By Neal Ganser
Replacing long-obsolete insulation materials with closed-cell or open-cell spray polyurethane foam is a big step in the right direction for energy efficiency. Of the two types of foam, it is closed-cell foam that truly answers all of the building envelope’s requirements for long-term sustainability. Compared to open-cell spray foam, closed-cell foam has a higher R-value and is less vapor-permeable.
Higher R-Value per Inch
Closed-cell foam encapsulates a high-R-value blowing agent which acts somewhat like the argon in a gas-filled insulated window. The most commonly used and safest blowing agent is made by Honeywell Corporation; it’s a non-ozone-depleting, environmentally benign chemical. This blowing agent produces a closed-cell foam with an aged R-value of about 6.2 per inch.
The blowing agent used for open-cell foam is carbon dioxide, which is off-gassed as the foam cures and is eventually replaced with air. This off-gassing also contains some sensitizing agents and various catalysts. The process takes about 30 days (or more, depending on thickness), and can have an impact on people working in the building. The R-value of open-cell foam is about 3.6 per inch.
While open-cell foam is cheaper to buy per inch, it takes nearly twice as many inches to achieve required R-values, and so it ends up close to the same cost per installed R-value. Accommodating the required thickness of open-cell foam may require oversized structural members, since even R-19 walls require 2×6 studs. A customer should buy insulation by its aged R-value — based on testing by a third-party laboratory — not by inches.
To avoid moisture problems in any wall or roof system, the indoor and outdoor climates must be kept apart. This requires that four criteria be met: thermal isolation, air isolation, moisture isolation, and reversibility.¹ The reversibility requirement accounts for daily and seasonal reversals in the direction of vapor drive through wall and ceiling assemblies.
Closed-cell spray foam meets all of these criteria, while open-cell foam does not. The vapor permeance of 2 inches of closed-cell foam is less than 1 perm, making it a natural moisture resister. The permeance of 3 1/2 inches of open-cell foam, on the other hand, is more than 16 perms — roughly ten times the permeance of closed-cell.
Because of its extremely high moisture permeance, open-cell foam fails at moisture isolation, forcing the builder to decide which side gets a vapor retarder. The outcome of this decision precludes reversibility — that is, the ability of the wall to resist moisture or to dry out in either direction.
Although air leakage is a more important mechanism of moisture transport than vapor diffusion, it is silly to assume that the laws of moisture diffusion are suspended for open-cell foams. Because open-cell foam allows moisture to diffuse through it, it can allow condensation to form behind exterior sheathing in winter or behind a vapor retarder or wallpaper in summer. Diffusion-related moisture problems in walls insulated with open-cell foam are not a lot different from similar problems with fiberglass-insulated walls — except that the moisture dries more slowly, because foam doesn’t allow air movement.
While it is possible to use open-cell foam successfully in some climate zones where vapor drives are predominantly in a single direction — in other words, climate zones where you can accurately predict which side of a wall to apply the vapor retarder — these zones tend to be hot, humid coastal areas subject to hurricanes and flooding. Closed-cell foam is the only insulation approved by FEMA for flood zones.
As a bonus, closed-cell foam imparts strength to whatever construction it is applied in. It can be used in any climate zone in the United States without a lot of extra stuff (like vapor retarders and thick walls) to get it to work and not cause moisture damage.
Look at the refrigerator in your kitchen. Refrigerators haven’t used fiber insulation for over forty years, and they don’t use open-cell foam either. Closed-cell foam is the insulation the others can only ‘wanna be.’
¹ Exerpted from “Walls that Work” seminar, © 2003 Corbond Corporation
To read another perspective on the issue, see “Open-Cell Foam Beats Closed-Cell Foam.”
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