Does it make sense to install two staggered layers of rigid foam above a roof with spray foam insulation – or just one layer?
We’re finally getting ready to replace the roofing on an old building that has recently been insulated with spray foam. The 2×6 walls were filled with open cell foam and shaved flush. We chose to use closed cell (Lapolla’s 4G) foam between the rafters due mainly to the fact that we were not going to put the minimum r-20 (zone 5) rigid foam on top of the old board sheathing. It was just too thick and would look way out of proportion with the building. The design of the roof didn’t lend itself to reducing the fascia height either. In fact, the thickness of the roof will be doubled with just 3″ of rigid foam, a 1 1/2″ vent space, and new plywood deck sheathing.
On a separate note, we decided to insulate before the tear off and reroof in order to keep the filth and contaminants (cement-asbestos tiles) from falling into the building through the spaces between the sheathing.
I’ve read here that it’s best to use two layers of rigid foam with taped and staggered seams when adding exterior insulation. I’m wondering if in our situation, with the spray foam beneath, if it makes more sense to go with just one layer to save time. We plan to use 3″ (total) of EPS foam which I had planned to seal the seams with canned foam during install.
I am aware of the risk we will have of the roof sheathing being sandwiched between two vapor impermeable layers. Therefore I’m also wondering if it would make sense to use a traditional asphalt impregnated felt for the underlayment between the old board sheathing and the EPS sheets. The thought being that there might be some drying potential through the EPS to the vent space above the foam.
Any thoughts or suggestions are appreciated. Thanks!
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