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Best adhesive to use for multiple layers of rigid insulation (foamboard)

michaelbluejay | Posted in Green Building Techniques on
In most cases you actually won’t need adhesive for multiple layers of rigid insulation (foam board).  Yeah, I click-baited the title to be about adhesive because if you found this article it’s probably because you were asking the wrong question, since you probably don’t need adhesive.  But, if you actually do need adhesive for your particular install, I will indeed address that at the end.
ROOFS, ABOVE THE SHEATHING.  The sheets don’t need to be adhered to each other.  Gravity keeps them in place until you put something else on top of them.
WALLS, EXTERIOR.  Hold the sheets in place with cap nails (or screws+washers, if you prefer easily disassembly if you think you might someday need to take it apart).
…(1) Cutting and fitting pieces of rigid insulation into rafter or stud bays is known as the “cut and cobble” approach.  It’s generally not recommended for sloped roofs because of the risk of condensation and rot of the roof decking.  See: and

…(2) First, try to get insulation that’s the same size as your cavities so you don’t need lots of thin sheets.  If your big box store doesn’t carry anything thicker than 1″ (like mine), Google “insulation supply [my city]” to find an insulation supplier that probably has what you want.  Even if you can’t get 3.5″ or 5.5″ foamboard, you can get 2″ and 4″, and maybe even 3″ and 5″.
…(3) Regardless of how thick your foamboard is, the normal way to install is to friction fit:  Cut it to fit tightly, then tap it in place.  Don’t make it *too* oversized, otherwise it might bow when you tap it in rather than remaining flat.
…(4) If you cut your foamboard too flat and it’s falling out, then you have some options to keep it in place:
……(a) Foil tape from the foamboard to the rafter or stud.
……(b) Latex caulk between the sheets.  It won’t ever cure because latex needs air for that, but it’s sticky enough to hold the boards in place, and because it doesn’t cure, you’re able to pull the boards apart in the future if you ever need to do so.
……(c) Construction Adhesive.  The foamboard manufacturers recommend “construction adhesive” if you do need to adhere, but usually aren’t very specific about what kind.  The Liquid Nails LN-704 (low-VOC) I used in my tests was actually the least adhesive of any of the various caulks I tested.  Probably the normal-VOC flavor of Liquid Nails would adhere better.
……(d) Spray foam.  It dries quickly and sticks tenaciously.  You won’t get it apart.  Well, you can pull the sheets apart, but the next layer of facing will come off with it.
……(e) Once your initial layers are installed, the layers that go on top of the edge of the rafters or studs are secured to the rafters or studs with cap nails (or screws with washers), and will hold the initial layers in place.
GBA editors, if I’ve made errors or ommisions, I can edit the original post, or delete it and start a new post.

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