed to flame and, in at least one reported instance, an electrician's trouble light. The best guarantee against improperly treated cellulose is the torch test (See illustration at left) applied to each bundle of cellulose before it is installed.

Unlike fiberglass or any of the foams, cellulose is hygroscopic. It�s able to soak up and retain liquid water. If undetected and untreated, it can be a growing bed for mold and mildew even though the borate treatment given to cellulose kills most mold and mildew. But the chemicals in wet cellulose can also corrode metal such as nails, pipes and electrical wires over extended periods of time. Of course, if you have this much water in your walls, insulation is probably the very least of your soon to be very many worries.

Because cellulose material is itself insulating, it can be dense packed for even more insulation effectiveness. Fiberglass can be dense packed only to a point, after that further packing actually decreases its insulating value as the tiny air pockets necessary for its effectiveness are eliminated.

Densely packed cellulose blocks air leaks better than blow-in fiberglass, and is second only to the foams at finding and filling all the voids in your walls. The process works much like the sugar dispenser at a restaurant. First the sugar flows freely. Then a large clump or two gets wedged in the opening. A few more smaller clumps collect at the opening, then individual grains pile up around these and the opening is blocked completely so no more sugar comes out. Annoying when you need that morning cup of coffee, but great when filling gaps in cavity walls. While nothing fills cavities like foam, cellulose is a strong runner-up.

At R-3.50 to R-4.0 per inch, cellulose blocks heat transfer better than fiberglass, and just about as well as most, more expensive, open cell foams. It is less efficient than closed cell foams, but also about 1/4 the cost - so the cost-to-value ratio is much better. In a typical 2"x4" stud wall (the wall cavity is usually 3 3/4" in old walls), the rated R-value of cellulose is about R-14, which complies with most modern building and energy code requirements in Nebraska. Taking into account the other materials in your walls, including the thermal breaks caused by your wood studs and other framing, the total clear wall insulation value of a cellulose-insulated wall is about R-15.35. (See below).

For another perspective on cellulose as insulation, read the University of Massachusetts (Amhurst) report on cellulose insulation: Cellulose Insulation � A Smart Choice.

Blocking Radiation
Reducing conduction and blocking convection are not complete solution to heat movement in and out of your house. There is also the problem of radiation. In the southern parts of the U.S., and especially in the desert southwest, radiation is a primary concern. Not so much in net heating climates like our own. Still, on bight, sunny summer days when sunlight is blasting through your west windows, you may wonder why you did not pay more attention to it.

None the traditional insulation materials are of any help. Radiation is not stopped by cellulose, fiberglass, or foams. The only way to combat the effects of radiation is to install a radiant barrier to reflect the radiation away from the house. In new wall construction this is accomplished by using foil-faced batts of fiberglass insulation installed while the walls and ceilings are still open. Foils reflect about 97% of the radiation that strikes them.

In windows the job of reflecting radiation is handled by low-emissivity ("low-e") coatings. These are just reflecting, usually metal, coatings applied to the glass to reflect radiating heat. The are tuned to the specific frequency of heat waves so they block most heat while allowing most light to pass.

For old houses, there is, unfortunately, no really good cure for radiation in walls (except to paint your house in a light color — light colors absorb less heat than dark colors). On the bright side, however, radiation through walls is not a prime producer of heat loss and gain. Studies of wall insulation in controlled laboratory settings by the Oak Ridge National Laboratories under contract with the Department of Energy found that including reflective foil in a well-insulated 3-1/2" wall cavity increased its R-value only slightly, from 13.9 to 14.4. In-wall convection, air leaks and heat conduction are much more important than radiation in producing heat transfer in walls.

Your roof, however, is another story. It takes a terrific beating from the sun in summer. So it can be a major contributor to your cooling load. Fortunately it's easier to install radiant barriers in your attic.

Radiant Foil Sheets Radiant barriers are usually in the form of paper- or plastic-backed foil sheets that can be stapled to your rafters or laid on top of your existing attic insulation. The Achilles Heel of these products is dust. Dust eventually settles on foil sheets reducing their reflectivity. Sheets stapled to your rafters collect less dust than sheets laid flat, but over time all will collect dust. The less reflective they are, the less they protect. In fact, when covered with a nice thick coat of dust, they become heat collectors, adding to the attic heat problem.

