subfloor in basement which is correct

Discussion in 'Remodel Forum & Blog' started by ctkeebler, Apr 5, 2010.

  1. ctkeebler

    ctkeebler New Member

    Messages:
    49
    Location:
    Massachusetts
    Hello

    I am remodeling/refinishing my basement in Connecticut. I have a question about putting down a subfloor. What would be the recommended or preferred way?

    (1) Put 1 inch XPS foam directly on the concrete floor then 3/4 or 5/8 plywood on top and tapcon both the wood and foam to the concrete floor.

    (2) Put down 6mm vapor barrier and use 1x4 pt sleepers with 3/4 or 5/8 plywood on top and use either 16 or 24 on center spacing for the sleepers. With 24 inch spacing can put 1/2 inch foam between the sleepers but there will be a gap between the foam and the plywood unlike option #1. where the plywood is directly ontop of the foam.

    (3) Put down a plastic dimpled membrane and put the plywood directly on top of the dimpled membrane.

    I ask becasue Im replacing a dricore floor that failed due to water. The water must have wicked through the seams of the dricore.

    Im also adding a sump pump but Ill post those questions in the proper forum. Thanks

    Attached Files:

  2. Dana

    Dana In the trades

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    Poly sheeting is far more vapor retardent than 1" of XPS (~0.05 perms vs. ~2.0perms), but if it's a bulk-water/high water table condition forcing liquid up through cracks the slab neither is going to save you completely. A hybrid of 1 & 2, with poly next to the concrete followed by 1" of XPS will be about as good as can be done. Minor amounts of water that wicks/seeps up the fastener holes can still easily dry through the XPS without creating mold conditions at the subfloor, and the high vapor retardency of the poly keeps the majority of the ground water where it belongs ( in the ground.).

    By putting all of the wood above the XPS (not sleepers on the slab) the wood stays warmer, and is less likely to retain or condense water on it. If you go with sleepers going directly on the slab they should be pressure-treated/ground-contact tolerant, since they'll likely be below the dew point of the interior air much of the year. Cool wood won't be able to dry readily, and will retain moisture.

    Separate sheets of poly should be lapped at least 8-12" and sealed with mastic at the edges, and extended up at least 6" on the foundation wall & mastic-sealed for best results. If you can't get at the foundation edges, use expanding foam to seal it best you can. The foam sealant is still semi vapor permeable in thin layers like that, but it's water proof and won't wick any ground moisture or condensation that might form on the foundation wall.
  3. ctkeebler

    ctkeebler New Member

    Messages:
    49
    Location:
    Massachusetts

    Dane Thanks,

    What are your thoughts on the use of a dimpled plastic membrane like the Delta FL or planton, then either 1/2 inch or 1 inch foam and then plywood on top of that. OR just the dimpled plastic membrance and plywood on top of that?

    Is OSB or Plywood a better option?

    Thanks Ill look forward to your responses

    Im not sure how strong the 1/2 inch foam is. I know the 2inch foam I used on the walls is very strong to stand on.
  4. hj

    hj Moderator & Master Plumber Staff Member

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    OSB swells in the presence of water, so I would NOT use it for a subfloor. You are putting a sheet of plywood over the foam, so your weight will be well distributed over the surface, so it is not as if you were standing directly on the foam.
  5. Dana

    Dana In the trades

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    I'm not sure what the advantage of the dimpled plastic would be in this application- the bottom of the subfloor will be the same temp & humidity with or without the air gap. With foam in there it's HUGE advantage, since it keeps the temp of the wood higher, which keeps it's vapor pressure up- it dries.

    With foam directly below the subfloor there's no structural load to spaeak of- the compressive strength of the foam is adding a LOT of structure. At 3/4" OSB is rated as subfloor under wood flooring even with 18" o.c. joist spacing. You can get away with 7/16" stuff under wood flooring if its a concrete/poly/XPS/OSB sandwich, no air gaps to flex. But if it's going to be under carpet or tile go with at least 5/8". XPS has a very significant compressive strength (~15psi / 10%), and is used under heavy concrete all the time- I wouldn't sweat it a bit under residential flloor dynamic loading with 5/8" of OSB on top.

