For the technical reader, polyurethane foam insulation results from the reaction of diisocyanates blended with polyols. Chemicals, such as catalysts, surfactants, and flame retardants, may be added to control the set time, workability, and flame-spread characteristics of the final product. It’s in the details of these chemical additives that foams differentiate themselves in both performance and environmental qualities.
Foams deliver superior insulation and air seal; several meet Greenguard indoor air quality standards and contain the most advanced propellants available with no impact on the ozone layer and improving global warming indices. Bio-based products have entered the foam insulation market to offset some of the negative effects, or at least perceived negative effects, of foam’s petroleum pedigree. But some misconceptions exist.
A common misunderstanding is that bio-based spray-foam insulations are made exclusively from rapidly renewable plant sources, such as soybean oil, castor oil, and sucrose-based oils from sugar beets. But most products with bio-based content contain less than 20%; in fact, the USDA’s threshold for claiming spray foam is made with renewable, bio-based materials is only 7%. Nevertheless, the inclusion of plant oils reduces the petroleum content of foam insulation; national green standards provide points for the use of foam insulation products that meet the criteria for the USDA’s Biobased Affirmative Procurement Program.
A study of the environmental benefit of soy-based polyols commissioned by the United Soybean Board in 2004 concluded that every pound of soy polyol that replaces a pound of petroleum has a positive carbon benefit of 5.6 pounds; however, it remains a minor component of foam insulation and some argue that the water and land consumed in growing crops, such as soybeans, has greater negative environmental impact than the petro savings. Neither camp has conclusive evidence of significant benefit or harm and most claims either way seem geared toward marketing rather than ecology. Overall, insulation of almost every type provides greater environmental benefits than costs.
Open-Cell Vs. Closed-Cell
The real distinction between types of foam insulation focuses on whether they are open- or closed-cell. In general, both are made from the same materials and work in the same way, trapping air or gas in a plastic matrix. The differences start with the “blowing agents” used to create bubbles and end with both varied performance and cost.
Open-cell foam costs slightly less for the same thickness, but offers lower per-inch R-values than closed-cell products. In some instances, this is a disadvantage, but where thickness is less relevant, or where higher R-values are not needed, then open-cell can provide the better choice. It also has some green advantages over closed-cell: The blowing agent used to install open-cell insulation is water, which reacts with air to become CO2—while closed-cell products use HFCs.
Because CO2 expands quickly, the bubbles tend to burst before the plastic sets, and hence the “open cells,” which produce a spongy, lightweight foam. The industry describes the foam as “half-pound” material, which simply means the foam has a mass that weighs 0.5 pounds per cubic foot. This density yields an R-value of approximately 3.6 per inch, equivalent to most traditional insulations. Because of the open cell structure, open-cell foam allows some vapor to pass through, making it a good choice in hot, humid climates, and under roof sheathing, such as in conditioned attics, where water vapor caught between insulation and sheathing could promote wood rot.