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Why is spray foam used for attic insulation?
Friday, September 21, 2018
We are seeing spray polyurethane foam (also referred to as “SPF”) used more often for attic insulation in the last few years, especially in homes built for high energy efficiency. Because the insulation is sprayed onto the underside of the roof sheathing, the attic becomes an insulated—but not conditioned—space.It’s a pleasure for us to inspect a spray foam attic in the summer, because the air is not blazing hot; which also means less energy loss from the air conditioning ducts running through the attic.
There are two basic types of polyurethane spray foam: closed-cell and open-cell. The open cell type is more common in the Gainesville area. Nick Gromicko, president of the International Association of Certified Home Inspectors (InterNACHI), provides an elegant explanation of the difference between the two types in this excerpt from his article posted at the InterNACHI website:
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/cubic foot (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 insulation value. InterNACHI inspectors may call out open-cell insulation discovered in moist areas, such as in external applications or below grade.
Because volatile, toxic chemicals are off-gassed during and for several days after installation, no one but a spray application technician wearing special protective gear should be in the house until it is determined to be safe for re-entry. Also, some people have a high sensitivity to the emitted gases, even in lingering, small quantities. Here’s what the EPA has to say:
Spray polyurethane foam (SPF) is an effective insulation and air sealant material; however, exposures to its key ingredient, isocyanates such as "MDI," and other SPF chemicals that may be found in vapors, aerosols, dust or on surfaces during and for a period of time after installation may cause adverse health effects such as asthma. Therefore, steps to control exposures and safety tips should be followed.
For more information about spray foam insulation, we suggest visiting www.whysprayfoam.org, the Spray Foam Coalition website. The SFC is a group of manufacturers of the chemicals and equipment for spray polyurethane foam insulation.
Spray polyurethane foam is also used as a roofing material. To read more about it, see our blog post ”What is an SPF roof?".
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