To combat the effects of dust, two or more sheets can be laid on top of one another. The top sheet may get dusty and loose reflectivity, but the bottom sheet is protected by the top sheet and does not get dusty. Some manufacturers make double and even triple sheet foils just for this purpose.

Radiant Chips Radiant barrier chips are being heavily promoted as better option than foil sheets because they are blown 6 to 10 layers deep on top of your attic insulation and are barely affected by dust. These are basically foil shards, although some are metal coated plastic. They work as well as and even better than flat foil barriers. Their drawback is that they are much more expensive than flat foils.

Radiant Coatings Even more interesting are the liquid radiant barriers. These are actually just reflective paints — although most manufacturers avoid using the word paint. They are sprayed, rolled or brushed on the underside of your roof. They can't get dusty. so the lose little effectiveness over time. They do not reflect as much radiation as foil, only 75% compared to 97% for new foil. But after a few years of dust, the two products have a similar effectiveness, and after a few more years, the paints begin to outperform many of the foils. So in the long term, the paints are probably more effective than either the sheets or chips. Paints need air space below them to work effectively. Which is fine, because all attic insulation needs air space to work effectively.

Attic Air Flow Attic insulation depends on their being adequate air flow from the vents in the eaves to the vents in the ridge. Insulation should never be packed right to the roof. This is the air that carries away the heat the builds up above the insulation and helps it work better. An air channel is needed to properly ventilate the attic. Without it, insulation can lose up to half its effectiveness.

Almost all building codes now require attics to have at least 1" of air space between the roof sheathing and any insulation. Often this is accomplished by the installation of ventilation chutes which protect the air way from accidental blocking by insulation.

Other Wall Materials
Besides insulation there are other materials in your walls that affect the heat barrier. We have already seen that thermal bridges caused by the framing in your

THE CLEAR WALL R-VALUE OF A 2X4 STUD WALL The study house included an extra 1/2" of plywood sheathing, possibly added when the house was re-sided. This layer is not typical and has been omitted from the table of results reported here.
R-VALUE FOR 2X4 WALL AT CAVITY AREAS			R-VALUE FOR 2X4 WALL AT STUDS
Component	Thickness	R-Value	Component	Thickness	R-Value
Inside Air Film	-	0.68	Inside Air Film	-	0.68
Interior Plaster	3/4"	0.45	Interior Plaster	3/4"	0.45
Blown-in Cellulose Insulation	3-3/4"	13.58	2x4 SPF Stud	3-3/4"	4.56
Horizontal Sheathing Boards	3/4"	0.93	Horizontal Sheathing Boards	3/4"	0.93
Felt Building Paper	-	0.03	Felt Building Paper	-	0.03
Cedar Drop Lap Siding	1/2"	0.81	Cedar Drop Lap Siding	1/2"	0.81
Exterior Air Film	-	0.17	Exterior Air Film	-	0.17
TOTAL R-VALUE (CAVITY AREA)		16.9	TOTAL R-VALUE (STUD AREA)		7.78

CLEAR WALL R-VALUE (CAVITIES AND STUD)	(% stud area x stud R-value) + (% cavity area x cavity R-value) = .17(7.78) + .83(16.9) =				15.35

wall reduce its effectiveness as a heat barrier. In contrast, your house siding, wall sheathing, inside plaster or drywall, and a thin film of dead air that clings to both the interior side and exterior side of your wall add some insulating value.

Curious as to how much insulation value a fully insulated old house wall might have when all of the materials in the wall are included in the R-value calculation, University of Oregon scientists did a study of a recently renovated 1913 four-square house to find out the actual R-value of insulated exterior walls. The table at left shows the study's findings.

The non-insulation components of the wall, including the interior and exterior air films added R-3.22 of insulation to the wall, a small increment, but appreciated nonetheless. The difference between the R-value of the insulated wall cavities and the thermal bridges at the studs was significant. Because of their greater conductivity, the studs had an R-value of only 4.56 for a 4" stud compared to R-13.68 for 4" of blown-in cellulose. Despite the loss of insulation caused by traditional old house framing, the average R-value for the clear wall, including both cavities and studs is 15.35, which exceeds the energy code minimum of R-13 in Nebraska by a safe margin.

