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Community and Q&A

Protecting bottom edge of exterior foam insulation board

Salesi | Posted in General Questions on

Which would be better to protect the bottom edge of 1-1/2″ exterior foam insulation board: using a piece of 1-1/2″ x 1-1/2″ treated wood around the perimeter of the house between the foundation and sheathing or using a piece of aluminum flashing bent in an L shape like a big drip cap? The foam insulation board would sit on top of either the treated wood or the L shaped flashing protecting it from insects and mice as wells as keeping it dry. Thanks.

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  1. charlie_sullivan | | #1

    How and where is the foundation insulated?

  2. Expert Member
    ARMANDO COBO | | #2

    I would leave 3/8” min. space between the foam and the drip cap, or flashing, or wood for proper drainage, if needed, either bottom edge would work. I would also wrap metal screen behind the foam and in front of the foam and/or rainscreen, to avoid bugs, termites, and mice.

  3. Expert Member
    BILL WICHERS | | #3

    You can get perforated metal strips ready made for this purpose. I prefer stainless steel here since certain soil conditions can corrode aluminum over time. I would try to avoid solid metal flashing here for fear of it collecting water. Perforated metal won’t collect water.


  4. Expert Member


    if your WRB is on the outside of the foam there is no harm in using a PT block underneath it. You still need a perforated flashing, or something like Cor--a-Vent, to seal the bottom of your rain-screen cavity though.

    Or you can do as Bill suggests and deal with both things at once with one piece of J-shaped perforated stock.

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