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PGH insulation and wrb details

Lukas Schwartz | Posted in Energy Efficiency and Durability on

Zone 5 new build would love some feedback. We are installing 2” foil faced hi-R on the exterior, with windows/doors installed on the outside, taped in (zip-tape), the intention being to make the (taped seams) foam our air barrier. Seems to me typar (my preferred wrb) is extraneous- opinions?
also currently debating roof/stud bay insulation. Leaning towards at least 3” of ccspf sprayed to the roof shathing. We have all cathedral ceilings and numerous skylights, making venting the roof difficult…
Thoughts?

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Replies

  1. Jon R | | #1

    IMO, lapped WRB is more resistant to water entry than non-lapped taped joints.

    > 2” foil faced hi-R

    If you were near the code minimum for exterior R (I don't think you are, even with proper de-rating), I'd allow a little outward drying by not using foil.

    > ccspf sprayed to the roof sheathing

    Think about replacement when the roof fails and there is some rotten sheathing. R has to be according to code and I'd use something like R5.6/inch for the very long term de-rated R value of ccspf.

  2. Walter Ahlgrim | | #2

    I do not like the idea of foam as your primary water barrier just too easily damaged. I think a layer of house wrap before the foam sounds like cheap insurance.

    Will you have a vented rain screen? Will you have large overhangs? How much rain do you get each year? I might feel differently if you have a rain screen 4 foot overhangs and lived in a desert.

    When I see the words “spray foam” and “new construction” together I think poor planning or laziness. Seems to me if you must have the sloped ceiling it would cost less to use sheet foam over the rafters and even cheaper if you located reclaimed foam.

    Walta

  3. Expert Member
    Akos | | #3

    I would also not trust taped foam as a WRB.

    With outies my preferred assembly is to use fiber faced roofing polyiso for the exterior rigid with the WRB over the foam. This puts the WRB in the same plane as your windows simplifying flashing details.

    The nice benefit of fiber faced polyiso (besides the fact that it tends the be the cheapest for $/R around me) is that it is slightly permeable allowing a bit of drying towards the exterior.

  4. Lukas Schwartz | | #4

    Thanks for your feedback!

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