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Large unvented flat roof overhang–Is full fill spray foam necessary?

user-6777025 | Posted in General Questions on

I’m currently building a modern structure with a substantial cantilevered flat roof overhang.  The roof over the interior living space will be unvented (flash and batt).  The architect thinks our soffit needs a full fill of spray foam between the joists as well, but she’s also unsure of what’s required here.  The soffit will be T&G hemlock.  In my mind, this is basically a large version of a typical roof overhang in an unvented cathedral roof assembly–the roof gets airsealed and insulated at the rafter blocking from the inside, and the overhangs are unvented and uninsulated.  I suppose the danger here is that warm air travels through the gaps in the hemlock and hits cold sheathing and condensates, but given this is all exterior space, will there ever be a risk of this temperature differential?  The flat roof is a gray PVC membrane, so there is tapered foam above the roof sheathing that should help with the temperature of the sheathing.  We’re in Seattle–climate zone 4c.

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Replies

  1. walta100 | | #1

    Spray foam is the most expensive way to buy R value.

    Consider eliminating the spray foam by replacing it with 2 or more layers of exterior sheet foam.

    Reclaimed used foam from a commercial building would be ideal.

    https://www.greenbuildingadvisor.com/article/insulating-low-slope-residential-roofs

    Walta

  2. Expert Member
    Akos | | #2

    Except for heavy snow country, as long as you air seal at the wall top plates, you don't need any insulation in the overhangs.

    If this is a sitting area, a bit of insulation can take the edge off solar gain there, so a bit of fluffy (R10 or so) does make a difference, definitely doesn't need to be spray foam.

  3. Expert Member
    MALCOLM TAYLOR | | #3

    User ...025,

    In the PNW the biggest threat to that sheathing is night-sky radiance - and the roof-top foam will cure that. I don't see much point in spray foaming the underside.

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