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Community and Q&A

Walkout basement floor

johnnsp | Posted in Energy Efficiency and Durability on

I have a walkout basement in MN that was never finished. I am finishing it now. I’ve read quite a bit on this site and have questions I’d appreciate help with.

The basement is drain tiled and dry. Three walls are concrete block with 2×2 furring strips nailed to them (installed in 1988 when the house was built). The furring strips couldn’t be removed or built out to the interior, so I concluded that spray foam was my best option. I now have 1-1.5″ of closed cell on the block walls between the furring strips. I also had them spray foam the rim joists and the walkout wall (2×6 construction).

I am trying to figure out what to do with the floor. I am open minded as to the final flooring material – carpet, LVT, and porcelain tile are all options I’ve considered (as are combinations of those). The ceilings are 9.5′ above the concrete slab, so ceiling height isn’t a big concern. However, there is a horizontal green treat 2×2 furring strip that runs along the slab (the rest of the furring strips are standard pine 2×2). Perhaps more significantly, there is a standard service door to exit from the walkout and one other door frame already in place on one interior wall. Putting down foam board with plywood over the top would completely cover the horizontal 2×2 and interfere with the service door and interior door. I could easily re-frame the interior door but the service door frame can’t realistically be changed.

Should I be concerned about the thickness of the floor as it relates to the horizontal furring strip and the doors? How should I accomplish the “step down” to the service door? That is, how should I “trim” the edge of the subfloor for the service door, which swings inward? I could theoretically replace it with a door that swings out, but I can’t see how I’d re-frame the door to install it on top of the new subfloor.

I found a great deal on Amdry LP 3/4″x24″x24″ subfloor panels. They aren’t insulated so we’re talking about an R value of 1.6. But I’m attracted to the 3/4″ thickness and the price (I could do the entire 1,500ft2 basement for about $500) and the idea that I wouldn’t have to drill holes in the slab. With the answers to the previous questions in mind, is there any merit to the Amdry LP product, or am I better off going a different route?

Thank you for your help and advice!

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  1. GBA Editor
    Martin Holladay | | #1

    I'm not quite sure what you mean by "there is a horizontal green treat 2x2 furring strip that runs along the slab (the rest of the furring strips are standard pine 2x2)."

    But I think I understand the gist of your dilemma.

    Most of the heat loss from the slab of a walk-out basement happens from the slab perimeter -- specifically, the side of the slab that includes the exterior door. So when it comes time to insulate the slab of a walk-out basement, the first order of business is to excavate a trench at the edge of the foundation on the walk-out side, and to insert vertical rigid foam to protect the slab edge. The rigid foam insulation should extend at least 2 or 3 feet below grade, and should be at least 2 inches thick -- 4 inches is better. You'll also need to install Z-flashing at the horizontal seam between the top of the rigid foam and the lowest course of siding, and you'll need to protect the above-grade portion of the rigid foam with a durable material (for example, metal flashing or pressure-treated plywood).

  2. johnnsp | | #2

    Thank you for the feedback, Martin. I am stuck with the design I've got and am trying to finish the basement the best I can without a major renovation. Perhaps I should have put this question in the interior design forum because while I want to maximize my efficiency and durability, I want to do that within the confines of the materials in place. That means I have to come up with the best floor system for the slab given the considerations I described above. I can live without ideal efficiency and comfort, but I do want to maximize it within reason (especially comfort) and I certainly want to avoid creating moisture problems. Thank you for your help!

  3. GBA Editor
    Martin Holladay | | #3

    My suggestion for insulating the perimeter of your slab is easier to implement than installing rigid foam above your slab -- precisely for the reasons you brought up (problems with existing doors).

    Adding insulation above the slab, as you propose, is more of a "major renovation" than installing vertical insulation at the exterior of the foundation (especially if you are only digging a narrow trench that is 2 feet deep).

  4. johnnsp | | #4

    I misunderstood - that does make sense. Thank you for clarifying. What should I install over the slab itself in that case?

  5. STEPHEN SHEEHY | | #5

    John: Have you considered just polishing the concrete slab as your finished floor?

  6. johnnsp | | #6

    Stephen, I hadn't. I'm not opposed to that at all -- I like the look. The slab isn't in perfect shape, but overall it looks pretty good for being 28 years old. I'd mostly be concerned about keeping it warm. I'm inclined to put carpet in the bedroom but a large rug could work... I appreciate the suggestion and will consider it. Any idea on a ft2 cost for stain/polish? Should I consider an epoxy or other type of finish if I go this route?

  7. johnnsp | | #7

    I should mention that the slab has ductwork underneath it. It's not horribly cold but it's stil concrete! There are two or three stress cracks in it but that's about it.

