Considering the Differences Between Closed & Open Cell Spray Foam
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.
The following article was produced and published by the source linked to above, who is solely responsible for its content. SBC Magazine is publishing this story to raise awareness of information publicly available online and does not verify the accuracy of the author’s claims. As a consequence, SBC cannot vouch for the validity of any facts, claims or opinions made in the article.
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 in. 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 opencell 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. Closedcell foam also has a low vaporpermeability rating (roughly 0.5 perms at a thickness of 3 in.) and is considered a class-II vapor retarder, meaning that it’s semiimpermeable.
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 2x4 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.