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Old 11-17-2015, 09:41 AM   #1
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Insulating Basement Walls in a Duplex


I'm in the very beginning stages of finishing my basement. I live in a fairly large side by side duplex that I own. I'm one of those people that does too much research for his own good, but here are my questions. I plan on using 2" Rigid foam board on all exterior walls (I'm in the northeast).

Should I use the Rigid foam board on the common wall between the units since technically that's not an exterior wall? It's really just an interior foundation wall, so can I use regular fiberglass insulation there or does that still pose a potential moisture problem because concrete is concrete?

Another concern that I have is we have a 4" septic/sewer pipe running along one of the exterior walls. It's basically running straight across about 3/4 of the way up the wall just below the windows (terrible placement). The pipe is right up against the wall so there's no way to insulate the underside of the pipe itself. When I use the rigid foam board on this wall, I guess we'll have to go around it. Lay the foam below the pipe, then separate pieces above the pipe. My concern is above the pipe there are metal brackets holding it to the foundation wall which also stick out 2-3". How do I create an airtight seal with with the foam board with all these little obstacles on this particular wall? Do I just insulate where I can and leave the brackets exposed? Do I try to fill it in with canned spray foam like tuff stuff or whatever it's called? How about the gap between the pipe and insulation? Do I fill that with spray foam as well? I also plan on using fiberglass insulation between the foam board and the drywall just to fill in the void/space in the wall cavity, so will that take care of these issues in itself, or will i still have moisture issues?
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Old 11-17-2015, 12:02 PM   #2
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No need for foam on the common wall. I would insulated it though, the entire wall with fibrous insulation, and pull out stud wall out from contact to help with the sound transmission.

Cut around them as best as possible and seal up with spray foam.

Best to fill the gap so there is no cold spot. Be sure to use a closed cell and low expansion foam.

Fill the cavities as well.
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Old 11-17-2015, 05:08 PM   #3
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Seal the edges of the foam board(s) to the concrete wall as best as you can.

You could have continuous large sheets of foam stopping a few inches shy of the brackets on top to facilitate attachment to the wall getting a good seal. Then you can glue on small pieces to fill the remaining space.

To prevent freezing of the pipe, there should be no insulation between the pipe itself and the finished interior wall.
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Last edited by AllanJ; 11-17-2015 at 05:20 PM.
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Old 11-17-2015, 05:14 PM   #4
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Being that it's a drain pipe, there shouldn't be any fluids or solids sitting in the pipe.
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Old 11-18-2015, 02:57 PM   #5
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Thank you for the replies and info! So, on that non-exterior foundation wall should I use some poly sheeting against the wall before putting in the fibrous insulation so that the insulation doesn't actually touch the concrete or is that not necessary?
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Old 11-18-2015, 06:47 PM   #6
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There shouldn't be any water in that wall. Use mineral wool either way. Seems to be less moisture sensitive.
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