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Old 02-24-13, 08:58 AM   #12
Helper EcoRenovator
Join Date: Feb 2009
Location: St. Louis, MO
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With your tips and other research, I think I have decided on doing the following:

1. Tear down the old wall and take out the old fiberglass.
2. Install 2 inches of XPS foam in each cavity. I will cut it a bit small and use canned expanding foam to get a good seal around the edges.
3. Build another 2x4 wall inside the first one. I will stagger the vertical 2x4's to reduce thermal bridging. I may even leave a 1/2 inch gap between the two walls to almost completely eliminate wood-wood contact (the top and bottom plates) and allow another 1/2 inch of insulation area.
4. Put up netting and then blow the remaining gap (~5-5.5 inches) with dense pack cellulose.

I have run the numbers, and this is the cheapest option (besides reusing the old fiberglass). It will run me about $275 (just for insulation, not wood or drywall) for a 240 square foot wall.

In regards to adding the foam to the outside of the house, I like the idea, but there are several problems:
1. I don't want to replace the siding at this time.
2. We have a walkout, so the top of the siding on the back and side of the house is about 30 feet in the air. Probably not a job I'm comfortable with. Adding in labor (even if just taking down then putting back up the old siding) would add too much cost to the job).
3. At least half of the reason for this job is to decrease outside noise infiltrating the wall. From my research, it seems dense pack cellulose is the way to go. The XPS doesn't decrease the sound much from what I understand.
4. When the siding does need to be replaced, I will still have the option of adding more XPS at that time.

My only question is: do I need to put up a plastic vapor barrier on the front face of the new 2x4 wall? I understand that the XPS is a vapor “retarder”, but I don't know if I need an actual vapor barrier on the inside part of the wall. I plan on sealing the new drywall pretty well, so I don't know if that would be good enough. I also have read some things that it's not good to have cellulose "sandwiched" between two vapor barriers. Any moisture that gets in there doesn't have a chance to escape. Any words of wisdom?

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