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dgeist

XPS insulation and fire blocking advice needed.

dgeist
8 years ago

I'm in the midst of a complete basement teardown and rebuild. After address all the water issues we've found, I plan on putting taped and sealed sheets of XPS against the perimeter walls followed by traditional 2-by framed walls (likely empty). Above the top plates of the new walls, I will run fireblocking fastened to the underside of the existing 2x10 joists flush against the inside edge of the mudsill and flush or just-proud of the fornt surface of the wall framing, followed by foam to fill the gap behind the framed wall between the cinder block wall and bottom of the fireblocking.

Here are the questions: What are the trade-offs between using 1" of XPS and more (like 2")? I'm in climate zone 3 (Atlanta) and the outside ground level is between 5 and 6 feet up on the wall. Suffice it to say, even with no insulation, the basement is one of the more temperate spaces in the house in both cooling and heating seasons. Most of my motivation is to prevent/control the humidity and vapor content in walls, which can get rather high any time there's rain and all through the warm months. I'm interested to know how much the vapor permanence will change with that extra inch of XPS. Also, does it make sense to bring the top edge of the XPS flush to the bottom of the horizontal fireblock or to leave a small "access gap"? The former seems like the obvious choice, but for some reason, I keep envisioning a small gap above the XPS.

With respect to the fireblocking itself, is there any (non-incendiary) advantage in 1/2" gypsum vs. 3/4" plywood vs. 2x lumber at the top of the assembly? I'm leaning towards the plywood since it's consistent an solid for fastening framing but will leave a bigger gap than the 2x for foam back against the top of the block wall once it's in place. Basically less thermal bridging, right?

Thanks.

Dan


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