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PolyIso necessary under metal roof in zone 4

D_C_H | Posted in Energy Efficiency and Durability on

I live in Portland, OR, which is region 4. We’re replacing our roof and highly considering a metal roof. The contractor included installing a layer of poly-iso under the metal roof as an option, but that would put us out of budget. The contractor put that as recommended, but not required, but I want to check how “not required” that really is.

We currently have r-50 cellulose insulation in the unfinished attic and have been told by energy efficiency contractors in the past that we have good ventilation as well. So I don’t know if everything is good given the attic, or if adding the poly-iso will be needed, or if we can skip the poly-iso, but install more ventilation to be safe.


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  1. D_C_H | | #1

    The metal roof, BTW, would have a cool roof certified paint/coating.

  2. Expert Member
    Akos | | #2

    Assuming you HVAC is elsewhere, and since you have a vented attic with lots insulation on the floor, the polyiso above the roof deck won't do much. Since you have a lot of insulation there already, reducing your attic temperature a bit will barely budge your cooling load.

    The polyiso above would only be needed if you are looking to converter to an unvented conditioned attic.

  3. D_C_H | | #3

    Thanks. There's return ducts up there, but that's it as far as HVAC goes (and they're also well insulated). So it sounds like we can skip the poly-iso. Thanks.

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