So it is well worth the cost to weather seal and insulate your old house, even though it was not actually built with effective insulation in mind.

Insulating Closed Walls

Whatever material you choose, the key to success is careful and ceaselessly vigilant application. Sloppy or unskilled application can defeat even the best insulation material.

To insulate a wall cavity, we need to gain access to the cavity. We can do this from outside your house through your siding, or from inside your house through the drywall or plaster. Generally, outside-in is the best choice since it leaves most of the mess outside. There will be some mess. It's unavoidable.

The process is roughly the same for all closed wall insulation materials. A hole between 3/4" and 2" is drilled into your wall at the top and middle of each wall cavity. A nozzle or tube is inserted and the wall cavity is filled with the insulating material.

If we are working from the exterior, we prefer to remove the row of siding where we drill. But sometimes we cannot remove old and brittle wood, or steel or aluminum siding without damage. If we cannot remove your wood siding then we have to drill through it. We use a wood plug to match your siding (cedar for cedar, pine for pine), glued in the hole, and sanded flush. When painted it is virtually invisible. If you have vinyl or steel siding, and for some odd reason you did not insulate when the siding was installed, then we will have to remove at least one row of siding per floor. This entails the risk of damage to the siding. If we cannot safely remove the siding, then insulating from the inside may be the only available choice.

Certain kinds of exterior finish are difficult to penetrate: brick and stone veneers, for example. We can do it, but it is time-consuming because these materials are slow to drill through, and we generally work with a smaller 1/2" hole. It takes four times as long as working through wood siding, and is, therefore quite a bit more costly.

If we are working from inside your house, we plug with a Styrofoam plug and apply drywall compound over that. The foam plug holds the drywall compound securely. When dry, we sand it to blend it into the existing wall. Again, after painting, it is invisible.

Safety and Personal Protection
All insulation materials can be dangerous if not handled properly. If fiberglass, rock wools or foam gets in your eyes, it can cause serious damage. Cellulose is more benign, but still an irritant. Eye protection in the form of wrap-around tightly sealing goggles are a must. Fiberglass in the lungs can be very serious. Once in, it never gets out. Fiberglass and rock wool were cleared of any role in cancer formation in 2000, but glass in the lungs cannot be good for you. Cellulose is just paper, so less of a concern. But never forget that Brown Lung disease is caused by breathing cotton fibers, so even relatively benign particles can cause damage. No matter the material used, always wear a good, UL-rated, particulate filter mask.

Protective clothing is not generally required for cellulose, foam or rock wool, but fiberglass is another story. Fiberglass in contact with the skin is an irritant, and itches like the very devil. Heavy clothing is optional. If you can stand the itching, then do without the clothing. The rest of us bundle up like mummies.

Be aware also that loose fiberglass fill in your attic remains a potential irritant for as long as it stays in your attic. If you need to go into your attic for any reason, wear the same protective gear you would wear for application.

Loose Packing and Dense Packing
Materials such as rock wools, fiberglass and cellulose are generally blown into attics and allowed to form a loosely compacted layer. This is "loose packing". Slightly more material than is needed to achieve the desired R-value is blown in because these materials in loose pack form will settle over time. In attics, that's not much of a problem because if they settle too much, more can be blown in later to bring the R-value back up. Expect about R-2.5 per inch from loose packed insulation.

Dense Packing In walls, settling is a bigger problem. If the material settles, it leaves a void at the top of the wall which cannot be fixed except by re-drilling and blowing in more material - a nuisance and expensive. So manufacturers and applicators have worked for years to develop the various techniques now known as "dense packing" to eliminate settling. The material is blown into the wall using a stream of air moving at a relatively high velocity — 100 feet per second or higher. These high-velocity blowers are a specialty tool. The insulation blowers you can rent generally do not have enough force to dense pack. They are for attic insulation — not for walls.

The mixture is deliberately kept very lean — much more air than material. In this environment the insulation behaves like a liquid, flowing around obstructions and filling all the nooks and bypasses in the wall. The aim is to reach the density recommended by the manufacturer for closed wall insulation. For cellulose, for example, this is usually an average density of 3.2 to 3.5 pounds of cellulose per cubic foot of wall cavity. This density not only provides excellent insulation, but blocks almost all air leaks and virtually eliminates settling. Not every square inch will contain just the right amount of material. The application is a blind process, we can't actually see what's going on in a closed wall. But, if, on average, we blow in 3.2 to 3.5 lbs. per cubic foot we know we have a well-insulated wall.

