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I have a lookout basement wall where the windows sit directly on the silplate. The foundation wall sticks out about 2 inches past the silplate and there is 2 inches of xpf directly glued to the concrete wall. On this wall I'm planning on putting a PT 2x8 up against the existing silplate tapcon'd into top of the exposed 2 inch section of concrete foundation wall. Then finishing the lower half with 2x3s on the flat side 16"OC so EMT can fit behind. EMT is code here. Once finished the windows will have drywall returns and a painted poplar top plate.

Firstly, is this a good plan? Should I put silplate foam under the 2x8 before it's mounted? Should I not put the 2x3s on the flat, or maybe switch to 2x4s on the flat? Or maybe just normally frame with the 2x3s and drill for the EMT.

The reason for the 2x3 is the top plate finished look. I didn't want compromise the insulation and the finished top plate will have a 1/2” overhang, so it will be a true 1x8 plus the 4" depth where the windows are. window

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I see no problems with your plan, depending on local code requirement that I'm not aware of.

  • Sill seal foam isn't needed unless you need to block airflow.

  • A treated sill plate extension is appropriate.

  • Short energy walls built with flat lumber is fine. You might find that 2x4 is actually cheaper than 2x3. I would use the sill plate as the top wall plate and just fill studs between it and a new treated bottom plate for stability and cost savings. (Halves of a ripped 2x4 would make a fine bottom plate. It doesn't much matter that it'll be slightly oversized.)

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