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Thread: subfloor in basement which is correct

  1. #16
    DIY Senior Member dlarrivee's Avatar
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    You want to lay what down twice? Hopefully not the poly...?

  2. #17

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    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

  3. #18
    DIY Member ctkeebler's Avatar
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    Quote Originally Posted by dlarrivee View Post
    You want to lay what down twice? Hopefully not the poly...?
    Im guessing laying down two layers of poly is not a good idea?

  4. #19
    DIY Member ctkeebler's Avatar
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    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

  5. #20
    DIY Member ctkeebler's Avatar
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    any thoughts on painting the floor with the drylock or similar

  6. #21
    DIY Member ctkeebler's Avatar
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    Just to make sure I understand the insulation rigid board I should use on the floor

    Dow 1/2" x 8' x 4' R-3.3 Polyisocyanurate Rigid Foam Insulated Sheathing is not the correct stuff right even though it has moisture resistant facers?

  7. #22
    In the trades Dana's Avatar
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    Painting or Drylocking the floor will seal it from bulk-water pressure when the slab is below the water table. But otherwise a single layer of 6 mil poly with 12" overlaps at sheet transitions is more than enough. Dual layers of poly are pointless- a single layer of poly reduces the permeance from 2.0perms (1" of XPS) down to ~0.05 perms. Foam insulation by itself is an excellent air-barrier, but at thicknesses less than 2" it's a LOUSY vapor retarder against vapor-diffused moisture. Polyethylene sheeting even at 0.006" is an EXCELLENT vapor retarder.

    And not to worry about floor loading issues with washer/dryers, water-heaters etc. on top of XPS. With an OSB layer to distribute the weight it can handle over 1000lbs/square foot. (XPS and EPS is often used as insulating pads directly under electric hot water heaters, after all.)

    By contrast polyisocynurate is downright WIMPY when it comes to pressure loading, and should not be used as slab insulation under a floor. It also can absorb water over time in a no-loading slab (no floor) application.

    Foil or poly faced insulation should never be used as foundation wall insulation either, since interior vapor retarders raise the moisture content of the foundation, increasing the risk of sill rot and above-grade spalling & efflorescence on the exterior. In perma-frost zones it's OK though. (Interior vapor retarders on foundations is one area where the general building codes in Canada may be at odds with best-practices for particular Canadian climate zones. In the US outside of the AK interior it's pretty clear that the foundation insulation needs to be at least somewhat vapor permeable to allow drying toward the interior. In both countries in all climate zones fiberglass & wooden studwalls without an intervening foam layer between studwall & foundation is a recipe for mold. Or maybe that would be "mould" in, Canada, eh? ;-) )

  8. #23
    DIY Member Agu's Avatar
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    I think you're making the vapor barrier too complex. Last basement I finished the concrete was cleaned/etched with muriatic acid and sealed with exterior concrete paint from Sherwin Williams. Moisture was contained and I didn't have to worry about plastic sheeting getting abraded and failing.

    jmo,

  9. #24
    In the trades Dana's Avatar
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    Quote Originally Posted by Agu View Post
    I think you're making the vapor barrier too complex. Last basement I finished the concrete was cleaned/etched with muriatic acid and sealed with exterior concrete paint from Sherwin Williams. Moisture was contained and I didn't have to worry about plastic sheeting getting abraded and failing.

    jmo,
    How far above the water table is your slab?

    How well drained is your foundation?

    What's the permeability rating on the paint at the applied thickness?

    Did you put down a wooden subfloor? If yes, did you put any insulation underneath it? If not, what is the average subsoil temp in your area?

    A lot of factors go into whether you actually need a vapor retarder on a slab, and how vapor retardent it needs to be. And there's a huge difference between making something waterproof vs. vapor retardent, the difference between liquids and gas. Most exterior paints have moderate to high vapor permeability (and its a good thing too, or you'd be trapping moisture in walls creating mold problems). If you think you've vapor-sealed the slab with the paint, read the fine print on the specs, 'cuz I doubt it. Sealers & paints can slow down capillary transfer of liquid moisture in masonry though, and maybe that's all you needed.

    In a basement with a history of bulk water incursions, in a climate zone with high summertime dew points and subsoil temps of ~50F, putting both a class-I vapor retarder and some insulation underneath a wood subfloor is a reasonable precaution, and the installation doesn't have to be perfect to make it work.

