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Ventilation, Insulation, and Water Management in a Conditioned Attic

christian_330 | Posted in General Questions on

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.

Venting:
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.

Insulating:
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?

Moisture:
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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Replies

  1. Expert Member
    Zephyr7 | | #1

    You won’t be able to fit code minimum levels of insulation in 2x6 rafters even using spray foam in a non-vented roof configuration. You could use foam board under the rafters, batts between the rafters with an air space on top, and build up the rigid foam layers until you get to your target R value. You’ll loose some headroom this way. You could put rigid foam on top of the sheathing to build an unvented assembly, without loosing headroom, but you’d have to replace your roof to do that.

    You can’t safely build an unvented roof without using spray foam or rigid foam. You’re going to have to use one of those. If you’re worried about spray foam (I really think the risk are greatly inflated due to people’s perception, not any actual reality based on actual data), you can use rigid foam. Rigid foam is very stable — that includes all three major rigid foam types. I would use polyiso here.

    Personally, I really think you should not try to use your attic for storage. If you want access to network cables, run them in an elevated cable tray that is above the level of the blown in insulation and/or build a catwalk above the blown insulation. This will be both easier and a lot cheaper to do well compared to a full conversion to a conditioned attic.

    If you want to go with a lightweight cable tray, I recommend the cablofil product line made by legrand. It is cheaper than the cooler bline system, and they have shorter leadtimes too.

    Bill

    1. christian_330 | | #5

      Bill have you ever heard of someone using insulation between rafters and insulation between ceiling joists? Would the r-value combine or possibly be better with a large air space between the layers on insulation?

  2. christian_330 | | #2

    Thanks again Bill. My house is already torn down to the studs and the roof is on track to be replaced with aluminum standing seam. Would it be reasonable for me to expect R7/inch with closed cell foam maybe getting me 4in of foam in the rafters for an R value of 28 on the rafters and then maybe add two layers of 2in R-10 rigid foam under the rafters for R-48 total? Maybe even the more expensive r13 boards? Would I be able the have rafter insulation and also blow in insulation between the joists, then just lay plywood down on top of them to have a clean attic but both rafter and joist r values could combine for the total ceiling r value? Also, what did you think of the radiant barrier. It just kinda stinks to give up all that useful space in a property that’s somewhat lacking in storage.

  3. GBA Editor
    Kiley Jacques | | #3

    Welcome to GBA, Christian! It's always a banner day when new members join the community. Regarding your project, another reader recently asked a related question in this thread: Insulation and Ventilation for a Low-Slope Hip Roof. Read through the answers to find a number of good resources for answering some of your questions.

    1. christian_330 | | #4

      Thanks Kylie. Sorry to clog up your servers with FAQ’s haha. I may have missed this in my research on here but I still have questions about venting the difficult low slop corners with very little airflow and also the question posted above on whether I could combine insulation between the rafters and insulation between the ceiling joists for a total R-value. For venting the hip areas under the decking, is there a reason I can’t cut a ridge vent all the way down the hip? Particularly in the areas like in the photo. I’ll be able to air seal most ceiling drywall joints on interior wall top plates. The slope is too tight for me to get to the exterior walls so I’m playing on sealing a 4 or 5 ft stop on certainteed membrane from the top plates on to the ceiling and then once the ceiling drywall is placed I can seal the membrane to the drywall from the top. I think I can just stuff batts into those hip corners while trying to keep so air gap underneath the decking. I tried to use the baffles as best I could there. I supposed I could also cut a hole in the membrane there and blow in cellulose and then tape it back up in those difficult areas. I’ll also include a picture of a hip ridge vent that I found.

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