Eric Dymond lives in a 1920s Baltimore row house that needs a new roof. He plans on replacing the low-slope, built-up roof with an EPDM membrane, and the question is how to insulate it correctly.
Currently, the built-up asphalt roof is installed over Homasote (or something similar) and a roof deck made of wood planks. Although there’s some “sparsely distributed” insulation in the space between the ceilings and the roof deck, it won’t meet current recommendations for Dymond’s Climate Zone 4 house.
A roofing contractor has recommended that the EPDM membrane be installed over 2 in. of rigid polyisocyanurate insulation. “Does EPDM over foam board make sense?” Dymond asks in this Q&A post. “It doesn’t make sense to me to have 7 in. of foam board to meet R-38, and I’d be concerned about walking on the roof with that setup.”
In addition, Dymond is concerned that 2 in. of insulation is woefully inadequate. Would it make more sense, he asks, to eliminate the foam board recommended by the roofer and insulate beneath the roof deck with fiberglass batts or open-cell foam?
Dymond’s request for feedback and suggestions on other options is the topic for this week’s Q&A Spotlight.
EPDM over foam will work
Despite Dymond’s trepidation, a roof consisting of an EPDM membrane over a thick layer of polyiso foam is common, Jesse Thompson replies.
He adds a link for Firestone roofing products, which include a number of the synthetic rubber products made from ethylene propylene diene mononer (EPDM). The company’s web site includes a number of drawings showing how the roofing should be installed.
Insulation can be installed in the attic or on top…