Insulating Above a Knee Wall
Hello, I’m planning another project on my 50’s story and a half house. I plan on insulating the slanted portion of an upstairs bedroom above the knee wall. Right now as far as I can tell it is only a 2×4 between this slanted wall and the roof sheathing and the insulation is in direct contact with the roof sheathing with no ventilation.
The plan is to remove the drywall in this spot and drop the ceiling down so I can fit ventilation chutes and more insulation. How I was thinking of accomplishing this is by adding parallel 4.5” furring strips to allow for .5 – 1” vent chutes and R30 rock wool (7.25”).
The issue with doing it this way is there is still thermal bridging with the 16” OC construction dropping my R value.
Can I use a 1” thick foam strip cut to the thickness of a 2x (1.5”) and then use a 2×4 (3.5”) as a furring strip on top of the foam? Kind of like exterior foam but not continuous it would only be under the studs making a 2×4 1” foam 2×4 sandwich. The wood furring strip would be countersunk with long structural would screws holding it all together.
Or option 2 is running the furring strips perpendicular to the studs now to help get rid of the thermal bridging but this also causeS issues with the vent chutes and the thickness required for R15 mineral wool it wouldn’t compress in the cavity with the chutes in there.
Northern Minnesota climate zone and I don’t like spray foam because of chemicals and fires.
Thanks!
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Replies
I’m going to suggest an alternate plan that might be less work for you:
1- put some 2” or 2.5” reclaimed polyiso in between those 2x4 rafters. This will leave either a 1” or 1.5” cent channel, respectively. Use some strips of plywood or 1x to frame out the vent channel so that you can just press the polyiso into place, then seal it into place with some canned foam. Since you have a vented roof here, the issues with cut and cobble in a roof don’t apply. 2” is R13, 2.5” is R19 if using polyiso.
2- put a layer of 2.5” foil faced polyiso under the rafters. If you used 2.5” polyiso between the rafters, a layer of 2.5” polyiso under the rafters brings you up to a total of R38. Tape the seams of this polyiso to seal things. Put drywall directly over the polyiso, or run furring strips and then drywall if you’re worried about hitting the rafters through the polyiso with the drywall screws.
This method eliminates the need for vent chutes since it creates it a own, it gets you more R value in less space allowing a higher ceiling, and it will probably be less work to install too.
Note that code requires minimum 1” vent channels for a roof.
Bill
Thanks for your suggestion but I’m not a fan of polyiso foam as the R-value decreases with temperature and I’m in a very cold climate.
It doesn't decrease as much as you might think, and typically bottoms out near the same value as XPS or EPS. Some types (like Dow Thermax), don't drop with temperature at all. The R value drop is due to some issues with one of the blowing agents used, but not all polyiso manufacturers use that particular blowing agent anymore.
I used to be of the same thinking as you are, and I'm a very similar climate to you. I had been thinking of a layer of XPS over a layer of polyiso for my exterior rigid foam. Dana Dorsett on here convinced me to just use all polyiso, and had good info to back up his reasoning. He seems to have taken a leave for a while, but if you search the archives you'll find a lot of info on the subject in his posts.
Bill
Understand going in that retrofitting half story buildings to be remotely energy efficient is very difficult if not impossible without spray foam.
https://www.greenbuildingadvisor.com/question/devils-triangle-update
https://www.greenbuildingadvisor.com/question/devils-triangle-and-cathedral-ceiling-insulation
https://www.finehomebuilding.com/2012/09/06/two-ways-to-insulate-attic-kneewalls
Walta