Another slightly different knee wall insulation question
What about putting rigid foam underneath the fluffy stuff on top of joists behind the knee wall?
My third floor is gutted. I am working to put it back together and then will move to gutting the second floor.
I reviewed other knee wall threads and watched Mike Guertin’s video. Two potential downsides to that approach for me:
1. If I install rigid foam blocking in joist bays below/in line with the knee wall, I will be trying to seal it at the bottom to old keyed plaster and lathe, which wont be an easy seal and which is getting torn out. Also I’m going to install HVAC duct in the joist bays and a couple of the second floor supplies will be under the third floor knee wall space.
2. I can’t put fluffy insulation in the joist bays, because it’ll all fall out when I gut the second floor, and I need open bays for ductwork.
What if instead, I put the vertical rigid foam blocking at the end of the joist bays between the top plate of the second floor and the bottom plate of the third floor, and then also put rigid foam (or drywall or plywood) horizontally on the uncovered joists, from where the tongue and groove flooring ends to the newly installed foam blocking at the end of the joist bays. Then I would put the fluffy insulation on top of that. The end goal being that my joist bays are air sealed but empty, and the fluffy insulation is above rather than inside the bays. That way, when I gut the second floor, the knee wall insulation stays put, the dust from the second floor demo can’t go up behind the knee walls, and I have uninterrupted joist bays to install ductwork.
Another question: if I do the above and then also create baffles that run continuously from the bottom of the roof all the way past the knee wall up into the attic above the third floor, do I still need to install vertical rigid foam on the knee walls to encapsulate the fiberglass insulation? I imagine not. I figure if I have to baffle at bottom of the roof and again where it intersects the knee wall, I might as well just connect them and have one continuous baffle…
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