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Exterior rigid foam question

Randy Mason | Posted in Green Building Techniques on

I live in the San Francisco Bay Area and am applying exterior rigid foam as follows:

plywood-2 layers of 3/4″ foil faced polyiso – WRB -rainscreen – fiber cement lap siding

In many of the articles I have read on the subject, it recommends applying an adhesive on the foam on the panel edges were it meets the bottom of the wall and the perimeter wall edges.

My question is if you apply an adhesive between the plywood and the bottom of the foam were the plywood meets the foundation – if any water made it past the foam to the plywood, the water would not drain out of the bottom because it would be stopped by the adhesive. Is the theory then that any such water would dry to the inside?

If you do not apply an adhesive, then couldn’t air move up through the foam boards?

If the idea of the adhesive is to stop air movement , if using two layers of foam, wouldn’t you also use adhesive between the two layers of foam?

Thanks for any thoughts you may have on the subject.


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  1. GBA Editor
    Martin Holladay | | #1

    You guessed correctly: with this assembly, the plywood should be able to dry to the interior. For more information on the theory behind foam-sheathed walls, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

    Whether or not you should use an adhesive or caulk at the perimeter of each layer of foam (or tape at the foam seams) is a judgment call. Sealing more than one layer against air leakage may not be necessary, but it isn't nuts. In general, I tend to admire obsessive air sealers -- they are a more admirable bunch than the "who cares?" builders one often encounters.

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