By Jim Coler
When shopping for spray foam insulation, many builders and homeowners are uncertain whether to choose an open-cell product or a closed-cell product. In my opinion, the choice is clear: for above-grade residential applications, the best choice is usually open-cell foam. In most cases, open-cell foam will be more economical, more flexible, and more forgiving with moisture than closed-cell foam.
Open-cell spray foam is often called “half-pound foam,” a reference to its usual density of 0.5 lb. per cubic foot. (The range of densities for open-cell foam actually extends from 0.5 up to 1.7 lb. per cubic foot.)
Open-cell foam was first introduced to the market in 1986 by Icynene. At that time, all of the available spray foam products on the market were closed-cell foams. Since about 2003, however, many manufacturers have been marketing both open-cell and closed-cell foams.
More volume from the same weight
Although closed-cell foams claim to add structural rigidity to a building, the structural contribution is not necessary. In fact, most structural calculations are based only on the strength of the wood or structural members, excluding any contribution from spray foam.
The weight of the raw materials required for open-cell foam is less than for closed-cell foam of the same R-value. As long as you have the cavity thickness to add more insulation depth, open-cell foam will cost less per job and per R-value measurement.
Closed-cell foam is between 3.4 and 4 times more dense than open-cell foam, so a barrel of open-cell foam fills 3 to 4 times the volume as a barrel of closed-cell foam. The R-value of open-cell foam is between R-3.5 and R-4 per inch, while most closed-cell foams are R-6 to R-7.1 per inch. Since open-cell foam uses one-third the material to produce a product which is more than half the R-value, open-cell foam is more economical than the same R-value of closed-cell foam.
Open-cell and closed-cell foams are air barriers. But open-cell foam will absorb and release moisture vapor more freely than closed-cell foam — which is a good thing. This means it will perform much like wood. Open-cell foam is compatible with wood construction; both materials are air barriers that can absorb and release moisture.
Closed-cell foam, on the other hand, blocks most of the moisture and tends to drive it to the wood studs or rafters. This can cause the wood to expand and contract more than it does with open-cell foam. Open-cell foam provides more moisture-permeable surface area for excess moisture to absorb into and dry out of; that means the areas are less likely to become moisture-saturated if the moisture within the building is within control. Open-cell foam acts as a moisture buffer; it helps maintain a more consistent moisture level within the building, which is healthy.
But moisture in insulation is bad, right? Well — yes and no. Too much moisture in any material can cause issues. Too much moisture in wood may create an environment for mold and mildew. But foam has no food source for mold and blocks additional oxygen which is needed for mold to grow. This creates a more stable and healthier indoor air quality and environment.
There is an industry trend towards using soy oil or other bio-based oils in open-cell foam formulas. Open-cell foam uses a water-based blowing agent; in other words, water is used to cause the foam to expand. A water-blown foam is more stable and more environmentally friendly because it is a natural product. In contrast, most closed-cell foams use a hydrofluorocarbon (HFC) blowing agent. Although HFC blowing agents increase the R-value of foam, they offgas over time, causing the R-value to decrease. This slight added benefit does not outweigh the environmental effects that may be caused by the HFC. The more natural our raw materials, the safer the products.
We are all obligated to do what’s right — what’s right for our health, for our finances, and for the environment. Open-cell foam helps us accomplish all of these with a great return on investment.
To read another perspective on the issue, see “Closed-Cell Foam Beats Open-Cell Foam.”