Insulating Your Old House, Part 2: 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. All insulation, no matter its composition, works the same way. It traps small pockets of air. In some forms of insulation, it is the trapped air more than the actual insulation material that provides the insulative effect.

Slowing Heat Transfer

Heat travels from hot to cold. In winter the heat inside your house is working very hard to get out, and in summer it is trying to get from the hot outside to the cooler inside of your house. This is the exact opposite of the way we would like heat to move but stopping it from moving is actually impossible. Ultimately all of the heat inside your house in winter will move outside and the heat outside of your house in summer will move inside. The best we can do is erect barriers to slow it down.

Those barriers are insulation.

Slowing Convection

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.

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

What is the R-value of your walls, and do you 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.

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 the 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 effect 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.

Go outside and 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

R-Value Table

Click to Enlarge Table.

For example, if the interior wall is 70° and the 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)

All insulation slows convection by dividing the one big air cavity in your wall into thousands of tiny cavities or cells. Generally, the smaller the cell, the better in insulation. 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 slow convection better than others but all insulation focuses primarily on slowing convection.

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 fairly 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, The material as a whole works pretty well — but not as well as foam or cellulose.

Reflecting Radiation

Heat 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 tunable radiant barrier; one that blocks only radiation moving in the wrong direction and can be reversed when the seasons change. 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.

Even 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.

Types of Insulation

So now we know that to be effective, insulation must

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

If you are building a new house, insulation is normally installed in the walls before the walls are closed 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, pumped, or sprayed into existing wall cavities without removing the wall covering — "closed wall" insulation. The usual options are:

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.

The Igloo Factor

How much heat would you have to add to your house if the walls and roof were nearly perfect insulators that allowed almost no heat transfer?

Half a lifetime ago while serving with the 11th Special Forces during an era that worried about the Soviets overrunning the Alaskan Pipeline from the Siberian Arctic Ranger Tab Steppes, some chair-warming strategic genius in the Pentagon thought it a grand idea for my unit to round out its Army experience by being trained in winter warfare.

So the Army sent us on an all-expenses-paid adventure to the Arctic Ranger School at Ft. Greely, Alaska.

Located about three blocks from the arctic circle, Ft. Greely was quite a change from fetid Asian jungles from which most of us had just returned.

But, we learned a lot of interesting 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 part of the training syllabus but since nature presented us with a rather impressive arctic storm, the opportunity was just too good to pass up — or so our instructors told us. We trainees were a whole lot less enthusiastic.

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.

Loose snow is an excellent insulator, containing millions of little air pockets. It allows virtually no heat transfer.

Very soon the cave warms to the point at which your heavy "Parka, Man's, Arctic, M-65, with M-55 Cover, Camouflage, White" 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.

The Bio-Foams

Some manufacturers have started a switch to bio-oils to replace some of the petrochemicals 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. The fibers are virtually fireproof 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.

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 or Slag Wool

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 manufacturer, 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.

The fibers are non-combustible and have melting temperatures in excess of 1800° F, so rock wool is used to prevent the spread of fire and is the primary material in fire-rated ceiling tile and sprayed-on fire-retardant proofing.

It is also used as 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, easier to handle, and generally less expensive than rock wool. But, rock wool 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 effective insulation.

It is available loose, in rolls and in batts. 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 usually a better choice.


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.


Unlike fiberglass, cellulose 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 naturally occurring minerals. There are virtually no petrochemicals in cellulose — another "green" plus.

Cellulose also uses less energy in its manufacture than any other insulation material. In eco-speak, it has less "embedded energy" than any other materials. 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 will 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 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.

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 essentially 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. At 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 1/5 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. 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.

Radiant Barriers

Reducing conduction and blocking convection is most of the solution but not a 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.

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 (ORNL) 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. None of 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 reflective, usually metal, coatings applied to the glass to reflect radiating heat. They are tuned to the specific frequency of heat waves so they block most heat while allowing most light to pass. To learn more about radiant barriers in windows. See: Your Old Windows.

In attics, radiant barriers are of three basic types: sheets or rolls, foil chips and heat reflecting paint.

Radiant Foil Sheets and Rolls

Radiant barriers are usually in the form of paper- or plastic-backed foil sheets, usually in rolls, that can be stapled to your rafters or laid on top of your existing attic insulation. (Think "large roll of aluminum foil.")

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 can 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 a 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 often 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 known as Interior Radiation Control Coatings or IRCCs. They are actually just reflective paints — although most manufacturers avoid using the word paint. The primary reflective elements are tiny highly polished aluminum flakes immersed in an infrared-neutral liquid binder. They are sprayed, rolled or brushed on the underside of your roof. They can't accumulate a thick coating of dust, so they lose little effectiveness over time.

They do not reflect as much radiation as foil, a little more than 75% compared to as much as 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 airspace below them to work effectively. Which is fine, because all attic insulation also needs airspace to work effectively.

Not just any shiny paint is an IRCC. To qualify as an IRCC the coating must reduce emissivity to 25% or less. Emissivity refers to the percent of radiant heat that a hot object emits or radiates. It is the opposite of reflectivity. A coating that reflects 75% of the Sun's energy and permits only 25% to "emit" into the attic is considered an IRCC. But, only if it has been tested according to the protocol specified in ASTM C1321 and certified by the testing organization. If it passes it is considered an IRCC and will bear the testing organization's mark. Otherwise, it is just shiny paint. One IRCC coating, Lo/Mit Max by Solec has been certified to block 86% of the sun's radiant heat, which is getting very close to the efficiency of foil sheets.

For comparison, Oriented Strand Board (OSB) used as sheathing on most modern walls and roofs admits 93% of the sun's heat into the attic. The average emissivity of attic insulation, including fiberglass and cellulose, is 85%. So, you can see that a little paint of the right kind can greatly reduce the heat flowing into your attic.

Attic Air Flow

Attic insulation depends on 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 that 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 airspace between the roof sheathing and any insulation. Often this is accomplished by the installation of rafter ventilation chutes which protect the airway 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 wall reduce its effectiveness as a 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.

The Clear Wall R-Value of a 2"X4" Stud Wall

Component Thickness at
Inside Air Film 0.68 0.68
Interior Plaster 3/4" 0.45 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 0.93
Felt Building Paper 0.03 0.03
Cedar Drop Lap Siding 1/2" 0.81 0.81
Exterior Air Film 0.17 0.17
Total R-Value at Insulation 16.9
Total R-Value at Studs 7.78
Clear Wall R-Value
(Combined Insulation and Studs)

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.

Insulation and 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.

The only insulation materials that are potentially harmful in a fire are the foams. They don't actually burn in the sense of sustaining flame but when in contact with high heat they give off toxic gases. The solution to the problem is to ensure that they do not come in contact with a potential fire by isolating them with a fire-retarding material. This is why every building code requires them to be covered with a fire-resistant barrier.

Rock wool and fiberglass 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.

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 (NRC) study. (Canada has much more interest in insulation than the U.S. and does a lot of studies. Much of what we know about insulation comes from Canadian research.)

Cellulose has a reputation as a fire suppressant. It supposedly helps smother fires in stud wall cavities. The thinking is 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. The difference is often cited by cellulose manufacturers to show that cellulose is superior to fiberglass as an insulation material.

But, various NRC studies have found that both materials provide more fire-resistance when compared to an uninsulated wall, and the difference between the two materials is minimal with cellulose having just a slight advantage.

So, here is what we know from actual fire research, as opposed to folklore:

Rev. 10/13/18