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Detailing basement insulation

user-7151685 | Posted in Energy Efficiency and Durability on

I live in central NH zone 6 and I have a question about details for insulating my basement by myself.  The house is a year old and during last summer the basement reached 64F and 80% RH until I got a dehumidifier.  The town calls for R-15 cont./R-19 cavity wall, so I am planning 3” of XPS or Polyiso and 1.5” XPS then subfloor over the slab.  My question is about my PVC waste line which is attached to the foundation perimeter wall.  I can get 1” of rigid foam behind it, but that’s all.  Do I need to move this line away from foundation, or is there an alternative?
Second question is about the rigid foam on the slab around interior staircase and support walls.  Does the staircase need to be removed (yikes) to be raised up to the new subfloor or do I have other options.

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  1. Expert Member
    Dana Dorsett | | #1

    Put what insulation you can between the drain and the foundation, add insulation above & below the drain tight to the drain (rock wool is fine). Leave a ~2" stripe of pipe on the conditioned space side uninsulated to ensure it doesn't develop frost/ice blockage over an extended cold snap.

    The stairs become another issue. It has to stay within code parameters for the amount of rise on that bottom step- if it's too short it's a tripping hazard. Sometimes it's better to build out a landing mactching the height of that bottom step and doing the transition to the new finish floor level with ramps.

    1. user-7151685 | | #5

      Thanks for the reply. If I can only get R-5 behind the waste line and leave it exposed won’t that be greater risk of condensation?

  2. GBA Editor
    Martin Holladay | | #2

    My article on this topic discusses the issue of stairs: "Installing Rigid Foam Above a Concrete Slab."

    The real question is, how can these problems arise in a house that is only one year old? Building codes require basement wall insulation.

    Is this builder error? Or was this an owner/builder issue?

    1. Expert Member
      Dana Dorsett | | #4

      >"Building codes require basement wall insulation.

      Is this builder error? Or was this an owner/builder issue?"

      I can't really say how many times I've seen 3" of XPS on the exterior side of foundations in southern NH and MA that ended at grade, with uninsulated exposed foundation above grade. It's a mystery to me how builders and inspectors manage to interpret the code that way, but it's fairly common.

    2. user-7151685 | | #6

      Thanks I read that article that’s where I got the details. I understand about the codes for the stair riser height. I was more wondering about The pressure treated bottom plates on the support walls. do I add 1.5” of rigid foam on top of them to match the rest of the slab? Add cavity batt insulation or just leave it?
      As far as code The basement is unconditioned with fiberglass batts in the overhead joists, I am looking to move it to conditioned space and bring the thermal boundary to the foundation walls and slab.

      1. GBA Editor
        Martin Holladay | | #8

        My recommendation is to leave the pressure-treated bottom plates right where they are, and just butt the new rigid foam up to the plates. If there is a gap, caulk it.

  3. Expert Member
    Dana Dorsett | | #3

    BTW: XPS is the least-green insulation in common use today, largely due to it's HFC blowing agents. Both EPS and polyiso are blown with fairly low impact pentane. R for R the polyisocycanurate polymer has less overall environmental impact than polystyrene (EPS, XPS), though EPS isn't anywhere near as impactful as XPS. As the HFCs diffuse out over time the performance of XPS drops to that of EPS of similar density (about R4.2/inch), though it is warranteed to 90% of it's labeled R. From a design point of view assume that it's R4.5/inch (the warranteed performance), not the labeled R5/inch.

    Installing 1.5" of EPS on top of the slab has about the same lifecycle performance as 1.5" of XPS, installing 2" of EPS has a higher performance than 1.5" XPS, and it's cheaper.

    There are several foam reclaimers in New England selling used foam in good condition at 1/3 or less the price of virgin stock foam. Reclaimed foam (any type) is the greenest possible solution, since no new blowing agent or polymer was created, you're just piling on to benefit side of the cost^benefit balance.

    1. user-7151685 | | #7

      I really love this site. I had already found out about several of the reclaimed foam dealers from this site. It really is a win/win for everyone.

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