Air barrier for compact, low pitch, unvented roof
Hi GBA community!
I am renovating a 1965 ranch where the existing roof assembly (1.5:12 pitch) is:
– exposed 2×6 car decking that extends to the exterior to create generous overhangs
– tar paper
– 2.5” fiberglas faced polyiso
– 20-year old hot mop built up roofing
My tentative plan for the new roof assembly is:
– leave 2×6 car decking as is
– hybrid roofing underlayment for air/vapor barrier (mechanically attached, but nail sealing)
– 4” of new fiberglas faced polyiso
– salvaged 2.5” fiberglas faced polyiso
– 1/4” DensDeck Prime for fire rating
– 60 mil fully adhered black EPDM
Will the new underlayment improve the air sealing of the assembly? It will be quite a bit of work to pull up the original polyiso, so I want to be sure it is worth the effort. I know that tar paper is the original “smart vapor barrier” but I haven’t been able to determine if it is an effective air barrier.
Also, does it make a difference to the thermal performance whether the old or new polyiso is closest to the interior of the home?
Thanks in advance for your feedback!
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