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Partially finished basement question

jay443 | Posted in General Questions on

I have received advice here in the past and read through all of the articles for finishing a basement. I live in Minneapolis and I had planned on using a 6 mil poly, 1” EPS, and 3/4 t&g plywood for the floor before adding the finished floor. I got to plan it, I was thinking about the implications of finishing this area as it is only a partial finish of the basement. I am adding a room to a finished space that has carpet on concrete. I do run the dehumidifier all summer to keep it <50% RH. If I finish the room in the way that I described, I would have to create a transition strip to go from the new floor to the old carpet. That’s not a big deal. But I was wondering if it was even worth it to insulate the floor in this way. It’s only a 70sq foot room. Should I just leave the concrete, throw down some rugs for now, and then add the poly/insulation in 10 years when I tear out the rest of the carpet? Thanks.

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  1. user-2310254 | | #1

    Do you have an insulation on the basement walls? If not, that would be a better place to start in my opinion.

  2. jay443 | | #2

    Hi Steve. I plan to finish the basement walls in accordance with GBA philosophies. I am using EPS foam attached to the wall, sealed all around with expanding foam. On the inside of the EPS I will be framing the wall. My unfinished area has two walls next to the concrete block, so I will be doing that on both of them.

  3. GBA Editor
    Martin Holladay | | #3

    If there isn't any horizontal insulation under your slab, you don't want to put down any carpeting above the slab. If you do, you're likely to grow mold between the slab and the carpet -- because the slab will be cold enough during the summer to encourage condensation.

  4. jay443 | | #4

    Hi Martin. Nope--no insulation under the slab. I hadn't planned on putting down carpet--maybe just some rugs. That way I could take them out from time to time and wash them if they get musty.

  5. Expert Member
    Dana Dorsett | | #5

    Simply throwing rugs down on the concrete would indeed be a mold hazard.

    If there were an inch of EPS under the already carpeted section (and the walls were all insulated) you wouldn't need nearly as much dehumidification in summer- even 60% would be sufficient to keep the must-basement smell at bay.

  6. jay443 | | #6

    Ok--so it sounds like really my only options are to:
    *leave it bare concrete with nothing on top, not even rugs
    *go with my original plan of poly, eps, and then plywood

    Otherwise I risk a moldy basement. Does that sound right?

    I guess the biggest reason why I was considering bare concrete was because of what happened to a neighbor down the street. He did the "tape poly on floor" test and found that there wasn't condensation underneath. Then he laid down some engineered flooring. A year later, it had buckled, and when he pulled it up, there was pooled water underneath. Based on my limited knowledge from reading here, I wonder if that was due to low vapor permeability of the engineered floor?


  7. Expert Member
    Dana Dorsett | | #7

    The R-value of the engineered floor and it's air-permeability (more so than it's vapor permeability were contributors, but there may have been other factors too.

    Did he even so much as install a poly vapor barrier between the concrete & flooring?

    With even ~R1 of flooring the temperature of the slab drops, usually to below the summertime interior air dew point, which is why you want at least R4 (and a vapor barrier) between the cool concrete and the subflooring. With 3/4" subfloor and 3/4" hardwood you would have about R1.5-R2 of wood between the bottom of the subfloor and the top finish of the hardwood. To keep the bottom of the subflooring above the summertime dew point requires real R value between the wood and concrete.

    Deep subsoil temps in Minneapolis are about 47F, and the summertime slab temp will end up about 50F or thereabouts when there are insulating layers above it. In a 70F summertime basement with R4 foam under R2 wood foam the temperature at the wood/foam boundary will be about 63F. The dew point of 70F 60% RH air is 55F, so the subfloor won't end up getting soaked if you dehumidify to 60% RH. (But if you thrown an R2 rug on top of the hardwood it can.) If left for indefinitely long periods of time the air at the 63F interface is humid enough to support mold, but the vapor permeance of 3/4" plywood is under 1 perm when dry - it takes a handful of months for the moisture content to rise to mold-supporting levels on the bottom from vapor diffusion alone. Conditioned basement air will be much drier than that in the fall & winter seasons, and any summertime accumulation will dry out over the winter (unless you actively humidify the basement to above 50% RH in winter.)

    Of course if you can spare another 0.5-1", more foam-R provides even better margin at subsoil temps as low as yours.

