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Rigid foam roof insulation and polyethylene vapor barrier inside — problem?

dirkgently | Posted in GBA Pro Help on

Was planning on a r-25 insulated roof deck for a ski house in lower zone 6 NH with cold roof (strapping with plywood) on top of that to help prevent ice dams.
Existing cathedral ceiling is actual 8″ rafters framed 24″ o.c. with 6″ paper faced fiberglass batts. In exploring the original construction (circa 1980) I also found a layer of POLY as an additional vapor barrier….which sabotages my R-25 rigid foam roof plan…..damn.
Considering cutting sections of plywood roof deck out in strips (? on width) to try and sabotage the existing poly and render it ineffective…….Is this a sound plan or pipe dream?
Hoping someone out there has an experience in removing existing fiberglass in catherdral ceiling from the topside that they would like to share. I would like to dense pack the rafter bays after roof deck is insulated.
Thanks all

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  1. GBA Editor
    Martin Holladay | | #1

    This is a judgment call. If you do a good job with your new rigid foam and roofing, your roof assembly should stay dry. While the polyethylene is not ideal, leaving it in place is probably a low risk. If I were you, I'd leave it in place and I wouldn't worry.

  2. dirkgently | | #2

    Thanks Martin.
    I really thought you would say I had to remove the poly.
    Any other opinions on this are most welcome.

  3. Expert Member
    Dana Dorsett | | #3

    If you're going to dense-pack the rafter bays, it's easier to do just BEFORE you add the exterior foam, drilling & filling from the exterior side, which means you don't have any interior re-finishing to deal with. Temporary patching the drill holes on the exterior if the dense packing happens a few weeks prior to the rigid foam isn't a big deal.

    It's also possible to insulate the exterior with fiber insulation under a vented nailer deck for the new shingles if you don't mind a thicker assembly. That way the structural roof deck can dry to the exterior- no moisture trap.

  4. dirkgently | | #4

    Hey Dana,
    thought u might comment, I'm sure u wanna be warm there next winter eh?
    by fiber insulation do u Roxul comfort boardl? I was wondering if that could be done....but only found it being used on side wall exteriors. I thought perhaps the weight of snow could complicate it being used on roofs.
    I sure would feel better if it could dry to 1 side.

  5. Dana1 | | #5

    ComfortBoard would work, as would a second set of rafters & purlins with standard density rock wool (or high density fiberglass) batts. If the new rafters needed to be fully structural to better manage snow loads, that's do-able too.

  6. dirkgently | | #6

    Stacking a 2nd set of rafters is not in the time budget, so it is out.
    I am concerned about additional dead load on roof. Since the roof will no longer melt snow load off I was hoping to partially compensate for additional weight by switching from ashphalt to metal roofing.

    I believe a 6" layer of comfortboard is would add 4 # per sq. ft. (psf) to deadload of roof.
    I find conflicting info for the PSF for 5" of XPS....anywhere between 1 psf (IRC) and 7.5 psf (google). Does anyone know the real psf per inch of rigid foam??

    Right now I am leaning toward trying to sabotage the poly VB and use reclaimed xps/eps.
    Still curious on thoughts of how many tears need to be in poly vb to become ineffective????

  7. GBA Editor
    Martin Holladay | | #7

    The effectiveness of polyethylene as a vapor retarder is directly proportional to its area, so slitting the poly does nothing. If you remove 50% of the poly, the poly will be half as effective as it was before you started.

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