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Is there a system that uses 2″ polyiso for the wall and 3″ for the roof that could be considered energy efficient for zone 6?

Luke Thompson | Posted in Energy Efficiency and Durability on

Is there a way to construct an energy efficient home for Northern Vermont using 2″ polyiso walls and 3″ in the roof? There are 3/4″ t+g pine boards on the interior, an aluminum vapor barrier, polyiso, stapping and clapboards. How does this work in zone six? Am I missing something?

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  1. Expert Member
    Dana Dorsett | | #1

    Sure- you're missing a lot, such as a third to half the required R-value:

    If you used TWO layers of 2" on the walls (4" total) and TWO layers of 3" on the roof (6" total) it would then be slightly better than code minimum performance using 2" polyiso on the walls, and 3" on the roof, assuming there were no framing or structural elements thermally bridging the foam layer. (The layer has to be uninterrupted by framing or timbers.)

    But that's probably not the answer you were looking for, eh?

    But that's reality.

    Building codes may not be enforced in VT, but the laws of physics are self-enforcing.

    To meet code minimums It needs a "whole wall-R" value of at least R21 for the walls (eg an 2x4/R13 wall with R12 continuous foam over the sheathing, or 3" of continuous polyiso with the t & g pine interior and some siding).

    A code-min roof needs to be R38.5 whole-assembly for the roof, say, R23 rock wool batts between 2x6 rafters tight to a 3/4" plywood roof deck with 4" of polyiso above the roof deck, and half-inch gypsum on the underside of the rafters.

    To be moisture safe at the sheathing layer at least half the center-span R between rafters has to be above the roof deck:

    For a wall to be moisture safe at the sheathing layer, at least a third of the center-span R between the studs has to be on the exterior:

  2. GBA Editor
    Martin Holladay | | #2

    It looks like you posted this question twice -- with somewhat different wording, but on the same topic.

    To keep all future comments on the same Q&A thread, I urge any GBA readers (and Luke) to post future comments on the other page, where most comments seem to have been posted. Here is the link:

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