Canned spray foam: a reliable air seal?
Simple question, folks … Canned spray foam is widely used to seal between a window frame and it’s rough opening and around all sorts of envelope penetrations. But are canned spray foams open or closed cell? Does this vary by product? And if they’re open cell (my hunch based on stated R values), then are they reliable air barriers if they’re less than 3.5 to 5.5 inches thick? See …
I’m guessing that a lot of window installations get canned spray foam that doesn’t reach the required thickness and so doesn’t really give an airtight seal.
We’re working out air barrier specs with builder of our new home, and I’m wondering if we should specify some minimum thickness of canned spray foam, wherever it’s going to be used.
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Replies
William,
The short answer is that one-component spray foam (also known as canned spray foam or moisture-cured spray foam) is not a perfect air barrier. It cuts down significantly on air leakage, but it can't be considered airtight.
If you want an airtight seal around your windows, use one of the European tapes sold by 475 High-Performance Building Products or Small Planet Workshop.
The density of cured one-component spray foam varies widely, depending on the brand you use and the moisture content of the air and surrounding materials on the day the foam was installed. In general, the density of cured one-component spray foam tends to be less than the density of closed-cell two-component spray foam.
-- Martin Holladay
Also consider issues like movement capability and long term shrinkage. IMO, skip all use of canned spray foam in favor of tapes and gaskets and some use of high quality sealants.
To expand on Jon R's comment regarding movement capability (and possibly related to Martin's comment regarding conditions during application), in my somewhat limited experience with canned foam I've found it can sometimes be extremely brittle once cured and have somewhat poor adhesion to surfaces.
In my experience, there are three substrates that canned foam reliably adheres to: skin, hair, and clothing. ;-)
--John
John,
Same with Tremco acoustic sealant. I'd add car upholstery to both lists.
I'd consider spray foam between windows and framing to be insulation, not an air barrier. If you then apply tape inside and out, you'll have a robust air barrier.
Thanks, all very helpful. OK, so we'll consider canned spray foam as insulation only, and use tape at windows. We're thinking about using 3M All Weather Tape on sheathing joints. Do you think this would be an acceptable alternative to the European tapes for window sealing? Or should we spring for the good stuff?
I second this question. William did you ever find an answer???
Great Stuff Pro™ Gap & Cracks Closed Cell Content, ASTM D2856, % 80
http://msdssearch.dow.com/PublishedLiteratureDOWCOM/dh_0943/0901b80380943321.pdf?filepath=styrofoam/pdfs/noreg/179-07323.pdf&fromPage=GetDoc
GREAT STUFF PRO™ Window & Door Open Cell Content, ASTM D6226, % <75%
http://msdssearch.dow.com/PublishedLiteratureDOWCOM/dh_095a/0901b8038095aeb5.pdf?filepath=styrofoam/pdfs/noreg/179-07324.pdf&fromPage=GetDoc
Erich,
The spec sheet for GREAT STUFF PRO Gaps & Cracks Insulating Foam Sealant that you linked to shows that the product has a density of 1.01 pound per cubic foot.
The spec sheet for GREAT STUFF PRO Window & Door Insulating Foam Sealant shows that the product has a density of 1.29 pound per cubic foot.
These densities fall between the usual density of open-cell spray foam (0.5 pound per cubic foot) and the usual density of closed-cell spray foam (2 pounds per cubic foot).
-- Martin Holladay
I have heard that Cutting the foam back increases its air permeability. does anyone know if this is true?
This is a very typical trade practice of overfilling a hole and then trimming it down. Anecdotally this makes sense as it seems to "open the pores" and the "skin" formed seems to better at "air sealing"
I agree that anecdotally that makes sense, but I don't know of any studies done on it. Most air leaks happen at the gaps and cracks at the perimeter of assemblies, not in the middle of the wall, and foam often pulls away from framing, so to me it's a moot point when talking about spray foam for air sealing.