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Vapour barrier under hardwood floor

Vwright | Posted in General Discussion on April 20, 2008 09:19am

I’m reposting this in the general discusion and see if I get any takers. I’m still working on my floor. It’s a small cottage with no foundation, just pier blocks. I’m going to put the vapour barrier down and started thinking about how to  seal the weather out. I’ve got the sub floor out and I’m looking at dirt and rock. I’m thinking of tacking cement board from the bottom of the outside joist down to the vapour barrier and then sealing it with caulking/expanding foam. First, does this sound logical? If it does, then do I have to put a vapour barrier below my hardwood? Thanks for all your help.

Vern.

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  1. fingersandtoes | Apr 20, 2008 08:12pm | #1

    Your subfloor acts as a vapour barrier. The floor assembly should be something along these lines:

    Hardwood floor, craft paper underlay, subfloor (3/4" minimum), joists with your choice of insulation, plywood sheathing on the underside.

    You might also want to run 1" rigid insulation on the underside of the joists before the sheathing.

    To mitigate damage to the plywood sheathing, make sure there is adequate airflow under the building - in other words don't close off the perimeter, which in effect creates a crawlspace, and make sure the ground has no low spots to hold water.

    1. Vwright | Apr 21, 2008 03:09am | #2

       I don't have room for the plywood under the joists. My understanding was that I've got to put a vapour barrier ( plastic ) under the hardwood. If I put the vapour barrier over the insulation, like a wall, do I still need a vapour barrier on the ground? Also, I keep getting told by the guys in town that I don't need the kraft paper. What is it's job?

      1. fingersandtoes | Apr 21, 2008 05:09am | #5

        We both posted replies at the same time...

        You should not install a plastic vapour barrier under the hardwood. The vapour barrier on a wall is designed to stop the movement of  moist air from interior heated space into the stud space where it may condense.

        What you are concerned with in your floor system is the ground moisture rising into your joist spaces. That is why I would suggest you provide a ground seal in the form of a plastic vapour barrier on the rock.

        The kraft paper, or building paper depending on your preference, serves a different purpose. It allows some slippage between the hardwood and subfloor as they move over time. It aids the installation and goes some way to minimizing squeaks.

        Since this discussion seems to have attracted more interest in the Construction forum, lets continue this there.

  2. fingersandtoes | Apr 21, 2008 03:10am | #3

    I see from your more detailed post in the construction folder that the rock is very tight to the underside of your joists. You won't be able to get at the underside to sheath it, and probably getting a well sealed vapour barrier on the rock will be difficult.  

    I'd suggest screwing 2"x2" to the bottom of each side of the joists,  then dropping in 1/4" plywood. You can then foam, or fill the joist spaces with batts. I still don't think sealing the perimeter is a good idea. Air movement under the building, coupled with installing as good a vapour barrier as you are able to on the rock will preserve your wood floor structure.

    1. Vwright | Apr 21, 2008 05:12am | #6

      I was actually going to staple some Tyvek between the  joists like a hammock to catch the insulation. No matter what I do I have to work from above.  From what you say, I'll probably do best if I just put one layer of vapour barrier above the insulation. Should I put paper between the sub floor and the hardwood?

      1. fingersandtoes | Apr 21, 2008 05:28am | #7

        Tyvek will hold the insulation, but without something more substantial like plywood protecting the underside you will end up with a perfect home for mice rats, squirrels wasps etc.

        I guess I wasn't very clear. You don't want ANY vapour barrier (besides the subfloor itself) in your floor system. No plastic at all above the insulation.

  3. pickings | Apr 21, 2008 05:02am | #4

    Have the joist cavities filled w/ a "spray on" closed cell foam insulation. If you can get to from below, do it after the sub floor is in, if not, do it from above before the sub floor, but you need to block in all tyhe cavities with plastic, plywood, or something.

    Not cheap, but gives lots of R value (I think 4/inch) and provides a vapor proof seal also.

    Then sub floor, pink paper (or tar paper) then finish floor.

     

  4. User avater
    user-246028 | Apr 21, 2008 06:46am | #8

    I would be more inclined to vapour barrier the ground and insulate the exterior wall with a vapour barrier over top of the insulation. Basement blanket works good for this application. It comes in four foot wide rolls. If you don't have that much room go with a rigid foam insulation like a 2" thick shiplap and cut to fit or a roxul insulation.

    This will help to keep the floor (and the room warmer) and keep the moisture in the ground ....in the ground.

     

    Dave

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