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Canned foam not airtight?

Is there more information regarding whether canned one part foam is airtight? Is there a study done on this? I believe it was mentioned in the cut n cobble blog. Is one brand better than another? Is window and door low expansion better than crack and gap/fireblock? I currently use the Great Stuff Pro brand due its the easiest to get at the big box stores. Any info would be appreciated!

Asked by eric nutis
Posted Fri, 12/06/2013 - 12:38


6 Answers

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I don't have any information that would lead me to believe that one brand is better than another.

If you are talking about two-component spray foam from a truck or trailer, closed-cell foam is more airtight than open-cell foam. But once you get about 1 inch of 1.5 inch of closed-cell foam, or 3 or 4 inches of open-cell foam, you've got a pretty airtight layer.

If you are using one-component canned foam to seal penetrations or the gap between the window frame and the rough opening, the foam will cut way down on air leakage. But it won't be airtight.

Whether or not you can accept the small amount of air leakage associated with canned spray foam depends on your air sealing goals. A lot of Passivhaus builders can't accept small air leaks, so they pay the big bucks for European tape (which definitely performs better than canned spray foam).

Many other builders are happy with the reductions in air leakage that they get with canned spray foam.

Answered by Martin Holladay, GBA Advisor
Posted Fri, 12/06/2013 - 12:59

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Thanks for the reply. I've used closed cell two part for air sealing the big areas but have used the cut ncobble with one part canned foam for small areas and air sealing attics.
I have noticed that some outlets and switches that I have used one part canned foam to seal the wire penetrations and back of electric boxes with are still having airflow issues. I think I'll be using caulk or tape from now on.
I just started using the Siga tapes and are very happy with them. But they are pretty pricey.

Answered by eric nutis
Posted Fri, 12/06/2013 - 13:14

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Dear Eric and Martin:

Great Stuff foam is excellent for sealing the ends of foam panels, holes and gaps -- but you have to be careful to achieve a good air barrier.

An important caution on the Great Stuff Fireblock product: it is just as flammable as standard Great Stuff, and will ignite at only 240 degrees F! This is far below the ignition point for wood, so the Great Stuff will begin to burn before your studs do. Many builders, electricians and inspectors take the "Fireblock" name at face value and are not aware of this problem. We switched back to non-flammable fireblocking caulks made by 3M and DAP, which are also sold at Home Depot and Lowes.

DOW's retail brochure for Great Stuff also highlights using it to seal enclosed light fixtures -- even though their tech team cautions against this due to the heat and potential for ignition.


Answered by Mark Hays
Posted Tue, 06/17/2014 - 18:32

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Is it understood how canned foam leaks? Is it actually through the foam, or is it because the foam does not achieve a good seal with the building materials? Does using more foam help? Does cleaning/wetting the surface help?

Answered by Nick Welch
Posted Fri, 06/20/2014 - 21:12

Helpful? 0

Q. "Is it understood how canned foam leaks? Is it actually through the foam, or is it because the foam does not achieve a good seal with the building materials?"

A. I think it is both.

Q. "Does using more foam help?"

A. Probably. Six inches of spray foam will be less leaky than one inch of spray foam.

Q. "Does cleaning/wetting the surface help?"

A. Yes. You don't want dust, dirt, or flaky surfaces. Concerning wetting the surface, check out this article: Justin Fink’s Canned Spray Foam Tip.

Answered by Martin Holladay, GBA Advisor
Posted Sat, 06/21/2014 - 07:52

Helpful? 0

There are definitely qualitative differences in can foam performance. Dow's Great Stuff actually does pretty well. In the pro-grade foams, Pur-Fill is also excellent. The lower grade foams have a lot of air bubbles inside the cured foam, with a noticeable difference in leakage through the foam.

For basic weatherization projects it simply isn't practical to optimize surface prep, so choosing a good foam is important.

Answered by Jesse Smith
Posted Sat, 06/21/2014 - 10:31

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