By Bud Coburn
Polyurethane spray foam is a versatile insulation material that is sprayed into building cavities where it quickly expands and molds itself to its surroundings. It is available in “closed-cell” and “open-cell” varieties, each of which offers advantages and disadvantages, depending on the requirements of its application. The following guide briefly explains the differences between these insulation options.
Closed-Cell Polyurethane Foam
Closed-cell polyurethane foam (CCPF) is composed of tiny cells with solid, unbroken cell walls that resemble inflated balloons piled tightly together. The cells are inflated with a special gas selected to make the insulation value of the foam as high as possible. Like the inflated tires that hold up an automobile, the gas-filled bubbles, when dried, create a material that is strong enough to walk on without major distortion. Wall-racking strength can by enhanced when CCPF is applied, and its strength makes it preferable for roofing applications. The high thermal resistance of the gas gives CCPF an R-value of approximately R-7 to R-8 per inch, according to the U.S. Department of Energy (DOE), which is significantly better than its open-cell alternative. It also acts as a vapor barrier, making it the product of choice if the insulation is likely to be exposed to high levels of moisture. Its density is generally 2 lb/ft3 (32 kilograms per cubic meter [kg/m3]).
Over time, the R-value of CCPF can drop as some of the low-conductivity gas escapes and is replaced with ordinary air, a process known as thermal drift. Research performed by the DOE revealed that most thermal drift occurs within the first two years after the insulation material is applied, but then the foam remains relatively unchanged unless it is damaged.
Open-Cell Polyurethane Foam
Open-cell polyurethane foam (OCPF) is a soft, flexible, spongy insulation with broken cell walls that permit air to fill them. They typically have a density of 0.5 lb/ft3 (8 kilograms per cubic meter [kg/m3]), which is significantly less than closed-cell insulation, as well as having a reduced R-value per inch, although OCPF still has excellent thermal-insulating and air-barrier properties. The foam is weaker and less rigid than closed-cell foams, too. It will require trimming and disposal of excess material as it expands to over 100 times its initial liquid size.
Builders often choose open-cell foam for the following advantages it affords, including:
- its low cost. Where economical yield is important, open-cell foam is generally chosen over its more costly alternative;
- providing a sound barrier. OCPF forms a more effective sound barrier in normal-frequency ranges than closed-cell foam. For this reason, OCPF is well-suited for installation beneath floors and around theater rooms;
- its flexibility. Open-cell foam is more flexible than closed-cell foam, which allows it to adjust to weather-induced expansion and contraction of framing members. CCPF, by contrast, may develop hairline fractures because it cannot flex sufficiently; and
- its permeability to moisture. While often cited as a reason to avoid the use of OCPF, in certain situations, it can be helpful for moisture to pass through insulation. Open-cell foam used in roofs, for instance, will allow a roof leak to make its way to the space below where it is more likely to be discovered. Closed-cell foam used in the same application would trap the moisture, hiding the leak and potentially leading to wood decay. In most situations, however, OCPF should not be used in any place where it might become wet, as moisture will diminish its insulative value. InterNACHI inspectors may call out open-cell insulation discovered in moist areas, such as in external applications or below grade.