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Unvented Roof Retrofit and Dormer Addition

atgambleassoc | Posted in Expert Exchange Q&A on

We need a new roof on our  CZ5 1998 house and I’m struggling to figure out the right approach.  Our existing wall and roof assemblies aren’t great:
– Walls are 2×6 construction with painted claps nailed directly through Tyvek to the plywood sheathing.  Fiberglass batts fill the stud cavity, and there is a poly vapor barrier behind the painted GWB on the interior.
– 9:12 Vented (ridge + soffitt) Roof: Asphalt shingles, plywood sheathing and 2×10 rafters w/ 2×8 collar ties.  R30 paper-faced fiberglass batts stapled in all bays.  The half-story under the roof is conditioned space, but the HVAC ducts are located behind the kneewall and right next to the soffitt vents, so it’s terribly inefficient.

In a perfect world, we would add rigid CI over the entire roof and remove all vents, but I’m having a hard time convincing any roofers in my area to convert the house to a hot roof.   My questions are:

1. Should I continue to push for an unvented roof assembly?  My PE has determined that my 2×10 rafters are at 95% of allowable loading, so I wonder if I can even support R20 of CI on the roof without significant structural work.
2.Since we plan to add 2 new dormers in conjunction with the new roof project, can these dormers be unvented if the rest of the preexisting roof remains vented?  My gut tells me that since I’m increasing the surface area on this story with 2 new dormers, it would make sense to construct these to be as airtight and well-insulated as possible, but given the rest of the house, maybe this is a mistake.
3.  Ideally, we would reside the house (with a layer of rigid CI over the sheathing at all walls and an airgap behind the new clapboards), but this isn’t in the cards right now.  For my new dormers, my standard practice is to backvent all new siding, install a layer of CI over the sheathing, and utilize intelligent WRB and vapor retarders like Mento and Intello outside and inside respectively.  The newly dormered side of the house will be exposed to weather here on the coast.  But is this strategy a waste of money for the new dormers if we aren’t doing it everywhere else on the house?

Thanks for helping me think this through!

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  1. plumb_bob | | #1

    If your rafters are already maxed out, adding roof insulation will mean snow does not melt, adding to snow load, causing risk of structural failure. Address the structural aspect first, then go after the envelope.

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