  8. jay443 | | #8

    Hi Dana. I don't mind giving up more space to more foam. I have about 92'' of space between the floor and joists, with no ductwork in the area. I was planning on installing some finished birch plywood panels as ceiling "tiles". My only concern about stacking up more foam is the interface between the new finished area at nearly 2'' and the old carpeted area. Granted, since it's only meeting at a doorway, the transition wouldn't be too hard to manage, but it will still be something to trip over.

  9. Expert Member
    Dana Dorsett | | #9

    If headroom is not a serious concern up to R10 EPS ( ~2.5") would usually still be financially rational on heating energy savings over the lifecycle of a house in zone 6, beyond which the lifecycle energy savings of additional thickness may have a lifecycle cost greater than the rooftop solar.

    With 2.5" of foam, 0.75" subfloor and 0.75 finish floor you would also be at the minimum recommended 4" riser height for stairways, which is less of a trip hazard than a shallower transition step.

    Scroll down to "THE RISERS AND TREADS":

    Both 2.5" and 3" are standard thicknesses for roofing EPS- you may be able to find used EPS or XPS from reclaimers for the same or lower cost than 1" new-retail.

  10. jay443 | | #10

    Thanks Dana. When I tear out the already-finished carpet in a few years and add poly/EPS to match my new finished space, will it be a problem that the bottom plates for the walls are on the concrete? I know it's not an ideal setup. But I don't really want to have to tear out the walls to place the bottom plate on top of the subfloor.

  11. Jon_R | | #11

    See here for why you don't want poly on top of a concrete slab.

  12. GBA Editor
    Martin Holladay | | #12

    Q. "Will it be a problem that the bottom plates for the walls are on the concrete?"

    A. Probably not, as long as the bottom plates are pressure-treated (as required by code).

  13. jay443 | | #13

    Martin, unfortunately I doubt that the previous owner used treated bottom plates when he finished the rest of the basement. I will be able to inspect their condition when I renovate that space in the coming years. If they look ok, I'll probably just leave them, considering that they've been in place for ~30 years.

    But now I'm confused about that Building Science article linked above that says not to use poly on top of concrete. Which method is correct?

  14. GBA Editor
    Martin Holladay | | #14

    I think that we can all agree that the best place for the polyethylene is directly under the slab.

    I know-- that's not what you're asking about. You're wondering what to do when there is no polyethylene under the slab.

    Joe Lstiburek is worried about moisture and mold in air pockets. As far as I can tell, this is the type of assembly he is worried about, from the bottom up:
    Slab / Polyethylene / Furring strips / Subfloor / Wood flooring

    If there is a continuous layer of rigid foam directly above the poly, I don't think there will be any moisture or mold problems. I'm talking about this type of assembly, from the bottom up:
    Slab / Polyethylene / Rigid foam / Subfloor

    If you are still worried about the polyethylene, you can safely omit it, as long as you have a continuous layer of XPS or EPS above the slab. After all, the rigid foam is a vapor retarder.

  15. Jon_R | | #15

    IMO, the most conservative approach is what Joe suggests, a fluid applied vapor barrier. With a dry floor, taped foam and a high perm final floor covering, no poly should also work (as it does on an insulated basement wall).

  16. jay443 | | #16

    So if a person were to not use carpet or rugs, and didn't mind the comfort/energy loss issue, is there a reason to use foam on the floor? For example, ceramic tile--just put it down on the slab? What about engineered flooring where the manufacturer only requires a liquid vapor barrier and some underlayment--is that enough?

  17. Expert Member
    Dana Dorsett | | #17

    Tile laid on the slab would be fine.

    A cardboard box or throw rug resting on the tile, probably not. Engineered wood, probably not either, since without the insulation the bottom of the wood would be below the summertime ventilation air's dew point.

    If you mechanically dehumidify in summer to to a low enough level it's fine, but if the goal is a low mold-spore count and odor-free basement without thinking about it or running the dehumidifier, the insulation is "worth it".

  18. jay443 | | #18

    Thanks Dana. I know you're right, that the insulation is worth it, and that I just need to get with the program. I just can't help but think of the two headaches I'll create for myself after adding insulation. The first headache being the transition from the new finished space floor to the old finished space. The second headache being in 10 years when I pull out the old carpet, add insulation and subfloor, and then have a code violation issue due to riser height differences on the stairs. That's what makes the whole, "comfort and energy be-damned" path of ceramic tile on the slab so appealing.

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