Conditioning attic
Currently remodeling a home in climate zone 6 with a living space in the attic. The interior is gutted down to the studs. The home just had a brand new metal roof put in before I took ownership. The home has gable vents and a ridge vent, no soffit vents.
I want to close off the vents and condition the space without having to add r25 of CI on top of the sheathing as I want to avoid disturbing the brand new roof. I have furred out the 2×6 rafters to 2×8 to create a level surface for Sheetrock and allow for additional insulation. I do not want to lose any more headroom in the living space by adding CI below the rafters. Is it feasible for me to do 2” of rigid foam in between the rafter bays pressed up against the sheathing (to allow for removal or repair of sheathing in the future) and then fill the rest of the cavity with closed cell spray foam along with any minor gaps not filled with the rigid foam?
The space will have central air with adequate returns.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
No, you should not "cut'n'cobble" rigid foam into the assembly. You need a fully adhered insulation layer below the sheathing, which means closed cell spray foam at least thick enough for moisture control, and then additional insulation to get up to code minimum R value if the spray foam layer alone isn't going to be that thick.
Leaving any gaps or voids between the impermeable insulation and the sheathing allows for moisture to potentially accumulate over time and cause problems. You need either a vented assembly, which would mean either continuous soffit-to-ridge vent channels in each rafter bay and adding soffit vents (plus leaving the ridge vent in place), or a fully adhered layer of closed cell spray foam over the entirety of the underside of the roof sheathing.
Bill
Bill,
I may be misinterpreting, but I read the OP's assembly as 2" rigid + 4"-5" CCSF. Would the CCSF not provide an air and vapor barrier in this case, even if adhered only to the rafters (and not the underside of the roof decking)? Is the concern that the CCSF could pull away from the framing over time, creating big pathways for warm air to hit cold sheathing (wouldn't this also be true in CCSF-only roof applications, with no rigid against the sheathing)?
I ask partly out of self-interest, as I'm considering something similar for a mudroom renovation I am (slowly) working on:
https://www.greenbuildingadvisor.com/question/another-unvented-ceiling-question
The issue is that anything that is not fully adhered to the underside of the sheathing introduces the potential for moisture accumulation, and that's where problems arise. Spray foam is fully adhered -- glued -- to the entire underside of the sheathing, with absolutely no gaps anywhere, so no where for moisture to accumulate or air currents to circulate. Any other insulating material cannot do this, and those gaps are a problem.
What would be better is use closed cell for most of the fill under the sheathing, open cell to finish off and bring things flush to the lower edges of the rafters, then polyiso under the rafters if additional R value is needed. This avoids any gaps or air spaces in the assembly under the sheathing.
Bill