    As for swelling concerns, thats only an issue if you anticipate it flooding, and in that unhappy eventuality you'd be looking at ripping it up whether it was plywood or OSB. With a class-I vapor retarder on the floor it won't be picking up ground moisture, and with R5 between it and the cool earth both the top & bottom will be pretty much at room temp & humidity. I wouldn't put either on a basement floor in CT with no insulation, vapor retarder only, since it's ~R 0.75 insulation value will keep the bottom side cold enough to retain moisture- under a thick carpet even the top side could be cold enough to grow mold. But with a total R-stackup of R6-8 (depending on finish floor materials), all of the wood will be much closer to room temp than ground temp, and can't stay wet enough to rot, mold or swell. If you keep the basement below 60% relative humidity (the high end of the ASHRAE recommended for health- keep it under 50% if anybody is allergic to dust mites) it'll be fine. In CT this usually means running a dehumidifier/AC and keeping outdoor air from entering during the summer, since outdoor dewpoints tend to be high, and outdoor-air ventilation into a cool basement raises, not lowers the relative humidity. At 70% RH & up the mold hazard skyrockets.
  6. ctkeebler

    ctkeebler New Member

    Messages:
    49
    Location:
    Massachusetts

    Thanks Dana, I guess the thought of the dimpled plastic was for air movement to keep it dry if there was any moisture or seepage. So your recommendation is a 6ml poly vapor barrier, then the XPS foam and 5/8 plywood/osb on top of that all tapcon to the concrete floor?

    One inch minimum on the XPS foam or can I get away with 1/2 inch? Well never mind looks like the ownes corning XPS pink board is 1 inch (R5) or 3/4 of an inch (3.8)

    So I guess it doesnt really matter the size of the foam as the 1/4 wont affect the height that much. Might as well spend the $2.00 more per board now and get the better R value. maybe Ill save money in the long run and the wood is that much higher (away) from the cold concrete floor.


    I'm also looking at the Zoeller M98 sump pump. Probably over kill, but want to make sure there is enough room to space if there is another huge rain storm.
  7. dlarrivee

    dlarrivee New Member

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    1,172
    Location:
    Canada
    I can't lose another inch in my basement, I'm using Delta FL with 5/8" plywood.

    OSB should be avoided.

    1" of XPS would be great if you have lots of headroom already.
  8. ctkeebler

    ctkeebler New Member

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    49
    Location:
    Massachusetts
    What are you trying to accomplish with the Delta? Just getting the floor off the concrete? Are you just going to tapcon the plywood to the concrete through the delta? I dont have alot of room, but the 1inch foam and the 5/8 plywood is only about 5/8 more than the dricore I had down so that would be okay. Im not sure how thick the delta is if I used that as well.
  9. Lightwave

    Lightwave New Member

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    98
    Location:
    Vancouver, BC
    BSC has a few documents on best practices for insulating basement floors for habitation use.

    Start here:
    http://www.buildingscience.com/documents/reports/rr-0308-renovating-your-basment
    http://www.buildingscience.com

    Given that you have enough of a water problem to compromise Dricore, you might need to take very substantial measures to reduce water ingress before thinking about what type of subfloor to put down. You need to keep the water out of the slab first and foremost. Try to find a building envelope consultant (typically an engineer) to identify your water problem first. Putting down another subfloor without dealing with ingress might result in yet another rotted floor and a lot of wasted money.
  10. nukeman

    nukeman Nuclear Engineer

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    Location:
    VA
    In addition, remember that it isn't just ceiling height that is impacted with your choice. Doors may have to be modified. The other typical issue is with the stairs. Although code uses a fairly wide range on what the rise/run of the stairs can be, once you choose a rise, the rise of all of the steps have to be about the same (usually within ~3/8"). People get used to a certain rise on the steps and it can throw them off if you have one that is much higher/lower than the others.

