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. (J.D. Ned Nisson, “Attic Insulation Problems in Cold Climates,” Energy Design Update, March 1992, 42-43)

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Convection Losses In Loose-Fill Insulation

Most people know that air penetrates the walls of a house. In fact, when the wind blows across some homes, its tenants can feel it. But what most people, including many engineers, do not realize is that there are very serious convection currents that occur within fiber insulations. These convection currents rotate vast amounts of air, but they are not fast enough to feel or even measure, with any but the most sensitive instruments. Nevertheless, the air constantly carries heat from the underside of the fiber pile to the top side, letting it escape. If we seal off the air movement, we generally seal in water vapor. That additional water often condenses and can become a moisture-source that rots the structure. The water, as a vapor or condensation, seriously decreases an insulation value — the R-value. The only way to deal with a fiber insulation is to ventilate. But ventilating means moving air that also decreases the R-value
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Air Penetration

The filter medium 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, air flows through a furnace filter very readily. All well and good for a furnace filter -- but can that same material effectively insulate a structure? Can you imagine insulating a house by stuffing furnace filters into the walls and ceiling? Tremendous air currents blow through the walls of a typical home. To demonstrate, hold a lit candle near an electrical outlet on an outside wall when the wind is blowing. That flame will flicker and may even go out. 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 fiber insulation in our house and bring the air infiltration close to zero from one side of the wall to the other, we still do not stop air from moving vertically through the insulation itself, in ceilings and walls.


Solid Insulations

The best known solid insulation is expanded polystyrene. Other solid insulations include cork, foam glass and polyisocyanate or polyisocyanurate board stock. The last two are variations of urethane foam. Each of these insulations is 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 of course 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, etc. 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 new houses. The R-value of the urethane board stock is of course better than any of the other solid insulations. All of these solid insulations perform far better than fiber insulations whenever there is wind or moisture involved.

Most solid insulations are installed as sheets or board stock, and most suffer from one very common problem. They generally don't fit tight enough to prevent air infiltration. And if the wind gets behind them, it matters not how thick these board stocks are. We see this often in masonry construction where board stock is used between a brick and a block wall. Unless the board stock is actually physically glued to the block wall, air will infiltrate behind it. When this happens, the board stock becomes virtually worthless, since the air flows through the weep holes in the brick and around the insulation negating its effectiveness. Great care must be exercised in placing solid insulations. The brick ties need to be fitted at the joints and then sealed to prevent air flow behind the insulation.

Spray-in-place polyurethane is the only commonly used solid insulation that absolutely protects itself from air infiltration. When it is properly placed between two studs or against a concrete block wall or wherever, the bonding of the spray plus the expansion of the material in place creates a total seal. It's almost impossible to overestimate this total seal. In my opinion, most of the heat loss in the walls of a home has to do with the seal, rather than the insulation.

Heat does not conduct horizontally nearly as well as it does vertically. Therefore, if a home had no insulation in its walls, but did have an absolute airtight seal, there would not necessarily be a huge difference in heat loss. But this would not be the case if ceiling insulation was missing.

Spray-in-place polyurethane can most effectively stop air infiltration. It is the only material that properly applied fills in the corners, cripples, 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.


Case Studies

During the 1970s in Idaho's Snake River Valley, my firm insulated the walls of many new homes with 1.25 inches of spray-in-place polyurethane foam. 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 fiberglass insulators. Today, using ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) published charts, 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 high R-value. But in reality, our insulation customers invariably thanked us for the savings in their heat bills. Many told us that their heating bills were half of what their neighbors paid. They felt that they saved the cost of the polyurethane in one or, at most, two years. Most of these customers were savvy people. They would not have paid the extra to get the urethane insulation if it had not been better. Nevertheless, what I call “Case Studies,” some people might call “anecdotal evidence.” That's okay. Anecdotal evidence is also compelling and very real in our world.

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"There is a problem with loose-fill fiberglass attic insulation is cold climates. It appears that, as attic temperature drops 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.(J.D. Ned Nisson, “Attic Insulation Problems in Cold Climates,” Energy Design Update, March 1992, 42-43)

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R-Value vs. Temperature

About mid 1975, I received a call from a division manager of a major fiberglass insulation manufacturer. The caller said, "I understand that you are spraying polyurethane in the walls of homes." I told him that was true. He was calling because we were cutting into fiberglass insulation sales in our area. He asked, "How can you do it?"

I knew what he meant. He wanted to know how I could look folks in the eye and sell them a more expensive insulation instead of cheap fiberglass. I told him the way I did it was with a spray gun. Of course, that wasn't the answer he wanted. He wanted to know why I did not feel guilty. I told him about insulating one of two nearly identical houses built side-by-side. We insulated the walls of one with 1.25 inches of urethane. Its near-twin was insulated with full, thick fiberglass batts by a reputable installer. Not only did we use just 1.25 inches of urethane as the total wall insulation, but we had the builder leave off the insulated sheathing. At the end of the first winter, the urethane insulated home had a heating bill half of its neighbors. Again, such evidence is not terribly scientific, but it is very real. I am not sure that the manager was convinced, but it should be noted: in the following year, that same company jumped into the urethane foam supply business.

One and a quarter inch of polyurethane sprayed properly in the wall of a house will prevent more heat loss than all the fiber insulation that can be crammed in the walls — even up to an eight-inch thickness. Not only does the polyurethane provide better insulation, it provides the house with significant additional strength.

Brent was an early client of mine for whom I had insulated several potato storages. He knew what spray-in-place urethane insulation could do. When he decided to build his new, very large, very fancy new home, he asked me to insulate it. The builder pitched a fit. He didn't need any of that spray-in-place urethane in his buildings. He made his buildings tight, and fiberglass was just as good.

Brent told the builder, "I know who is going to insulate the building. It is not as definite as to who is going to be the contractor. You can make up your mind. We are going to have the urethane insulation and you build the building, or we are going to have the urethane insulation, and I will have someone else build the building." It didn't take the contractor long to decide he wanted to use urethane insulation.

R-value tables are truly part of the fairy tale. They chart solid and fiber insulations side by side, implying that they can be compared. The fact is, without taking installation conditions into account, comparisons are meaningless. Spray-in-place urethane foam provides its own vapor barrier, water barrier and wind barrier. None of the other insulations are as effective without special care taken at installation. Fiber insulations must be protected from wind, water and water vapor. Again, the tables need a second table to state installation conditions.

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