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Can I use a table saw to cut a 4x4 at an angle to replace the top plate in this knee wall to salvage the wall?

Background: I have an attic bedroom which lacked insulation. The rafters are only 2x4s so my sister convinced me we should install these 2x3 along the rafters to place the insulation between. Unfortunately, now I'm realizing that this make the rafters into thermal breaks and I'd much rather remove the 2x3s and layer foamboard directly over the 2x4 rafters to avoid this. Problem is, going back to the normal way to frame the kneewall would require all new lumber. I'd like to save the wood and work already done by just replacing the top plate. Is it possible to do that?

enter image description here

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  • Why not just run your insulation along the rafters as is? It would be a bit thinner where it passes the top plate, but significantly better than nothing, wouldn't take any extra work, and there is some (though not a huge amount) of insulating value in the wood itself.
    – FreeMan
    Dec 6 '20 at 23:11
  • I'd like to avoid significant thermal breaks. Wood is ~ 1.5R/in. The insulation I'm planning for is 6.5R/in. I'd prefer to maximize the insulation's effectiveness while I have the chance.
    – HiHi
    Dec 6 '20 at 23:52
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It might be easier to nail a 2x4 to the roof joists and then cut the tops of the vertical studs to fit the angle of the top plate. You could then easily nail the studs to the top plate.

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  • That's what I'm trying to avoid if possible since it would require dismantling the frame and completely rebuilding it. I certainly can do it but if I can just replace the top plate it's a lot simpler
    – HiHi
    Dec 6 '20 at 23:41
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enter image description here

Cut up through the 2x3 with a handsaw or oscillating saw flush with top plate.

Remove 2x3 from rafters leaving the angled block above top plate. Knee wall is now a bit taller.

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  • How do I ensure the left-over 2x3 angled block is attached correctly to the rafters and current top plate? Just add nails/screws at weird angles?
    – HiHi
    Dec 6 '20 at 23:43
  • Pilot a hole angled up through the block into rafters then use a screw long enough to catch into rafter at least an inch. Also could add a board on back side to rafters. Screw to rafters and wall top.
    – Kris
    Dec 7 '20 at 0:13

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