    Just some things to think about when making your choice.
  11. Dana

    Dana In the trades

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    I take issue with the Building Science statement about 1" XPS being a vapor retarder, therefore poly isn't needed. At 1" thickness XPS is a fairly WEAK vapor retarder (Class-III, ~2.0 perms, which is comparable to the vapor retardency of the half-inch plywood or OSB itself!). It takes at least 2" to get it up to Class-II (1.0 perm) performance. Depending on just how far the water table is below the slab and how well it's drained below the concrete, with a permeance of 2.0perms there can still be significant moisure coming up through the slab and into the wood. XPS can and will take on water in a high-saturation environment. Poly sheeting is a cheap & effective Class-I vapor retarder ( ~0.05 perms at 6 mils), which takes the guesswork out of it. While vapor-sealing foundation walls could have negative consequences (rotted foundation sills, efflorescence & spalling on the exterior above grade), there's no such issue with vapor sealing below-grade slabs- indeed, it can even reduce radon levels.

    You're dead-right about controlling bulk-water issues- a few days of flooded floor can destroy it.

    Another issue I'd neglected to mention: For best-practices, the finish floor surface should be at least semi-vapor permeable, (1perm or higher) or you will trap water in the subfloor leading to long-term degradation. Many asphalt tiles and all vinyl flooring has a permeance of less than 1, as do some laminates. Be sure to look up the specs- while a permeance of 1 or higher is fine, even 0.5 can work as long as there is a poly layer on the concrete, but that's about the lower limit.
  12. ctkeebler

    ctkeebler New Member

    Messages:
    49
    Location:
    Massachusetts
    is the 3/4 inch xps foam covered with 5/8 plywood strong enough to support a dryer and front load washer? They are non stackable and are side by side.
  13. dlarrivee

    dlarrivee New Member

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    1,172
    Location:
    Canada
    Delta FL is 5/16" so you end up with just under an inch above the slab with your subfloor. The reason I'm avoiding dri-core is because of the OSB.

    A good 8mil and 2 sheets of 5/8" is another good option and nets you 1 1/4" or so.

    I don't have any moisture issues at all in mine though, so my choice has little to do with vapor and water permeance.

    You NEED to take care of the water issue before moving forward.

    Do you have good drainage away from the house?
  14. dlarrivee

    dlarrivee New Member

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    1,172
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    I'm not sure if you're familiar with XPS practically or not, but with the plywood over it I wouldn't worry one bit.
  15. ctkeebler

    ctkeebler New Member

    Messages:
    49
    Location:
    Massachusetts
    I am working on solving the water issues, sump pump, gutters draining into 4 inch pipes under ground and pipe running 100 feet away from house. Sealing cracks in floor, looking at drains around the sides of the house.

    But also want to do what is the best on the inside of the house. I'm leaning toward the 6ml vapor barrier, maybe put down twice in opposite direction, the 3/4 inch XPS pink rigid foam board them 5/8 plywood over that. Is the 5/8 the thinnest that is recommended?

    Water was never an issue before, but I dont want water issue again and just because this was a 100 year storm, doesn't mean its going to wait another 100 years.
  16. dlarrivee

    dlarrivee New Member

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    You want to lay what down twice? Hopefully not the poly...?
  17. drick

    drick In the Trades

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    392
    What do folks think about painting the floor with a "water proofing" paint first? I know they don't actually keep out water - or very little anyway. However a couple of years ago I painted the walls in my basement and with some type of water proofing paint from a big box store (can't remember the name now) and it made a noticeable difference in the amount of moisture in the air in the basement. My dehumidifier runs about half as mush as before now. Why would I not want to do a similar thing with the cement floor before laying down a 6 mil vapor barrier?

    -rick
  18. ctkeebler

    ctkeebler New Member

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    49
    Location:
    Massachusetts
    Im guessing laying down two layers of poly is not a good idea?
  19. ctkeebler

    ctkeebler New Member

    Messages:
    49
    Location:
    Massachusetts
    Drick

    I thought the same thing and if I remember correctly the answer was that you want the concrete the breath and the paint wont let the concrete breath
  20. ctkeebler

    ctkeebler New Member

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    49
    Location:
    Massachusetts
    any thoughts on painting the floor with the drylock or similar
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