Exterior foam for mansard roof with cathedral ceilings
I need a sanity check on the proper application of exterior rigid foam with a complex Mansard style roof.
We just closed on a home built in the 80’s that needs an entirely new roof. The current roof appears to have been installed in 1986 and is well past it’s use by date.
The existing structure is unique and poses a couple of challenges.
The architecture includes a large Cupola in the center, 2 large barrel shaped dormers that taper up the roof and an eyebrow section over the front door with a complex, convex shape.
The interior is a vaulted ceiling with exposed structural trusses and a tight drywall ceiling. (see photo)
Only 2 recessed can lights on the entire upper level, so it appears to be fairly airtight at the ceiling. The roof truss appear to be 2×10’s with R-13 or R-13 in the space between the trusses, above the drywall, appears to include 2×8’s with R13 or R19.
I have zero visibility into the roof insulation, just judging by thermal images. There is only a small 8×8 “attic” space in the entire upper level of the home, and of course it houses an HVAC unit. There are no ducts in the attic.
I am assuming that there is some airspace between the current insulation and the roof deck, per common practice and gravity since it’s 32 years old.
I’m having difficulty finding roofers who understand the concept, much less the execution, of exterior rigid foam over the structural sheathing.
The discussions start with “the roof needs to breath” then I point out the current roof is unvented.
Followed by “we can use some cool roofing shingles, it will do the same thing”.
Then it devolves into adding radiant barriers, covering the entire Mansard wall in peel and stick, etc.
I’m using the detail image found in the detail library (attached) to try and relay the concepts.
Is this detail suitable for adapting to a Mansard roof?
I can’t think of any reason why it would not apply.
I’m asking to have the structural sheathing taped or fluid applied at the seams, staggered polyiso foam of 2-3″, again taped at seams, followed by nailbase, sympathetic underlayment and shingles.
Am I over thinking this or missing something? It just doesn’t seem that complex.
I understand that the roof will be “thicker” than normal and that brings additional fascia detailing. We might even have to get some sort of tapering of a couple of panels on the eyebrow dormer but nothing over the top.
Finally, I know I’ll be replacing some of the roof decking based on the condition of the shingles. I assume since we are using exterior foam, I can just add insulation to the exposed cavities and not worry about any air channel below the structural sheathing given the exterior foam.
GBA Detail Library
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