The R-value of an Insulation Product, is commonly misunderstood and ignored.


An insulating material’s resistance to conductive heat flow is measured in terms of its thermal resistance or R-value. The higher the R-value, the greater the insulating effectiveness. The R-value depends on the type of insulation, its thickness, and its density.

Consider the R-value of an insulation after it has been submerged in water or with a 20 mile per hour wind blowing through it. The R-value of fiber insulations would go to zero. Under the same conditions, the solid insulations would be largely unaffected. R-value numbers are “funny” numbers meaning they are meaningless unless we know other characteristics. None of us would ever buy a piece of property if we knew only one dimension. Suppose someone offered a property for $10,000 and told you it was a seven. You would instantly wonder if that meant seven acres, seven square feet, seven miles square, etc. In other words, one number cannot accurately describe anything. The use of an R-value alone is irrelevant yet we have code bodies mandating R-values of 20’s or 30’s or 40’s. A 25 R-value fiber insulation placed in a house not properly sealed will allow the wind to blow through it as if there were no insulation at all. The R-value may be accurate in the lab, but it is not even remotely part of the real world. We must start asking for some additional dimensions to our insulation. We need to know its resistance to air penetration, free water, and vapor drive. What is the R-value after it is subjected to real world conditions? The R-value is a fictitious number supposed to indicate a material’s ability to resist heat loss. It is derived by taking the “K” value of a product and dividing it by the number one. The “K” value is the actual measurement of heat transferred through a specific material.

insulation for timber framing

Test to Determine the R-Value

The test used to produce the “K” value is an ASTM test. This ASTM test was designed by a committee to give us measurement values that hopefully would be meaningful. A major part of the problem lies in the design of the test. The test favors the fiber insulations — fiberglass, rock wool, and cellulose fiber. Very little input went into the test for solid insulations, such as foam glass, cork, expanded polystyrene or urethane foam.
The test does not account for air movement (wind) or any amount of moisture (water vapor). In other words, the test used to create the R-value is a test in non real-world conditions. For instance, fiberglass is generally assigned an R-value of approximately 3.5. It will only achieve that R-value if tested in an absolute zero wind and zero moisture environment. Zero wind and zero moisture are not real-world. Our houses leak air and they often leak water. Water vapor from the atmosphere, showers, cooking, breathing, etc. constantly moves back and forth through the walls and ceilings. If an attic is not properly ventilated, the water vapor from inside a house will very quickly semi-saturate the insulation above the ceiling. Even small amounts of moisture will cause a dramatic drop in fiber insulation’s R-value — as much as 50 percent or more.

Vapor Barriers

We are told that insulation should have a vapor barrier on the warm side. Which is the warm side of the wall of a house?

Obviously, it changes from summer to winter — even from day to night. If it is 20 degrees below zero outside, the inside of an occupied house is certainly the warm side. During the summer months, when the sun is shining, the warm side is the outside. Sometimes the novice will try to put vapor barriers on both sides of the insulation. Vapor barriers on both sides of fiber insulation generally prove to be disastrous. It seems the vapor barriers will stop most of the moisture but not all. Small amounts of moisture will move into the fiber insulation between the two vapor barriers and be trapped. It will accumulate as the temperature swings back and forth. This accumulation can become a huge problem. We have re-insulated a number of potato storages which originally were insulated with fiberglass having a vapor barrier on both sides. Within a year or two the insulation would completely fail to insulate. The moisture would get trapped between the vapor barriers and saturate the fiberglass insulation to the point of holding buckets of water. Fiber insulation needs ventilation on one side; therefore, the vapor barrier should go on the side where it will do the most good. At very cold temperatures, when the temperature difference across the attic insulation reaches a certain critical point, convection within the insulation can reduce R-value.

We understand air penetration through the wall of the house. In some homes when the wind blows, we often can feel it, but what most people, including many engineers, do not realize is that there are very serious convection currents that occur within the fiber insulations. These convection currents rotate vast amounts of air. The air currents are not fast enough to feel or even measure with any but the most sensitive instruments. Nevertheless, the air is constantly carrying heat from the underside of the pile of fibers to the top side, letting it escape. If we seal off the air movement, we generally seal in water vapor. The additional water often will condense which becomes a source of water for rotting of the structure. The water, as a vapor or condensation, will seriously decrease the R-value. The only way to deal with a fiber insulation is to ventilate. But to ventilate means moving air which also decreases the Rvalue.

Air Penetration

The filter type for most furnace filters is fiberglass — the same spun fiberglass used as insulation. Fiberglass is used for an air filter because it has less impedance to the air flow, and it is cheap. In other words, the air flows through it very readily. It is ironic how we wrap our house in a furnace filter that will strain the bugs out of the wind as it blows through the house. There are tremendous air currents that blow through the walls of a typical home. As a demonstration, hold a lit candle near an electrical outlet on an outside wall when the wind is blowing. The average home with all its doors and windows closed has a combination of air leaks equal to the size of an open door. Even if we do a perfect job of installing the fiber insulation in our house and bring the air infiltration very close to zero from one side of the wall to the other, we still do not stop the air from moving through the insulation itself vertically both in the ceiling and the walls.

