Best thin basement subfloor?

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

  1. Jotun

    Jotun New Member

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    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

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    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

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    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

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    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

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    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

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    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

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    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!
     
  9. Geran

    Geran New Member

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    Interesting, I am trying to figure out the same thing for my basement.

    I have 1.5" of height (using tile in front of basement door, bathroom and laundry room; all others are getting carpet) and I'm trying to decide the best way to insulate the top of the basement floor without using 6mil plastic.
     
  10. Dana

    Dana In the trades

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    What's your issue with 6 mil plastic?
     
  11. Geran

    Geran New Member

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    Not really an issue, just seen too many bad installations (mold growth mainly) of it to the point it makes me nervous to install it correctly.
     
  12. Dana

    Dana In the trades

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    With any finish flooring over a slab there needs to be some sort of vapor barrier to keep ground moisture at bay, and that's even more true if there is a wood subfloor under the finish floor, and a mold susceptible carpet. The air in the soil under the slab is pretty much at 100% relative humidity all the time, even when there isn't liquid water at slab level, and the moisture drive is always from the soil toward the drier air of the conditioned space- the finish flooring never dries toward the slab. Six mil plastic (between polystyrene insulation & subfloor is best practice, between slab & polystyrene insulation is usually OK) is a cheap & reliable vapor barrier. As long as the seasonal average temperature at the bottom of the wood subfloor stays sufficiently above the seasonal average dew point of the conditioned space air, the moisture content of the wood won't reach mold/rot supporting levels.

    The deep subsoil temps in MD are well below the outdoor summertime dew point averages, and the sensible cooling loads of basements are low, so there isn't a whole lot of help from the AC to keep the basement air sufficiently dry. Putting even R2 foam between the subfloor & slab raises the average temp at the foam/subfloor boundary by quite a bit, lowering the average moisture content of the wood.

    I've heard of people using cement board tile backer designed for walls instead of wood subfloor over the foam when the intended finish flooring is tile, but I've never done it or seen it myself. I'm not sure how well that would hold up over time, but I suspect it wouldn't do very well.
     
  13. Geran

    Geran New Member

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    Understood. So my plan of 3/4" foamboard and 3/4" subfloor (with 6mil poly) would be fine then for both carpet and tile then? Or would 1" of foam and 1/2" for subfloor be better?
     
  14. Dana

    Dana In the trades

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    3/4" foam + 3/4" subfloor eats up your entire 1.5" clearance budget, with no room for tile or rug. Don't you need another half inch for the thinset + tile or rug?

    If the 1.5" budget was for just from slab to the top of the subfloor, sure 3/4" foam + 3/4" subfloor (TapConned to the slab, with at least 3/8" clearance around the perimeter to accomodate seasonal dimension changes) works for either rugs or tile.

    Fire code issues get in the way with having only 1/2" subfloor under a rug (though some people do it.) Foam board needs a timed thermal barrier against ignition.

    Even though polysiocyanurate has a higher R value (~R5 @ 3/4") , only polystyrene foam can be used (EPS strongly preferred over XPS on environmental grounds) in this application, since polyiso can wick & retain moisture over time or in the event of minor flooding, losing quite a bit of performance when saturated.
     
  15. Geran

    Geran New Member

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    Yes the 1.5" budget was just from the slab to the top of the subfloor. I figured if the doors become an issue, I can trim the bottom of the doors to make them fit instead of going with 1/2" EPS + a double-thickness of half-inch sheathing.

    I am curious though with the 1/2" EPS + a double-thickness of half-inch sheathing. Would the two layers of half-inch sheathing need to be fastened to the concrete or just the first one and the second would be only fasten to the first one?
     
  16. Dana

    Dana In the trades

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    With a double layer of half inch sheathing screwed to each other with the seams between layers overlapped by a foot or more it can be floated on top of the foam board without TapCons. If after it's fully assembled it feels bouncy anywhere due to imperfections in the levelness of the slab, shoot in a few Tapcons to bring the waves under control before installing tile on it. (With rugs it doesn't matter unless it feels like a trampoline.)

    Almost all half-inch EPS comes with facers (it's too fragile without them), and usually 0.9-1lb density "Type-I" which is fine if there's an inch of plywood on top. It runs about R1.8-R1.9. Box stores near me carry it for about $9/sheet. With plastic or aluminium faced EPS it's fine to just use housewrap tape or HVAC tape over the seams and skip the 6 mil polyethylene. Tread lightly when walking on Type-I EPS before it's covered with plywood. It's pretty easy to leave permanent footprints or heel marks in it, even with the facers. While Type-I EPS can be used as a "walkable" roof insulation under membrane roofs, most roofers prefer to use 1.25lbs density "Type VIII" for better ruggedness in handling.
     
  17. Geran

    Geran New Member

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    Dana,

    Thank you for the explanation. I found one of your older post in a thread of mine that said going thinner than R-3 for the insulation could increases the mold risk to the subfloor.

    With this in mind and wanting to keep the height of the subfloor assembly around 1 3/8", I found some 11/32" plywood that I could use with 1" EPS foamboard which would give me about 5/32" of space. So the plan now would be 1" foamboard, 6mil poly, 11/32" plywood tapconed to the floor with 3/8" gap all around the perimeter and 1/8" gap between each sheet.

    Since the floor isn't structural I figured the plywood is really only serving as a place to install the finished floor that's why I picked 11/32" plywood.
     
  18. Dana

    Dana In the trades

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    And that's true, depending on what your subsoil temps and summertime outdoor dew points really are. R3 is somewhat conservative, and works pretty well even in colder climates with with subsoil temps in the low 40s or high 30s.

    The amount of air conditioning/mechanical dehumidifcation makes a difference too. The dew point of 40% relative humidity 75F air conditioned air is about 49F, which is probably cooler than your slab temp, and way cooler than foam/subfloor interface that matters, so there would not be a moisture accumulation or mold risk even without the insulation. But 60% RH @ 78F room air has a dew point of 63F, and need at least some insulation to keep the bottom of the subfloor from getting moldy in an area with subsoil temps in the 50sF. Local outdoor air dew points in most of MD average in the mid to high 60s during the peak weeks of summer.

    If the house is NOT air conditioned during the dog days of summer and you rely on throwing the windows open at night it might still need R3 under the portion with the rug, since rugs are insulative, lowering the temperature of the subfloor.

    Unless it's something really stiff like Baltic birch it's possible that 11/32" plywood won't be thick enough to completely protect the foam from permanent deformation in high load areas (say, under the leg of your antique slate pool table.) But I have no experience with anything thinner than half-inch.
     
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