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Community and Q&A

Sealing gaps between rafters and rigid foam

mfleck | Posted in General Questions on

Hi all,

I’m cutting and cobbling in Rigid foam panels in my rafter bays to insulate and create 1 inch air gaps to ventilate. Most of the gaps between the edge of the foam and the rafters are between 1/4 inch and 3/4 inches and I plan to use great stuff to seal these gaps.

There are stretches where the foam is right up against the rafter or less than 1/4 inch away, and I’m wondering how to deal these smaller gaps. Is there a specific caulk anyone would recommend?  Any other technique?

Also, I’d appreciate anyone’s advice on sealing these gaps in general.  I’m doing my best to clean out the dust where the seal will be with a stiff bristle broom. I’ve heard it helps to wet the surfaces a little too. 

Thanks again,

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  1. Expert Member
    BILL WICHERS | | #1

    I’d try to cut the foam so that there are no gaps too small for the great stuff straw to fit. Great stuff is really the best way to seal this, and you don’t want any gaps for air to leak through.

    Clean surfaces are important, and a light mist of air does help the foam to cure. Most people apply with a small spray bottle. You just want a light mist, don’t overdo it.


  2. GBA Editor
    Brian Pontolilo | | #2

    Hi Matthew.

    My experience is similar to Bill's. I find it much easier to make the canned spray foam work when there is a bit of a gap at the edges of the rigid foam. But how do you hold the foam centered in place to fill the gaps on each side of the panel? One way is to cut the foam to friction fit the space, but bevel the edges. Then the foam has a gap to fill and lots of surface to bond to. I've found that this works well.

    Of course, you can also use caulk. But since I have always used canned spray foam, I'll let someone else weigh in on that.

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