Installing a vapor diffusion port in zone 4
My house has a cape cod style upper floor where the roof creates the ceiling for a portion of the living space. The current attic is vented by 3 box vents and the eave space is unvented. There is little insulation and in some places no insulation is provided at the eaves. Rafters are 2x4s at 24” O.C., roof geometry is a cross gable with “T” footprint, and roofing material is asphalt shingles with hopes to replace with a metal roof in the future.
My plan is to demo the attic ceiling, add some skylights, and do an unvented roof assembly. Exterior roof insulation is out of budget, so I plan to use plywood gussets to extend the rafter depth at every other one to deepen the roof cavity to provide the code minimum R-38 (zone 4C) insulation requirement using the flash and Batt method.
One option I’m considering is using spray foam (R-20 air impermeable required by code) and use blow in insulation for the remainder. But have mixed feelings on using spray foam. So, my other option is to use rigid foam. But I worry about the installer error (me) and potential air leaks.
And then I read about the research on vapor ports and it makes me wonder….
I understand that fundamentally a vapor port is not needed if I am providing air impermeable insulation against the sheathing up to the dew point , but would it be prudent to install one at the ridge anyway? Would it help mitigate the risk if I did flash in batt or cut and cobble with rigid insulation?
Insulating a Cape Cod House
Vapor Diffusion ports (Rats, I’m zone 4C)
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