Dense packing requires skill and a lot of practice. You have to use the sound and feel of the hose, and your sense of how long it should take to fill a cavity to determine when enough material has been inserted, but not too much so it does not blow-back and go all over the place. It's tricky, and, despite what the tool rental places and big lumber stores may say, it is not a job for the inexperienced. The usual result when untrained applicators try it is that not nearly enough material is applied, many large voids are left in the walls, and the insulation is not fully effective.

Cellulose can be dense packed, fiberglass cannot be. Dense packing fiberglass reduces its insulation effectiveness. Heavy rock and slag wools tend to settle even when dense packed due to its relatively great weight, so they are rarely used in closed wall applications in the U.S. these days. In Europe and Asia, however, they are still widely used. ("Dense" fiberglass is not dense packed fiberglass, although the terms are often confused. Dense fiberglass is composed of especially small particles and usually intended for blow-in wall application. Many companies call the material "prime fiber" fiberglass.)

In dense pack applications, cellulose has an R-value of about between R-3.5 and R-4.0 per inch — about R3.75 on average. Fiberglass formulated for blow-in application runs about R-2.75 to R3.50, depending on composition and manufacturer, and is about twice the price. Some companies make a special fiberglass product for wall and claim that it equals and even exceeds the R-value of cellulose. But if any outside verification of these claims exist at all, they are usually from company-funded studies, and suspect. We know of no truly independent studies of these products that support these high R-value claims. If you do, please let us know.

When and Where to Insulate Your Closed Walls

Photo: U.S. Department of Energy

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The Energy Star brochure for DIY insulators is full of useful facts, safety precautions, and helpful tips.

R-Value of Insulation and Other Common Materials
Insulation Materials (PDF Table)
Material	Application	Center-of-Cavity R-Value per Inch	R-Value in a 3-3/4" Wall Cavity
Fiberglass Batt (Standard)	Open wall	3.14	11.78
Fiberglass Batt (Dense)	Open wall	3.85	14.44
Fiberglass Batt (Dense - Foil Backed)	Open wall	4.30	16.13
Fiberglass Blow-in (Loose Pack)	Attic	2.20-2.75	-
Fiberglass Blow-in (Prime Fiber)	Closed Wall	2.75-3.14	10.31-11.78
Rock Wool Batt	Open wall	3.10	11.63
Rock Wool Blow-in (Loose Pack)	Attic	3.10	-
Rock Wool Blow-in (Dense Pack)	Open/Closed Wall	3.70	13.88
Cellulose Blow-in (Loose Pack)	Attic	3.13-3.50	-
Cellulose Blow-in (Dense Pack)	Open/Closed Wall	3.71-4.00*	13.92-15.00
Expanded Polystyrene (EPS, Beadboard)	Attic, Open Wall	4.00	15.00
Extruded Polystyrene (XPS)	Attic, Basement, Open Wall	5.00	18.75
Polyurethane Foam (Closed Cell)	Attic, Basement, Open/Closed Wall	6.25	23.44
Construction Materials
Material		R-Value per Inch (If Applicable)	R-Value
Concrete Block 4"		-	0.80
Concrete Block 8"		-	1.11
Concrete Block 12"		-	1.28
Brick 4" Common		-	0.80
Poured Concrete		0.08	-
Spruce, Fir, Pine Lumber		1.40	-
Cedar Lumber		1.33	-
Plywood		1.25	-
Fiberboard		2.84	-
Hardboard		-	0.34
Wood Lap Siding		-	0.80
Aluminum, Steel, Vinyl Siding		-	0.61
Aluminum, Steel, Vinyl Siding with 1/2" Insulating Board Backer		-	1.80
Felt Building Paper		-	0.03
Gypsum Board Drywall (1/2")		-	0.45
Wood Paneling (3/8")		-	0.47
Particle Board		1.31	-
Hardwood Flooring		0.81	-
Vinyl Tile, Vinyl Sheet, Linoleum		-	0.05
Carpet (Fiber Pad)			2.08
Carpet (Rubber Pad)			1.23
Asphalt Shingles			0.44
Wood Shingles, Shakes			0.97
Single Panel Window Glass			0.91
Single Panel Window Glass with Storm Window			2.20
Double Pane Window Glass (Sealed)			2.10
Triple Pane Window Glass (Sealed)			3.20
Still Air and Dead Air Films
Description		R-Value per Inch (If applicable)	R-Value
Dry, Still Air		3.6	-
Interior Ceiling Film		-	0.61
Interior Wall Film		-	0.68
Exterior Wall Film		-	0.17

