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Insulation around basement drain pipe?

user-1144527 | Posted in General Questions on

We are buying a new house with a basement that we’d like to finish. We are in climate zone 4A, and there are no moisture problems in the basement. The floor cracks in the picture are (reportedly) from 6 years ago when the sump pump failed. We will be installing a backup pump.

I’ve read Martin’s article on insulating a basement wall, but I’m not sure how to handle the 4 inch PVC drain pipe that runs along two of my walls. The long wall is about 48′ and the short wall is about 14′. I can’t re-route the drain pipe because the other wall has the stairs.

I could probably squeeze 1 inch XPS behind the pipe, then build a 2×4 uninsulated wall to the inside. Should I use 2 inch XPS in the other locations and just tape the transitions? Any other suggestions?

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  1. user-1144527 | | #1

    The picture link didn't work... See attached.

  2. GBA Editor
    Martin Holladay | | #2

    You don't have to re-route the drain pipe; you just have to move the pipe a few inches inward.

    PVC pipe can be cut with a hacksaw. Joints are made watertight with a cleaner and PVC glue. If you can't do the work, hire a plumber.

  3. Expert Member
    Dana Dorsett | | #3

    In tight spots it's usually easier to cut PVC or ABS pipes with a string than a hacksaw:

    Using 2" XPS would get you to code-min R10 performance from a labeled R perspective, but it's not a very green way to get there due to the climate damaging HFC blowing agents used. Since you're finishing the basement you can beat code more greenly & cheaply by using half-inch (or more) of polyiso or 3/4" (or more) of EPS against the concrete, and installing unfaced (or kraft faced if it's cheaper) batts in the studwall. You need at least R3 of foam for wintertime dew point control in winter for the above-grade part of the foundation, and the vapor retardency of the foam impedes ground moisture vapor flows from creating an issue during the summer. Since there is at least some potential for flooding (sump pumps & power can fail during a hurricane), remove the facers on any kraft faced batts to at least a foot above the slab, maybe more.

    Even though it's a dry basement, unless there is foam under the slab it's a good idea to put at least 3/4" of EPS (and not polyiso) under the bottom plates of the finish studwall as a capillary and thermal break. The temperature of the slab during the dog days of summer will be below the outdoor air's dew point, but unlike wood the concrete isn't damaged by moisture adsorption. Putting insulation between the bottom plate and the slab keeps the wood at the room-air temperature, which is above the room-air dew point (unless it's pea soup fog down there.) Run the subfloor & slab foam all the way to the wall foam, with the stud plate on top of the subfloor.

    If you're insulating on top of the slab a minimum of 3/4" of EPS would be needed to be safe for say, a rug on top of a wood subfloor, but ceramic flooring would be fine to install directly on the slab.

  4. kenorakq | | #4

    works great///as an alternative to the string theory ")

  5. GBA Editor
    Martin Holladay | | #5

    Just make sure that no one flushes a toilet while you're working on the plumbing.

  6. Expert Member
    Dana Dorsett | | #6

    Tim- that's a great tool for 1.5" or smaller PVC, but we're talking a 4" drain here... (still a "string theory" job.) A small diameter cable (think bicycle shifter or brake cable), or a guitar string works pretty well too.

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