Insulating over the top of exterior walls
Hello All,
The House Now
I am retrofitting a 1960 ranch home 26×44 with a full walk out basement and a 24′ garage on the 45th paralell in Wausau Wisconsin. This house was a mold nightmare. After the complete gutting of the exterior walls and removal of all non load bearing interior walls and then remediation we are going to start the reconstruction. This is 2×4 construction walls with 2×6 rafters on a 4/12 pitch, the roof 44′ length runs north to south with a 24′ garage on the . The exterior wrapped in r24 polyiso down to the footings with epdm water protection and a drain tile installed. Derated polyiso is R19 and R15 batts installed inside nets out at R34 walls below the rafters.
My question lies in how to max out insulation over the exterior walls and the first couple of feet in from the exterior wall due to the 4/12 pitch, until I get to a 24″ layer of cellulose in the rest of the attic. So far what I came up with is not very satisfactory. Polyureathane closed cell foam over the exterior walls which will only get me to roughly R18 with a 3/4″ ventilation pan for ventilation to the ridge vent. The closed cell polyureathane does help in making sure that everything is air tight at the wall/ceiling intersection. I also thought of running an inch of polyiso above the ceiling drywall which doesnt really address the problem over the exterior wall, but does help on the next 2′ in and losing a inch on 8’walls is relatively unnoticeable. I can’t be the only one stuck in quandry.
I could say the heck with it and replace the rafters with trusses and instead of a 44′ long hip roof running north to south, I would have a 44′ long mono pitch for solar with the garage is on the south end of the structure so the mono pitch would run into the 90-degree oriented of 24′ roof of the garage. That way I would get the 24″ energy heals for cellose and a vaulted ceiling in throughout the house but that is not cheap by any means. It does allow me to reorient that roof to add a lot of solar panels Also the rafters are sheathed in 1″ rough cut boards that are 12″ wide, so ripping the roof of is not going to be a piece of cake either. A lot of work/cost with deconstruction and I am just not coming up with good answers to the problem. Thanks in advance for your input.
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Replies
Trying to slip rigid foam out over the top plates won't work as well as you might think. I've actually tried that (with XPS in my case, but same basic concept). It's surprisingly difficult to get a tight fit against the top plate, and without a tight fit, you get circulating air currents that cancel out a lot of what you were trying to accomplish.
Spray foam is going to get you the most R value out at the eaves, and it will help with air sealing that area. You have to make sure you block off the soffit and vent channels (I'm assuming you have a vented attic here), otherwise the spray foam has a habit of getting into places it shouldn't, and then sealing things that shouldn't be sealed. Getting spray foam applied well out at the eaves of a shallow roof isn't going to be easy unless you have access from areas besides the attic, so keep that in mind. It will be VERY difficult to get a good application if you're trying to maneuver in very confined spaces.
You could blow cellulose out in the eaves, and "dense-ish pack" it. Dense-ISH packing is just get it a little denser out there than in the other areas so that it won't settle as much over time. You still need to block off the soffit and vent channels, but it's not as critical with cellulose as it would be with spray foam. Cellulose is much less of an issue than spray foam if it goes where you don't want it, since you can easily remove it if needed.
If you have the walls open and can air seal the old fashioned way with caulk and canned foam, I would not use spray foam above the top plates. If you do NOT have any other way to air seal those top plates, I would consider spray foam if you can get sufficient access to install it well. You could try raised heel trusses, but that means an entirely new roof structure, so that's a major structural project that probably isn't worth doing just to gain a little extra R value over the exterior wall top plates.
Bill