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Community and Q&A

Framing Walls for Finished Basement

lonjalken | Posted in General Questions on

Hi All,

I’m looking to partially finish out the basement in our 1952 house in the Seattle area. I’d prefer to avoid using organic materials as much as possible, to prevent mold growth. We had an interior drain tile and sump pump system installed with Delta MS running up the exterior walls to the top of the foundation. I’m uncertain how to best frame the exterior walls considering that the thin concrete patch over the drain tile trench is about 12” from the walls. My initial idea was to place 2” XPS against the Delta/exterior walls, seal joints with spray foam, then use metal framing against the XPS and just use construction adhesive to connect the metal bottom plates to the floor and fasten top plates to joists. But then the thought of how to properly air/vapor seal the floor came into play with this method. I was considering using DMX with LVP on top. The ceilings are low and we need every bit of space we can get, so insulation foam board/OSB on the floor isn’t doable.

Is it best run the DMX on the floor up to/under the XPS on the walls and spray foam the connection to create a seal (Or taping the seam to seal?) and then frame the exterior walls on top of the DMX in the same manner – using construction adhesive to fasten the metal bottom plate?

Or would this way be better: Instead of using metal studs to frame walls against the XPS, employ metal furring strips? That way there is no bottom plate to worry about? (PS: I have to clue how to frame out an exterior door using this method).

If there are better ways to do this, please let me know. Thanks in advance!

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Lon.

    I have seen Dricore subfloor panels installed before/under the wall framing in finished basements, but not dimple mats. The reason for this is to keep the wall framing away from a potentially wet floor. Is your floor wet or likely to get wet? If so, I'd take every precaution to keep all materials, even steel studs away from it and would find a way to bring the dimple mat under the wall.

  2. lonjalken | | #2

    Thanks, Brian. Is there any reason not to run simple mat (or DMX) under the framing?


    1. GBA Editor
      Brian Pontolilo | | #6

      I can't think of one. Since these walls aren't carrying any weight, but I've never done it or seen it done, so I'm not sure exactly how to detail it. The Dricore products gives you the same air space as a dimple mat and a subfloor though, in one shot. It's a nice system.

  3. Alex D | | #3

    What's your total height there? have you considered building a floating floor, I believe there is an article somewhere on GBA detailing this? 1" EPS and then plywood/osb ontop of that, you lose about 2" this way (less, if you only do one layer of T&G ply/osb), but judging by the windows you're already not at a conforming finished space (no egress in that picture) so as long as no one in the household is super tall ~6'6" finished will feel short but plenty of headroom.

    Also I think advice in this forum will generally be to always avoid new XPS, it's much less environmentally friendly and does not provide much of an advantage over EPS.

  4. lonjalken | | #4

    Thanks Alex,

    From slab to ceiling joists, we only have 6’8” in height currently, on average. The tightest spots are around 6’7” and the most height is around 7’2”. Clearly the floors aren’t anywhere near level. Point is, the space is already non-conforming and we have little to work with. The room where that pic is taken actually has an exterior door, which can serve as an regress. But the ceiling height isn’t going to meet code, which is one reason we don’t want to spend oodles of $$$ doing this. That said, we want to partially finish the basement the right way and avoid any potential problems down the road. The main reason to partially finish the basement is to increase living space for our kids.

    So, XPS vs EPS against the Delta MS on the exterior walls?

    Is DMX not a good way to go on the slab?

    Thanks again!

  5. Bob Irving | | #5

    Polyiso works well on basement walls and, with the aluminum skin is vapor impermeable so will help keep out moisture migrating through the concrete. Rockwool batts would work for stud insulation.

  6. Expert Member
    Dana Dorsett | | #7

    >"My initial idea was to place 2” XPS against the Delta/exterior walls, seal joints with spray foam..."

    Of the things that are wrong with that picture, in zone 4C the IRC code minimum is R15, whereas 2" XPS is labeled R10, only warranteed to R9 @ 20 years, and at full depletion of the blowing agents after several decades would perform at R8.4, the same as EPS of the same density. Worse still, those HFC blowing agents are EXTREME greenhouse gases (and banned for that use under the Kigali amendment to the Montreal Protocol, as yet unsigned by the US), making XPS hands-down the least-green insulating material in common use in the US today about an order of magnitude more damage per R as EPS (using same polymer, different blowing agent):

    By contrast EPS and polyiso are blown with relatively benign hydrocarbon blowing agents, which do not contribute to the overall performance of the foam. In the EPS case most of the blowing agent escapes the foam at the factory, where it recaptured, not vented to the atmosphere.

    Then, can-foam is a lousy sealant for foam board. As the foam shrinks the can foam will pull away and no longer seal. Using foil faced foam board allows you to seal the seams with the appropriate tapes, and the air tightness holds up well over time.

    To hit code-min R15 with EPS would take 4", with polyiso only 2.5".

  7. lonjalken | | #8

    Thanks for the reply, Dana! Very helpful info! Just FYI: Was considering rock wool batts between the metal studs in crease the R-value. OK, so now it’s either PolyIso vs EPS??? Which is better and why?

  8. lonjalken | | #9

    Also, would just a dimple matting of some kind (DMX, etc.?) against the concrete subfloor and then LVP on top of that be acceptable? None of the built walls on the exterior basement will be weight bearing. Just wondering if framing walls on top of dimpled mat is wise or not?

    Thank you!

    1. Alex D | | #10

      This probably depends on the lvp you use, some lvp is very flexible and would probably telegraph the contour of the simple mat.

  9. lonjalken | | #11

    Another question:

    Returning to a basement finishing project: Should I run the existing Delta membrane that is on the basement wall all the way up the top edge of the foundation and the terminate with a Delta Molding strip and seal? Then apply the Polyiso to the Delta? Or can I just leave the Delta membrane where it is, at grade, and use the molding strip to terminate and seal there? Then apply the polyiso against the Delta membrane and run it all the way up to the foundation top and seal all the way into the rim joist? Any issues with any of this/ Thanks.


    1. DaniMarin | | #13

      Curious about your final decision about running the dimple mat under the frame.
      I.m on the same situation of low ceiling and I was thinking to run the dimple mat against the concrete wall or leave 1", insulate wall with 2" foam board, can-foam the gap and seal tape the transition between foam board and dimple mat. ON top of dimple mat, directly frame the walls as they are not load wall.
      Flooring I'm thinking on a thick LVP (7mm th min) on top with no sub-floor as I can,t afford to loose more ceiling height

  10. Chad McNeely | | #12

    In a similar situation as you, Zone 5 with ~7’ ceilings and a central duct chase at 6’-2”, I wanted the thinnest basement floor system I could get. I dealt with past wetness by excavating and waterproofing (Blueskin, new properly sloped non-corrugated tile, dimple/filter mat) on the outside. Inside, I’ve put down dimple mat on the floor (taped seams), 1/2” foil-faced polyiso (big box store “R-max”, taped seams), then a layer of 5mm flooring underlayment ply laid on top with the seams just dust-sealed during construction with masking tape. I’m planning a snap together laminate floor, so I’m at 1 1/4” total thickness. The wood-framed walls rest on this assembly just fine.

    Basement walls are 1 1/2” foil-faced eps glued to the concrete, set on the floor’s dimple mat but before the 1/2” floor foam, the two foam planes can-foamed then taped. A 2x4 wall with unfaced batts comes last.

    The “basement” smell is totally gone, the room feels like normal living space, and I’m confident the wet funk is very well separated from the wood materials.

    (The walls were previously framed against the concrete basement wall, batt insulation, then covered in sheet plastic under drywall. It didn’t end well...)

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