Best thin basement subfloor?

Discussion in 'Remodel Forum & Blog' started by Jotun, Jan 8, 2020.

  1. Jotun

    Jotun New Member

    Joined:
    Jan 8, 2020
    Location:
    Hampstead, MD
    Hi everyone, I live in Maryland, and am wondering what my best option for insulating the basement floor is. The basement slab had carpet padding directly on the slab, with berber carpet on top totalling 3/4". In one area I will keep it carpet (new carpet when this is all done), but most of the basement will be laminate wood flooring and I've removed most of the carpet. The home is about 25 years old, and has no current signs of visible water/moisture issues that I've seen.

    Unfortunately I only have about 1.25" of additional clearance to work with because of the inswing walkout doors. This is on top of the current carpet area (and tile near the back door) so I have about 2" total for insulation and material everywhere else.

    I'd like to put some kind of insulation down but I just don't have the height for something like 2" foam and plywood without having to raise the doors. I have to make some calls but I think I can get EPS foam locally for a decent price. Would something like 1/2" or 3/4" EPS even be worth it, and what would be the best plywood/OSB option if so?

    What are my best options? Sorry for yet another basement thread, but the information is a bit overwhelming and it seems everyone has a slightly different situation. I know Dricore is out there or something like DMX with OSB, but I'd like to get all my options. Thanks!
     
  2. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609
    It's worth installing 3/4" EPS (R3-ish) which is enough to keep the subfloor above the summertime outdoor dew points (most of the time), which will keep the subfloor from rotting and minimize mold risk in the carpeting even if you don't do much mechanical dehumidification. With out the foam the dehumidifer is a must.

    Deep subsoil temps in your area are about 55F, outdoor summertime dew points are above that mark 90% of the time.

    [​IMG]

    But with R3 under the subfloor & carpet the subfloor will be pretty near room temperature, and won't be taking up moisture from the room air, even if it has R1-R2 of carpet or other flooring above it. Even half-inch EPS (R2-ish) can be protective of the mold-susceptible materials if the flooring is wood laminate.

    Since the subfloor is fully supported by foam & concrete everywhere, it doesn't need to be 3/4"- it's not a structural subfloor the way it would be if spanning joists. Tack the EPS to the concrete with thin beads of foam-board construction adhesive, then a layer of 6 mil polyethylene over the foam. Half-inch CDX or OSB sheathing is usually a sufficient subfloor for flooring fasteners, even for hardwood flooring. Keep a foot of overlap of the subfloor seams relative to the foam board, and Tap-Con the subfloor to the slab 16" o.c. or tighter. Walk around on it a bit to test it, and apply a TapCon any bouncy sections to keep it from moving.

    Leave 3/8" - 1/2" gap of the to any walls to allow for seasonal expansion and lap the poly sheeting up the edge of the subfloor (or up & over, stapling it to the top.)

    If it's known that there is a plastic vapor barrier under the slab (likely, for a house built in the 1990s) it's possible to skip the polyethylene sheet and glue the subfloor to the foam. It still has to be secured to the slab with masonry screws to avoid "potato-chipping" curls and waves with seasonal humidity changes, but it's a bit easier than dealing with polyethylene between the foam & subfloor.

    With half-inch EPS and half-inch CDX/OSB and 3/8" laminate flooring you are still 1/8" over the 1.25" clearance budget at the doors. There are 1/4" and 3/8" fan-fold siding underlayment products that could be used rather than EPS, but it's not rocket science to burn 1/4" off the bottom of most doors. Insulated steel exterior doors could be a problem, but treating those door swing areas with something other than laminate flooring, say, ceramic floor tile on half-inch tile-backer directly on the slab is one possible solution, with some hardwood casing trim to ease the transition to the built-up laminate floor areas.
     
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  4. Jotun

    Jotun New Member

    Joined:
    Jan 8, 2020
    Location:
    Hampstead, MD
    First of all thank you so much for the detailed response. I was hoping you would reply after reading some of your other reponses! I may not have done the best job explaining the flooring situation in detail so here's a bit more info.

