Originally published by: Fine Homebuilding — May 21, 2016 by Mr. Rob Yagid, a former editor at Fine Homebuilding. Excerpted from Mr. Rob Yagid’s article with contributions from ABTG Staff.
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In an article by Rob Yagid for Fine Homebuilding, which was sponsored by Versi-Foam Systems, the question addressed is what is open cell versus closed cell foam? Rob delves into the debate about the properties of open-cell versus closed cell with the following points:
Much of the information you’ll find about spray foam is dedicated to its R-value and its permeability.
These traits have an overarching impact on the performance of open-cell and closed-cell foams. In most closed-cell foams, an HFC blowing agent is captured in the foam’s cell structure. This gas has a better thermal performance than the air-filled open-cell foam and gives it a higher overall R-value.However, while HFC-blown closed-cell foam might initially have an R-value as high as R-8 per in., as the blowing agent evaporates through the cell walls and is replaced by air, its R-value diminishes.
Closed-cell foam’s “aged” R-value is roughly R-6 per inch. Some manufacturers produce water-blown closed-cell foams. These foams have the same performance properties as HFC-blown foam, but slightly lower R-values at around R-5.5 per in.
Closed-cell foam’s greater density, 2 lb. per cu. ft. compared with open cell’s 1⁄2 lb. per cu. ft., also increases its R-value and offers it the rigidity that open cell foam lacks.
Structural testing, by a variety of spray foam manufacturers has confirmed that closed-cell foam increases the lateral shear and wind pressure strength of conventionally framed walls. Closed cell foam also has a low vapor permeability rating (roughly 0.5 perms at a thickness of 3 in.) and is considered a class-II vapor retarder, meaning that it’s semi impermeable.
Open-cell foam has a greater expansion rate than closed-cell foam. It expands 100 times its initial volume (closed-cell foam expands only 30 times its initial volume), so less of the foam is needed to insulate a house.
Although both foams will dry if they ever get wet, open-cell foam is vapor permeable and dries much faster than closed-cell foam.
Open cell’s one major weakness is its lower R-value, roughly R-3.5 per in. This means that when used in a 2×4 exterior wall, it will create an assembly that’s approximately only R-12, which won’t meet code in most parts of the country.
Spray polyurethane-foam manufacturers can rely upon several facts when it comes to marketing their products. According to the U.S. Department of Energy, up to 30% of a home’s heating and cooling costs are attributed to air leakage. Spray polyurethane foam is an effective air barrier and significantly reduces energy loss. Combined with a higher thermal resistance (R-value) than most other forms of insulation, it’s no wonder spray foam is often relied on to help make houses ultra-efficient. The key to proper use is knowing your climate, construction practice, wall and roof assembly types and building code requirements with a particular focus on continuous insulation. For more resources on the value of spray foam, visit continuousinsulation.org.