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Permeability of asphalt roof assembly

woobagoobaa | Posted in Green Products and Materials on

Folks,  I’m looking for a sanity check re: what is the vapor permeability to the exterior of an asphalt roof assembly for a medium pitch unvented shed roof.  Best I can tell it is essentially 0 (vapor barrier).

I’ve got a re-roof happening in a couple weeks (zone 5 MA), and the roof assembly the contractors are converging on is (outside to in):

– Asphalt shingles
– Underlayment.  Leaning towards 100% peel and stick ice and water shield.
– board sheathing
– 7″ closed cell spray foam in the existing, irregular 2×8 rafter bays. Conditioned space below for ductwork.

I ask because (I think) this assembly is a water/air/vapor barrier on both sides of the sheathing.  I have concerns that, if (when) water gets into the sheathing, there is no drying path.  I started to look at permeable underpayments to maybe allow some exterior drying, but seems the shingles are vapor closed so no point in worrying about a permeable underlayment.

Thanks a bunch.

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  1. Jon_R | | #1

    About .65 perms, although I'm sure it varies with the shingles. That's slow drying, but better than none. Some info on even smaller amounts of roof drying is here.

  2. Expert Member

    Wooba Goobaa,

    The question is whether the increased risk of wetting outweighs the benefit of a small amount of drying. RDH concluded an impermeable underlayment makes more sense.

    However that was in the context of a ventilated roof which could also dry to the inside. Maybe the equation changes when it's an un-vented roof? Or maybe the case is more compelling as the consequences are worse?

  3. Expert Member
    Akos | | #3

    Experince has shown that a roof deck between two impermeable layers is fine as long as the roof is dry during the install. There are a lot of cc SPF shingled roofs out there and they seem to work fine.

    Unless you have a low slope roof or a lot of ice dams, I would skip the full ice and water, install it where needed and use standard underlayment elsewhere. Since the SPF is already air sealing your roof deck, the peel and stick is not doing much.

    There could be an argument for a vapor permeable underlayment to allow a bit of drying. I always tend to use this as it is stock at my local roofing supply.

  4. azad_lassiter | | #4

    Reading up on this a bit I found a study by owens corning. One limitation is that it was done for Vented roof assemblies. But I assume the vapor permeance would not change for vented vs sealed. Is that a correct assumption?

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