The R-Value of a Material is Irrelevant if Air Can Get Past it.

The Test to Determine the R-Value

Obviously, that changes from summer to winter — even from day to night. In a winterly 20 F below zero environment, the inside of an occupied house will certainly be the warm side. But during sun-shiny summer months, the outside will be the warm side.

Sometimes a novice owner or builder will put vapor barriers on both sides of the insulation. Vapor barriers so placed generally prove to be disastrous. It seems the vapor barriers stop most of the moisture but not all. Consequently, small amounts of moisture move into the fiber insulation, between the two vapor barriers and become trapped. The moisture accumulates as the temperature swings back and forth. This accumulation can become a huge problem. It can eventually total buckets of water that saturate the fiberglass. Fiber insulation needs ventilation on one side; therefore, the vapor barrier should go on the side where it will do the most good.

Convection Losses in Loose-Fill Insulation

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 fiber insulation is to ventilate. But ventilating means moving air that also decreases the R-value.

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.

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)

A Real World Example

“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.”

With the lowest K-factor and highest R-value, urethane foam can provide more thermal resistance with less material than any other insulation.

Insulation has two purposes: to cut heat loss and to control surface temperature.

Thicker insulation is absolutely necessary to maintain higher interior surface temperatures

This graph illustrates a building's reduction in heat loss when it is insulated with various thicknesses of spray-in-place urethane foam. Note: Above 3 inches, the insulation benefit tops off quickly. 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 that, in turn, prevents mold growth.

Underground Housing — Surface Temperature Control vs. Heat Loss Control

Most underground housing gets in trouble from 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. Mold makes people sick. The only solution is using lots of insulation for temperature control and ignoring total heat loss since it is not a factor.

Advantages of Spray Foam Insulation

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:

• Closed and open cell foam 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.

• Urethane’s thermal resistance means that only one thickness of material is needed for most jobs.

• It has low moisture permeability (1-3 perms).

• 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. Or 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.