*this document adapted from sprayfoam.com. Additions and edits made in blue.

It is important to know the differences between open and closed cell urethane foams. If you are purchasing or specifying urethane foam for a home or commercial building, you must identify whether you will use .05 lb. /cu. ft., open cell foam, or 1.7 to 2.0 lb. /cu. ft. closed cell foam. This makes a difference in performance, application methods and cost.

With the open-cell vs. closed-cell issue, there are two major factors to understand and consider. The first is the nature of the foam. It is either open-cell foam, where the tiny cells of the foam are not completely closed. They are broken and air fills all of the “open” space inside the material. This makes the foam weaker or softer feeling than closed-cell foam and also makes in permeable to moisture and moisture vapor.  Open cell foams should have an auxiliary vapor barrier attached to the warm side of the insulation. Closed-cell foam differs in that all of its tiny cells are closed and packed together. These cells can be formulated to obtain many characteristics, the most common being size and density.

Density is measured by weighing one solid cubic foot of foam material. Open cell foams typically weigh in at .05 lb. /cu. ft. Closed cell foam for insulation applications range in density from 1.7 lb. /cu. ft. to 2.0 lb. /cu. ft.  Roofing applications typically use a 3.0 lb. /cu. ft. to support traffic and loads better. The higher the density the foam, the heavier, or stronger it becomes. Some polyurethane foams are molded into decorative interior molding and painted or stained for a simulated wood effect. These “higher density” foams are typically in the 10 lb./cu. ft. to 40 lb./cu. ft. density range.

Closed cell urethane foams have been used in the construction industry for over 35 years and open cell foams are a recent addition to the range of available products. The advantages of closed-cell foam compared to open-cell foam include its strength, higher R-value, and its greater resistance to the leakage of air, water or water vapor. The disadvantage of the closed-cell foam is that it is denser, requires more material, and therefore, is more expensive. Even though it has a better R-value, typically the cost per R is still higher than open-cell foam. The choice of foam can also be based on the requirements for application specific characteristics such as strength, vapor control, available space, etc. With the recent increases in the cost of fuel, the “R” values attainable with open cell foam are not as cost effective when compared to the creation of enough space to achieve the required insulation.

Both types of foam are commonly used in most building applications and the choice for which to use can depend on many of the factors discussed above. Some foams are inappropriate in specific applications. For example, you typically would not use open-cell foam below grade or in flotation applications where it could absorb water; this would negate its thermal performance because water is a poor insulator compared to air. Closed-cell foam would be a good choice where small framing sizes need the greatest R-value per inch possible. Closed-cell foam would be used for roofing applications.