The best known solid insulation is expanded polystyrene. Other solid insulations include cork, foam glass and polyisocyanate or polyisocyanurate board stock. The latter two being variations of urethane foam. Each of these insulations are ideally suited for many uses. Foam glass has been used for years on hot and cold tanks, especially in places where vapor drive is a problem. Cork is a very old standby often used in freezer applications. EPS or expanded polystyrene is seemingly used everywhere from throw away drinking cups and food containers to perimeter foundation insulation, masonry insulations, and more. Urethane board stock is becoming the standard for roof insulation, especially for hot mopped roofs. It is also widely used for exterior sheathing on many of the new houses. The R-value of the urethane board stock is of course better than any of the other solid insulations. All of the solid insulations will perform far better than fiber insulations whenever there is wind or moisture involved.

Most of the solid insulations are placed as sheets or board stock. They suffer from one very common problem. They generally don’t fit tight enough to prevent air infiltration. It does not matters how thick these board stocks are if the wind gets behind it. We see this often in masonry construction where board stock is used between a brick and a block wall. Unless the board stock is physically glued to the block wall, air will infiltrate behind it. In this case. as the air flows through the weep holes in the brick and around the insulation it is rendered virtually useless. Great care must be taken when placing the solid insulations. The brick ties need to be fitted at the joints and then sealed to prevent air flow behind the insulation.

The only commonly used solid insulation that absolutely protects itself from air infiltration is the spray in place polyurethane. When it is properly placed between two studs or against the concrete block wall, the bonding of the spray plus the expansion of the material in place will effect a total seal. This total seal is almost impossible to overestimate. In my opinion most of the heat loss in the walls of the home have to do with the seal rather than the insulation. For physical reasons, heat does not conduct horizontally nearly as well as it does vertically. Therefore, if there were no insulation in the walls of the homes, but an absolute airtight seal, there would not necessarily be a huge difference in the heat loss. This would not be the case if the insulation was missing from the ceiling. Air infiltration can most effectively be stopped with spray in place polyurethane. It is the only material that will fill in the corners, the cripples, the double studs, bottom plates, top plates, etc. The R-value of a material is of no interest or consequence if air can get past it.

During the 1970s, my firm insulated a large amount of new homes in the Snake River Valley of Idaho with 1.25 inches of spray in place polyurethane foam in the walls. In 1970, the popular number for the R-value of one inch of urethane foam was 9.09 per inch. Using this value, we were putting an R of 1.25 x 9.09 = 11.36 in the walls. This was much less than the R = 16 claimed by the fiberglass insulators. Today, using the charts from an ASHRAE book, we would only be able to claim an R-value for the 1.25 inches of 7.5 to 9. Neither of these numbers make for a very big R-value. The reality is that the people for whom we insulated their homes invariably would thank us for the savings in their heat bills. They would tell us their heating bill was half of their neighbor’s. They felt as if they saved the cost of the polyurethane in one, or at most two, years. This is anecdotal evidence, I know, but anecdotal evidence is also compelling and very real in our world. These customers were savvy people. They would not have paid the extra to get the urethane insulation if it had not been better.

The problem with loose-fill fiberglass attic insulation is cold climates. As attic temperature drop below a certain point, air begins to circulate into and within the insulation, forming “convective loops” that increase heat loss and decrease the effective R-value. At very cold temperatures (-20F), the R-value may decrease by up to 50%.” In full-scale attic tests at Oak Ridge National Laboratory, the R-value of 6 inches of cubed loose-fill attic insulation progressively fell as the attic air temperature dropped. At -18 F, the R-value measured only R-9. The problem seems to occur with any low-density, loose-fill fibrous insulation.*

Urethane Conserves Energy

Excellent thermal resistance is the primary performance benefit of urethane foam insulation, but it is not the only one. Urethane also has these advantages as a construction material: Its closed cell structure makes urethane most effective initially and in the long run. When protected by skins or other covering, urethane will not absorb water.Consequently the X factor (thermal conductivity) is virtually constant. Sprayed-on foam has the advantage of no seams or joints.

