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Thick polyIso as site-built vent baffle

Joe Norm | Posted in General Questions on

I’ve just read the article on site built vent baffles for roofs.

I’m wondering if there is any problem using thick polyiso, say 3 inch at the vent baffle in a vented cathedral roof. In this case the foam would not be foil-faced.

I understand that an airtight ceiling is the most important part. If the rigid insulation was thick, you’d get a higher R-value and the underside would not be cold, therefore less likely to condense. Right?

This is an approach I have not seen covered here yet. It would be similar to the Cut and Cobble method but be vented.

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  1. Expert Member
    Zephyr7 | | #1

    Would this thick polyiso be serving as only a vent baffle which might not even go all the way to the ridge, or would it be part of an insulated roof assembly?

    In the first case, I don’t see a benefit to the thick polyiso. Just use whatever is thick enough to be self supporting (probably 1/2”). If the baffle doesn’t go all the way to the ridge, such as the case with a baffle that serves only to keep the soffit open, there is certainly no benefit to really thick foam.

    If the polyiso is actually part of an insulated roof assembly, then thicker polyiso gives more R per inch (compared to batts). Cut and cobble is ok in vented assemblies, but it is a LOT of work to install in a large area.


    1. Joe Norm | | #2

      thanks Bill, It's the second case, using as part of the insulation as well as the vent baffle.

  2. Expert Member
    Michael Maines | | #3

    Joe, you could increase the foam thickness, up to a point. You're correct that it would minimize condensation risks, and increase the overall R-value. Moisture that does accumulate migrates to the exterior through the rafters, so you don't want to get too thick, or you'll extend that drying path. Unless the foam's R-value was enough to prevent condensation, but then you'd have the same issue as all cut-and-cobble installations: they're hard to do properly, and tend to lose airtightness over time.

    1. Joe Norm | | #4

      Begs the thick is too thick? The reason I ask is because I found a good deal on 3.3 inch boards. Marine Zone 4c.

      I could use this to make a sealed roof, but that seams just as complex, plus the added strapping, screws, metal flashing will add up in cost.

      Cut and cobble doesn't seem too bad if doing it yourself. Again this would have a vent channel.

  3. Expert Member
    Dana Dorsett | | #5

    When going with a 3" cut'n'cobble + interior fiber the ratio of the foam-R/total-R starts to matter, since the trying path through 3" of rafter isn't very vapor permeable and moisture could potentially accumulate in the fiber insulation. In zone 4C that would be a minimum of 20% of the total for condensation control per the IRC prescriptives, so you'd be in pretty good shape with 3.3" foam.

  4. Joe Norm | | #6

    Dana, note this would be a vented assembly, with at least 1" of airspace above the rigid foam.

    It's a hybrid Cut and cobble/vent baffle. Do the drying paths still matter?

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