Rigid Foam for Residential Roofs
Hello,
The not for profit land trust I work for’s office is in an old farm house. It is time to re-roof and we would like to take the opportunity to upgrade insulation.
The 3rd floor has sloped ceilings (paneling attached directly to the bottom of 2×6 rafters) so I have suggested:
-stripping entire roof
-filling cavities with insulation (high density fiberglass batts or mineral wool)
-covering outside of rafters with several inches of rigid foam bringing the assembly to at least minimum code (preferably foil faced polyiso)
-attaching 2×4 strapping on its side to the rafters, using long power lags, on top of the foam
-installing plywood roof deck on top of strapping
-ice/water shield
-shingles
essentially the first image in this link: https://continuingeducation.bnpmedia.com/courses/multi-aia/new-options-for-insulating-and-ventilating-wood-framed-sloped-roofs/3/
or this image: https://www.protradecraft.com/sites/protradecraft/files/Roof-insulation-retrofit_0.png
I’ve read about this design for years on forums like this and others. I’ve actually done this assembly on 2 of my houses myself.
The first roofing contractor we called to give us a quote was fine doing the roof but told us that “rigid foam is only approved for residential roofs if covered with metal roof”.
I’ve never heard/read this before. Is this true or is he just not familiar with the technique or doesn’t want to do the work?
Our town follows the 2015 IBC and the 2015 IRC. I have a call in to my town’s building inspector asking the same thing; but, who knows if/when he will get back to me so I figured I’d pose the question here for all you very wise folk.
Thank you!!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
First, the code does allow you to install shingles on a second layer of plywood sheathing.
Both details will work, if you want a ventilated assembly. The first detail has NO peel and stick membrane on top of the wood decking, which I think it should have. Then install the rigid foam, followed by plywood sheathing and the ventilated channel. Finally, install a second plywood sheathing, a roof underlayment and shingles.
The second detail does installs the peel and stick membrane on top of the wood decking, then the rigid foam followed by the 2x4s on top of the rigid foam, then a plywood decking, a roof underlayment and then shingles.
A third option, it’s like the second detail, but skips the venting channel. Install a peel and stick membrane on the wood decking, then the rigid foam, then the plywood decking, then a roof underlayment and finally the shingles. Details 9-00800 and 4-05050, here on the GBA.
That roof contractor is imagining the roof material will be installed directly on the foam. Call it a lack of knowledge, lack of imagination, or a miscommunication.
Like Armando says above, a second layer of roof sheathing makes anything possible.