During the late 1970s, the FTC went after the urethane foam suppliers for misleading advertising especially with regard to fire claims. A consent decree followed. It destroyed a tremendous amount of confidence in the use of urethane. Up to that point, Commonwealth Edison would give Gold Medallion approval for homes insulated with 1.25 inches of spray in place urethane in the side walls of masonry constructed homes. True, that was anecdotal evidence, but it worked. Much work was done in the early 1970’s using 1.25 inches urethane as a replacement for wall insulation in a home. Not only did it replace the wall insulation, it also replaced the exterior sheathing. The buildings are stronger and better insulated when sprayed with the 1.25 inches of urethane. Understanding the two purposes of insulation gives a standard to measure the insulations:

A. Heat loss

There is a substantial difference between insulation for temperature control and insulation for heat loss control. For instance, the graph below shows the heat loss control of the spray-in-place urethane foam insulation. Any insulation will have a similar graph but with thicker amounts of insulation. This graph points out that more insulation is not necessarily cost effective. There is a point where more insulation is pointless from a total heat loss perspective. Where circumstances demand thinner walls or roofs, Urethane — with its superior insulating capability — makes it possible to reduce the thickness of the insulation component with no loss of thermal resistance. The thermal resistance of an assembly can be increased without enlarging the size of the member. Urethane helps to offset the design restrictions imposed by the fact that most building materials are constant in thickness and weight.

This graph illustrates the reduction in heat loss from a building when it is insulated with various thicknesses of spray in place urethane foam.

Note: the insulation benefit tops off very quickly above three inches. The graph is not exact, but it shows in general what happens as additional insulation is added to the surface temperature. In other words, by super insulating, the surface temperature of the inside of the exterior walls comes very close to the room temperature. This prevents condensation, which prevents the growth of mold.

The graph shows that 70% of heat loss from conductance is stopped by a one inch thickness of spray in place urethane foam. Remember we are going to stop nearly 100% of the heat loss from air infiltration with the first one-fourth of an inch of urethane foam. The second inch of spray in place urethane stops about 90% of the heat loss and the third inch 95% and so forth.

Thermal diffusivity and Heat Sinks

It should be noted that when the urethane is used on the exterior of a heat sink, such as concrete, the actual effective R-value is approximately doubled. This is why with the Monolithic Dome, we are able to calculate effective R-values in excess of 60. A heat sink is any substance capable of storing large amounts of heat. Most commonly we think of concrete, brick, water, adobe and earth as heat sink materials used in building. The property of a heat sink to act as an insulation is called thermal diffusivity.

The simple explanation for the way it works is: As the temperature of the atmosphere cycles from cold to hot to cold to hot the heat sink absorbs or gives up heat. But because the heat sink can absorb so much heat it never catches up with the full range of the cycle. Therefore, the temperature of the heat sink tends to average. Large heat sinks will average over many days, weeks or even months.
An example is the adobe hacienda with its 2 to 6 foot thick walls. By the time the adobe walls begin to absorb the daytime heat it is night time and the same heat then escapes into the cooler night. Therefore, the temperature would average. Because the mass of the adobe is so large the temperature averages over periods of months. Adobe acts as an insulation even though adobe has a minimal R-value.
You can see from the graph that urethane thicknesses beyond four or five inches is practically immaterial. We use three inches for most of our construction. Two inches will do a very superior job. We have insulated many metal buildings with one inch of urethane and the drop in heat loss is dramatic. Obviously the first quarter inch takes care of the wind blowing through the cracks. (It usually takes an inch to be sure the cracks are all filled.) The balance of the inch adds the thermal protection.

Surface temperature control

Surface temperature control is the second reason for insulation. In many cases it is the most important reason for the insulation. I first noticed this phenomena while insulating potato storage buildings . We had various customers ask us to insulate the buildings with two to five inches of urethane. The buildings insulated with two inches would hold the temperatures of the potatoes properly, just as well as the buildings insulated with five inches. The difference came in the condensation. Potato storage buildings are kept up at very high humidity levels. The buildings with the two inches of urethane would have far more condensation than those with the five inches. An engineer from the Upjohn company explained this to me. He stated that thicker insulation is absolutely necessary to maintain higher interior surface temperatures. One and a half inches of urethane on the walls and ceiling of a potato storage building would control the heat loss from the building, but it would take a minimum of three inches of urethane to control the interior surface temperature. Four inches would be even better. With five inches the difference is practically negligible. The only place where we have felt the need for five inches of urethane was insulating the roof or ceiling of a sub-zero freezer.

Underground Housing

Surface temperature control vs. Heat loss control.

Most underground housing is susceptible to mold and mildew growth. The cause is not enough insulation to control interior surface temperatures. Rarely is there a problem with total heat loss. Water vapor condenses on the surface allowing mold to grow. The only solution is a lot of insulation for temperature control and ignore total heat loss.

My experience is that R-value tables can be used as indicators. They need modifications to make them equal to real world conditions. There needs to be allowances made. They must show equivalents. These equivalents will be more like one inch of spray in place urethane equal to four inches of fiberglass in a normal installation. Footnotes to the table will need to define degradation of insulations in real world conditions. Only then will the R-value become a real world success story.

– Bert Sarkkinen


Source: Nisson, J.D. Ned, JLC, “Attic Insulation Problems In Cold Climates” March 1992, pp. 42-43 / Urethane Foam as an Energy Conserver”, How to ConserveEnergy: in commercial, institutional and industrial construction, Mobay Chemical Corp. Pittsburgh, PA: 1975, p 3