Ventilation, Insulation, and Water Management in a Conditioned Attic
Hi again everyone. I’ve freeloaded enough off this site that I’ve finally subscribed haha. I’m located in climate zone 5/6 in the US. My house was built in 1969, has a hip roof with a very low slope, and has 2×8 ceiling joists and 2×6 rafters. I’m planning on having an Englert standing seam aluminum roof installed and my roofer is planning on using Englert’s thick high temp underlayment all the way from the soffit to the ridge (that will be cut in). I was originally planning on air sealing the ceiling and blowing in insulation. I’ve since become interested in insulating the rafters and being able to use the attic for storage and keep easy access to all electric and networking cables. I have no mechanicals or ductwork in my attic. I know this will be difficult to do properly with my hip roof and small rafter size. I don’t think insulating and venting on top of the roof decking is an option but I can ask. I have vent baffles installed from the top plates to the decking for ventilation into the attic at the soffit so I think I would be doing the vent channel under the decking method.
I’ve researched a bit and read the stickied post on conditioned attic building so I think I have an idea on what to do if I go this route. I think I would start by extending the vent baffle channel all the way to the ridge. I would then wall off the distal portions of the roof and ceiling, especially at the hips and corners with either rigid board or rockwool and blow in insulation behind that wall. To ventilate the hip portion of the roof, especially at the eaves where the run from the soffit to the ridge is very short, am I able to cut a ridge directly on the hip all the way down to the corner? I guess another option would be to install box vents and group all the rafter bays in that portion together to vent out of those, like the way it was done when the entire attic was vented. I feel like this is probably not going to vent those corner portions of the hip well especially after insulating. I shudder at the amount of work this would take but would it be possible for me to use a radiant barrier foil in the same way I used my certainteed membrane but stapling the foil inside the rafters about 4in deep so there will be at least an inch or so air gap between the foil and decking? That way I would presumably have a radiant barrier, air barrier, vent channel, and with space left over for r15 rockwool batts.
I’m trying to stay away from spray foam, polyurethane foams, and rigid foam board as best I can. This is probably unscientific but I’m, skeptical of the chemicals and longevity of such products. I realize though this may need to be used to achieve what I need in such a small space. I’m more drawn to rockwool products for this reason and for the fire and acoustic performance and for the non-permanent aspect (could easily take down or move batts or boards to address a repair). Would I be able to use the r-15 in the 2×6 rafters and then either furr out or just build an assembly to hold more or thicker batts on top of that to get a better r-value?
Would I still seal all airgaps on the ceiling as if it were a blown in application? Would the use of the Attic Foil need to be a vapor permeable option? Would I need to build a return and supply pathway for the HVAC to have access to the attic to condition the space and remove humidity?
Total amateur with a tenuous understanding of this stuff but hey, I’ve gotten this far in my complete reno haha. Thanks for any advice.
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