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Exterior Foam Insulation as Drainage Plane

Edithanne | Posted in General Questions on

Is it very risky to simply use the exterior foam (2″) as the drainage plane? i.e. no building wrap on the foam.

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  1. kbentley57 | | #1

    I would consider it risky, when such cheap and available products could be installed beforehand. A single, shingled layer of #15 felt behind the foam for backup might cost you $100 in materials on a whole house. Skipping that step is a mistake, in my opinion. If you're asking about adding another layer in front of the foam, while already having one in place behind it, I would say that is perfectly acceptable.

  2. Expert Member


    Think through how you will integrate the window and door flashing. If you use the foam as a WRB, you will be relying on a taped joint. If the WRB is behind the foam, the flashing need to go back to that. If the WRB is installed over the foam, you can rely on lapped joints.

  3. Edithanne | | #4

    Thanks everyone. I agree that it would best to bring the window flashing back to the WRB between the foam and the sheathing. But if taped joints would suffice I might go that route (or add number 15 asphalt felt).

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