You can insulate your walls from the outside, or from the inside. From the outside is easier and creates less mess and disruption to your regular routine. The best time to insulate walls from the outside is when you are already doing something major to the exterior of your walls — replacing the siding, or painting the exterior, for example. Insulating requires almost the same preparation as siding or painting, so it is a minor addition to the process compared to the work, mess and bother of either siding or painting.

If you are adding new siding, also consider adding an insulated sheathing. We don't mean the 1/4" foam board siders often use under new siding, but R-5 rigid insulation board. For Zone 5, which is where we are (just barely — Zone 4 starts at the Kansas border), the Energy Star program recommends R-13 in a 2"x4" stud wall and R-5 rigid foam sheathing under you new siding.

Insulating walls from inside your house can be a lot messier and require more preparation than insulating from outside. But if you are painting the inside of your walls, it may be a good time to insulate from the inside. You will need to drill holes through the interior plaster or gypsum board walls, but Styrofoam plugs and a little spackling will hide the holes nicely, and once painted they will be invisible. The preparation you do for painting is usually the same as preparation for insulating — drop cloths cover the floor, the furniture is moved out or covered — so insulating is not much of an added step — and certainly well worth the added work and modest expense.

Insulating Your Attic
If your plan is to insulate your walls in a few years when you paint your siding, don't wait that long to increase the insulation in your attic. Even if your blew in some extra inches just a few years ago, it's time to add more. The recommended level of insulation has changed. The Nebraska Energy Code now requires a minimum of R-38 in your attic. The EPA Energy Star program recommends up to R-60.

Attics tend to be very hot in summer and very cold in winter, so spring and fall are the best times to insulate. If you are doing it yourself, wear the proper protective gear for the material you are using, and, especially in summer, drink plenty of liquids.

We recommend against trying to insulate a closed wall yourself. But, insulating an attic is well within the capabilities of a seasoned do-it-yourselfer. But first read the very useful D-I-Y guide published by the federal Energy Star program (See sidebar at left). The brochure is full of useful facts, safety precautions, and helpful tips. Also, do something a little different this time: read and follow the instructions and safety precautions that come with the material you are using.

The R-Value of Insulation and Other Materials Common in Old Houses
The reported R-value of materials seems to depend largely on how R-value is measured and the economic interest of the person or entity doing the reporting. Every manufacturer tends to extol its own product, and dismiss competing products. Fiberglass manufacturers exaggerate the R-value of fiberglass and understate the R-value of cellulose. Cellulose producers are no better. We see constant comparisons stating that blow-in fiberglass has an R-value of 2.25 while blown-in cellulose has an R-value of 3.9. Both statements are true, but what's being glossed over is that the fiberglass is loose pack attic insulation being compared to dense pack cellulose wall insulation. If wall insulation is compared to wall insulation, fiberglass and cellulose are much closer in R-value.

It's just the way things are marketed here in the Good Ole' U.S. of A. If you lie a little, it's advertising, and if you do it well, an jury of your peers may award you a Clio. If you lie a lot, it's fraud, and a jury of your peers may award you a stretch in Leavenworth.

All these half truths, while not exactly the big lies, do make it hard for the owner of an old house to figure out the R-value of various materials, and which materials to use to have the best insulating effect.

To eliminate some of this confusion, and to educate ourselves, we decided to try to ferret out the actual R-value of common insulation and construction materials. We discounted manufacturers' claims unless supported by independent studies. Where several different R-values are reported, we tried to determine how, and by whom, they were calculated. If the calculations were made roughly the same way, but different results are reported by disinterested parties (e.g. universities or government organizations, Consumer Reports), we took the average of the different results.

The R-values in this table may not be completely accurate, but they are probably pretty close. And at very least, they are not self serving because we don't have any economic interest at all in promoting any particular product or material.