Many builders use spray polyurethane foam as an air barrier, raising the question: How thick does the spray foam layer have to be to stop air flow? There’s a follow-up question, of course: Is the answer different for open-cell spray foam than for closed-cell spray foam?
As with most building science questions, there is a short answer and a long answer. The short answer is that closed-cell spray foam needs to be at least 1 or 1.5 inch thick to act as an air barrier, while open-cell spray foam needs to be between 3.0 and 5.5 inches thick to act as an air barrier.
We’ll get to the longer answer after some presenting some definitions and data.
What’s the definition of an air barrier?
Most building codes define an air barrier material as a material that has an air leakage rate below 0.02 liters/sec-m² @75 Pa (0.004 cfm/sf @ 1.57 psf) when the material is tested according to either ASTM E2178 or ASTM E283.
This definition was adopted by the 2009 International Residential Code as the basis for the code definition of an “air-impermeable insulation.”
Joseph Lstiburek, a principal at Building Science Corporation in Westford, Massachusetts, told me that he helped develop this definition. “The .02 number was based on a suggestion from Gus Handegord,” Lstiburek told me. “He suggested that an air-barrier material should be defined relative to drywall. We tested drywall and the air leakage rate for drywall was a bit under .02. So that was the basis of the standard.” (For more information on the development of this definition, see Is OSB Airtight?)
Differences between open-cell spray foam and closed-cell spray foam
The main difference between open-cell spray foam and closed-cell spray foam is foam density. Open-cell spray foam usually has a density of 0.5…
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