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Community and Q&A

Polyiso over sheathing

Tarkus | Posted in Energy Efficiency and Durability on

Building a new home and am interested in adding a continuous insulation layer over the wall sheathing but have questions/concerns I can’t seem to get a straight answer on.

Home is in Zone 3, 2×6 exterior walls, R21 fiberglass, 1/2″ Advantech sheathing covered in mostly brick, some shake.
– Will adding 1/2″ Polyiso in lieu of housewrap “choke” the wall moisture. The Polyiso I’m looking at says it is allowable as a water barrier, but doesn’t seem like it would breathe like a house wrap.
– I like the idea of a continuous insulator for the thermal break over framing, and the foil side is a radiant barrier given the air gap behind the brick. Any drawback here?
– A downside is the windows will need the jams extended 1/2″
– So, does this sound worthwhile or just overkill?
– Any problem applying cedar shake over polyiso on sheathing?

Thanks in advance

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  1. GBA Editor
    Martin Holladay | | #1

    In Climate Zone 3, you can use 1/2-inch polyiso as you propose. The foil facing won't cause any problems. Here are links to three articles on this issue:

    Calculating the Minimum Thickness of Rigid Foam Sheathing

    How to Install Rigid Foam Sheathing

    Using Rigid Foam As a Water-Resistive Barrier

  2. GBA Editor
    Martin Holladay | | #2

    Q. "Any problem applying cedar shake over polyiso on sheathing?"

    A. Cedar siding shouldn't be installed directly over polyiso insulation. You need an intervening air space -- a product like Home Slicker.

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