  8. GBA Editor
    Martin Holladay | | #8

    There are lots of issues here. While insulating the perimeter of the slab is a good idea, it won't necessarily make the slab warm enough for carpeting. (When you install carpeting on a cool slab, you run the risk of moisture accumulation and mold between the carpeting and the slab.)

    It's a really bad idea to run ducts under a slab, because of moisture accumulation and mold. I strongly advise you to relocate those ducts if you possibly can.

  9. Expert Member

    You probably don't want to be seen as constantly blowing your own horn, but this is another discussion where your answer might be more persuasive if you linked to either your blog or the one here on GBA.

  10. johnnsp | | #10

    I'm not excited about possibly changing the ductwork. It's not a great setup, I know, but it seems to work well and if it's ever had water in it I can't tell (the PVC is clean/odorless/white/dry and I put a camera in accessible portions of the ductwork to check). The house is built on a significant slope with sandy soil and a pond at least 20 feet below the basement elevation. Everything around the house appears to drain very well.

    Ideally, I'd like to put drywall up over the 2x2 furring and 2x6 framing (now all spray foamed), put some kind of flooring down on the slab, and start using the space. I get that it won't be perfect. I'm just hopeful that it can be good.

    Anyway, PVC ductwork under the slab is what I've got. I have two furnaces on a common return. The basement furnace is configured as a downflow and sits directly on the concrete. There is a/c upstairs but not down. Is there a reasonable strategy I should consider for modifying the HVAC?

  11. GBA Editor
    Martin Holladay | | #11

    Q. "Is there a reasonable strategy I should consider for modifying the HVAC?"

    A. That's a big question, and it's one that is impossible to answer without more information. Whether or not you want to change aspects of your HVAC system depends on your goals and budget.

    The use of PVC for forced-air ductwork would make many people nervous, because of potential off-gassing of the PVC.

  12. johnnsp | | #12

    I'm comfortable with the off-gassing risk since it's been in place for 29 years and is limited to the basement HVAC supply.

    I'd love a recommendation for a flooring system! Polished concrete is interesting to me but my wife isn't excited about the look.

  13. user-2310254 | | #13

    Polished concrete is not inexpensive (when done correctly). If you are considering that approach, verify that your concrete is hard enough to deliver the type of final finish you want. The installer should have the tools to perform this test.

    If you ultimately decide to install flooring over the existing concrete, be sure to check the moisture level in the slab ahead of time. You can use a Tramex (or similar) moisture meter for an instant reading or tape small squares of poly to a few locations. You should also check your slab for level. Any out-of-spec high or low points will need to be addressed.

  14. johnnsp | | #14

    Let me try asking differently...

    I have an occasionally damp but never wet concrete slab in my basement. Sill plates have been installed directly on the slab around the entire perimeter.

    It sounds like plywood over foam board would be a great way to ensure a dry, comfortable floor. But shouldn't I be concerned about the fact that this system would be taller than the sill plates? Also, how could I address or accommodate the in-swing exterior door that would not clear the tall floor?

    Is there a significantly thinner product or system out there that would still protect against moisture/condensation and give me a slightly more comfortable floor? I'd like to put carpet down in some areas, and tile in others. I'd consider rugs over tile. Polished concrete is not an option. The in-swing door is in an area where I'd be happy with tile.

    I am on a budget but do have the flexibilty to get it done right within reason. Durability/moisture is my primary concern, followed by comfort, followed by efficiency. I'd like to do the work myself if possible and truly appreciate your input. Thank you!

  15. GBA Editor
    Martin Holladay | | #15

    I'm not sure what you mean by "sill plates." Are these the bottom plates for 2x4 or 2x6 walls? If so, I'm envisioning 4 walls -- 3 of which are adjacent to existing concrete walls, and one of which is the wall on the walk-out side. Is that right?

    If so, I assume that at least one of these walls is in place (the wall on the walk-out side). On the other three sides -- the sides with concrete walls -- have these 2x4 or 2x6 walls already been built? Or have you just installed the plates?

    If you install a continuous horizontal layer of rigid foam above your concrete slab, followed by plywood or OSB, you don't have to worry about the bottom plates of your walls -- as long as these plates are pressure-treated (as required by code). If you installed non-pressure-treated bottom plates, without any capillary break between the bottom plates and the (occasionally damp) concrete, that's not ideal. Pressure-treated would have been better.

    Concerning what to do about an in-swinging door, your options are limited. You can either (a) leave a 3'x3' area in front of the door without any rigid foam (leaving the exposed concrete in that area), or (b) temporarily remove the door and re-frame the rough opening, adjusting the header as needed.

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