    My basement slab is typically less than a foot from the water table, the slab temp (uninsulated) stays in the low 50s year round, and sealing it with a masonry sealer has pretty much halted capillary draw. But a cardboard box left on the floor will develop mold on the bottom if left for months, even in the drier-air winter months. In the humid summer when the room temp is ~65-68F and mechanically dehumidified to 60% RH, a box left on the floor even with a sheet of poly under it will develop mold after 6-10 weeks, an artifact of the temp at the bottom of the box being at or near the dew point of the room air is that it eventually accumulates enough moisture to grow mold. His place in CT isn't that far from my place in MA- temperature conditions at the slab and summertime air humidity are likely to be more similar than different.

  10. #25
    DIY Member ctkeebler's Avatar
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    Well I am finally putting down the new sub floor in my basement here in Connecticut.

    After the advice I got here this is the plan:

    6mm of ploy over the concrete floor. Then 1/2 inch XPS rigid foam board with 3/4 plywood (probably tongue and grove) on top. Then either power actuated nails or tapcons through the plywood and foam into the concrete floor.
    There is a lot of advice on vapor barriers that they should be on the warm side of the insulation. Does this mean I should put the XPS on the concrete and the vapor barrier on top of the XPS. This way the vapor barrier is between the XPS and the plywood?

    Tom

  11. #26
    DIY Member Rughead's Avatar
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    Just listen to Dana.

  12. #27
    DIY Junior Member bobgio's Avatar
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    Dana.......PLEASE HELP ME!

    It seems that you are the only person that actually knows what they are talking about. Here is my question...I want to lay laminate in one room in my basement...my house is 4 years old with no water or moisture issues...I did a moisture test on the concrete and all is well. Also, if it matters at all... all my basement walls were sprayed with 2lb foam approximately 3 1/2 inches...and I live in Niagara Falls, Ontario, Canada. I do not want to put rigid foam on the floor but instead would just like to lay the laminate floor on top of some type of moisture barrier. The manufacturer and everyone else I speak to tell me to just lay 6 mm poly over the slab. The laminate I purchased does have a foam backing but no moisture barrier...the laminate is very thick...13 mm. The manufacturers instructions say that I should put 6mm poly down and run it up the walls a few inches...my question is...is this system going to be adequate?? I assume that with change in weather there will be condensation on my concrete floor at some point...how does moisture under the poly dry??? do I assume it gets reabsorped into the concrete?? is having the poly run up the wall going to create a moisture problem in my walls??? I have read that I should use a high grade poly??? Wouldn't tar paper or typar/tyvek be better if I put it with the print side facing the concrete...that way it will stop moisture from wicking into the laminate but also allow some drying from the room down to the concrete...I AM CONFUSED! Please tell me the best system without using rigid foam....Thanks
    Last edited by bobgio; 10-27-2010 at 07:08 AM.

  13. #28
    In the trades Dana's Avatar
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    The layer of poly will protect the laminate from ground moisture. The concrete doesn't care- it can stay wet forever without damage. The real issue will be how cold the bottom of the laminate becomes. If the ground is below the dew point of the room air, the wood will accumulate moisture, even with the poly in place. 13mm thick laminate will have an R value of only about R0.5 though, so it may not be much of an issue. But if you put down even 5-6mm fan-fold XPS under the laminate your risk is pretty much eliminated. (The fan-fold goods are sold in box-stores as siding underlayment, and unfold into a very long sheet.) If the slab doesn't have poly underneath it or you're no sure, put poly underneath the thin XPS.

    Typar/Tyvek are vapor permeable- they would provide a slipsurface, but have zero moisture contol benefits, and would allow copious amounts of ground-water vapor to reach the cold under-side of the laminate.

    The thick layer of 2lb foam on the walls is more than adequate as a vapor barrier- bonding the poly to that would make a continuous vapor barrier with even the edges of the laminate protected.

  14. #29
    DIY Junior Member bobgio's Avatar
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    Dana,

    Does the XPS need to be secured in anyway? Do the seems need to be taped? Also, can I put the laminate directly on top of the fan-fold XPS??

    Thanks again!

  15. #30
    In the trades Dana's Avatar
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    There's no need to tape the seams or glue it to the concrete in this application, assuming the fastening system for the laminate secures it to the slab in multiple places(?) XPS has pretty good compressive loading characterisics. It would take a sledghammer to permanently deform it through 13mm of laminate.

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