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Adhering XPS foam to plywood

user-5538665 | Posted in General Questions on

I have built a double stud wall with plywood window bucks. I will be adding 2″ XPS foam on the inside of the window buck. How would you adhere the XPS to the plywood?

Metal fasteners through foam ( minimally ) defeats the purpose of the foam.

Some adhesives will hold the foam off of the plywood. Think how drywall adhesive leaves a gap. 

Some adhesives may melt the foam.

What would you do?

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  1. Expert Member


    Use the appropriate foam adhesive and it won't leave gaps or melt it. However fasteners may be a better solution. What are you covering the foam with, and how is it being fastened?

    I'm curious about your window buck. How have you detailed it so that it can have 2" of foam inside and not protrude into the glazed area?

  2. Expert Member
    BILL WICHERS | | #2

    Fasteners aren’t enough of a thermal bridge to worry about. I actually ran the thermal calculations and posted the results here in the GBA forums a few months ago. For the typical nailing schedule for a 4x8 foot sheet, the total thermal losses due to fasteners is only a few BTUs. Almost nothing. You'll likely lose more with air leaking through the keyhole of your door lock.

    I’d use glue here. Use the PL foamboard adhesive and you won’t have any trouble. If you apply a bead, stick on the foamboard, and then slide the foam around a little it will smear and spread out the bead of glue making the insulation fit tighter to the plywood.


  3. user-5538665 | | #3

    Malcom, I will try to post back some photos but the window will sit on the foam so is it a consistent plane. I will add additional foam around the sides and top of the outside of the frame when installing. Not the bottom so as to not interfere with the weep holes. I will be taping Proclima membranes over the foam to the window both inside and out with Tescon Profil tape. The sill will be covered with Extoseal Encors, connected to the bottom of the frame with Contega Solida EXO-D

    The PL foam board adhesive was at the top of my list for what to try. I have never used it. Good to hear your opinion is that it will go on thin and not create a stand off.

  4. Expert Member
    BILL WICHERS | | #4

    PL foam board adhesive comes out about the consistency of caulk. If you don’t do “squish and wiggle” step it might hold the foamboard put a bit more than you’d like. If you do squish and wiggle the foamboard, it’s easy to get a tight joint. I’ve used the stuff to hold rigid foam to wall sheathing and also to stack layers of foam into a thick block for attic hatch covers. It works well, and it’s easy to use. Note that you’re likely to need more tubes than you’d think — you’ll burn through the stuff pretty fast on a project of any size.


  5. user-5538665 | | #5

    I have been reading PVA glue works well for adhering foam. It is pretty inexpensive in bulk and can easily be applied to with / both plywood and foamboard.

  6. Jon_R | | #6

    Designed for bonding foamboard insulation and polystyrene to most porous building materials."

    But I used Titebond III and it also worked well.

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