Insulating kneewalls with obstructions
I’ve got knee walls in several rooms of my 1990s era zone 5 Michigan home. See photos attached. At present the 2×4 bays are filled with R-13 faced fiberglass batts. A thermal camera shows these rooms are losing a lot of heat through the knee walls, particularly at the ceiling and floor level.
After much reading of Martin’s articles, my plan is to screw 1.5″ foil-faced R9.3 polyiso to the joists and to seal around the edges with canned foam. However, the joist bays are obstructed by what I believe are structural supports for the roof. How should I apply the polyiso boards here for maximum thermal efficiency and minimum labor? Should I cut them to fit between the structural supports so they can be placed close to the fiberglass, or is it possible/desirable to attach the boards to the structural supports?
Additionally, for reasons unknown one area of the knee wall doesn’t appear insulated, but instead has home wrap. Any thoughts on what might be going on there and how to address that area?
Thanks very much
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Replies
Easiest would be to just cut the polyiso to fit between the framing where necassary, leaving enough gap (~1/4" or so) that you can seal it with canned foam. The plastic tips for the "pro" gun for Great Stuff work great for filling gaps like this. The downside to this method, while it saves a lot of labor, is that you'll have thermal bridging from the stud. If you want to eliminate that thermal bridging, you can "box over" the framing with polyiso strips so that the polyiso effectively wraps the outside of the problematic framing members. You want to seal the polyiso at the bottom and top too, since it's beneficial to have the polyiso act as an air barrier here. You can glue a strip of polyiso along the top plate too so that the top plate has polyiso on the top and exterior side, which will help with heat loss in that area. I usually just air seal the lower edge of the polyiso with canned foam and then bury it will loose fill (blown) insulation there.
I can't really tell what is going on with the section of wall that has the house wrap. My first guess is that that is an inside corner on the exterior of the wall, so some of the outer house wrap was tucked inside during construction. I can't really see enough to know what's going on there though.
Bill
Thanks Bill. Just to check if I've understood you, would I put the foam board against where the red arrow is pointing, and then put a covering strip of additional foam board where the green arrow is pointing to prevent thermal bridging?
I would continue the foam board down past that red arrow, all the way to the "floor", which looks like maybe drywall for the ceiling of the level below. I'd leave a gap there too and seal it with canned foam. The idea then is for the foam board to cover ALL exposed framing between the problem pieces (your green arrow is pointing at one of those), and sealing it will provide an air barrier for whatever is behind it.
To "box over" the problem framing pieces, you'd need to use three pieces of foam, to insulate ALL exposed sides. Note that that's a lot of work for limited gain, so it's the last step you want to do, and do it using leftover pieces of material.
Bill
Brilliant, thanks so much for the detailed analysis. I really appreciate it.
Actually, one additional question: the knee wall attic space does not have its own vents. Rather, the vents are near the peak of the roof, about 10 feet further up the roof. Do I need to account for that by not running the foam board all the way to the top of the roof sheathing so that air can flow from the soffit vents up through to the roof vents? I've not seen much discussion of how to achieve that.
You need to put vent chutes at the top of the kneewall then. That can mean leaving a gap of at least 1" between the roof sheathing and the top of the foam, or it can mean a "real" baffle extending up a way above the top of the wall, which is what I usually do (since I use loose fill insulation on the upper level attic floor).
Bill