    My total budget in height is 2" so it sounds like I can definitely do EPS and OSB. I would just have to raise the tile near the back door (which is only about .75" height from the slab) to match it which isn't a problem.

    So I should be able to go with 3/4" EPS, 1/2" OSB, and laminate flooring for the non carpet areas. Then the same EPS and OSB with carpet for my open area, and I could just add a layer of tile to get it closer to uniform. Am I right in thinking I'd be better of going thicker with the EPS rather than the OSB here?

    I would just rather not go above 2" total or I would either have to "drop down" to the tile area, or deal with cutting/raising the doors.

    I can try to find out about the vapor barrier but I don't know; the previous owner custom built the house so I'd have to either check the slab or get in touch with him to see if he knows. If I can't find out then I can just put it down as a precaution.
     
  5. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609
    Yes, going thicker with the EPS will be more thermal/moisture protective than a thicker subfloor, but with a 2" height budget there are other considerations to think about.

    If the slab isn't very flat it's sometimes useful to to put a double layer of half-inch to avoid the issue of trampoline sections if one of the TapCons pops. A double layer of half-inch glued and nailed together (seams lapped) the subfloor can usually be floated, without having to anchor it to the slab.

    This approach can't work with only a 1.25" thickness budget. But with 2" it can: Half-inch EPS + a double-thickness of half-inch sheathing + 3/8" laminate flooring leaves 1/8" to spare.

    Drilling a half-inch hole through the slab in some unimportant location with a hammerdrill can usually find the vapor barrier, if it exists. Filling the inspection hole back up with polyurethane caulk keeps it air & moisture tight.
     
  6. Jotun

    Jotun New Member

    Joined:
    Jan 8, 2020
    Location:
    Hampstead, MD
    Thanks again, I will check the vapor barrier before I get started. It looks like 1/2" EPS and 3/4" OSB is going to be my plan, because with the carpet I plan on using I won't quite be able to do 1/2" EPS and 1" of subfloor everywhere.

    One other question for now... there is an area of tile about 12' x 8' that I was planning on tiling over. How would this affect my basement by having tile (over tile) in that corner compared to ripping it up and putting 1/2" EPS/subfloor down under first?

    I'd rather not deal with removing it, but would I be defeating the purpose the other floor insulation by leaving that area as is? It is in an open area next to my media room if that matters. Thanks again for your help! Like most other DIY homeowners, I'm learning a lot as I go...
     
  7. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609
    From an energy use point of view leaving ~100 square feet with no insulation will add (a small amount) to the basement's heating load.

    The floor temp will be low enough that mold-susceptible materials (such as cardboard boxes or throw-rugs) may develop mold on the bottoms if resting directly on that section of floor over the summer, unless there is active dehumidification is used to take the dew point of the room air down to below the tile temp, which will probably be between 55-60F in July.

    In a 70F room a relative humidity of 60% correlates to dew point of 55F, which should be enough to mitigate that risk (and it's healthier & more comfortable to hold the line at 60% RH.)

    Neither of those issues are very consequential.

    By no means does leaving a section uninsulated be "...defeating the purpose..." of the insulation over the rest. The insulation will keep both the wood subfloor and the rugs & laminate flooring on top of it warmer and less prone to mold, dust mites, etc., both summer & winter, and there will be a modest reduction of heating energy needs, with higher barefoot comfort in winter (except on the uninsulated tile portion.)
     
  8. Jotun

    Jotun New Member

    Joined:
    Jan 8, 2020
    Location:
    Hampstead, MD
    Thanks Dana, I was hoping it wouldn't be too big of an issue to leave that area of tile alone. I'm disappointed that the prior owner didn't insulate the basement when the rest of the house is so nice, but I think this will all be worth the effort.

    Thanks again for your input!
     
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