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adjacent dissimilar subflooring

daveyg | Posted in General Discussion on May 26, 2009 09:19am

I’m developing a large crawlspace into a living space. Due to the head clearance, and the grade slope and surrounding stepped foundation/stem wall, half of the new floor in one large room will be a poured 4″ concrete slab. This slab will poured up to the surrounding (on three sides) stem walls with 1/2″ min. expansion joints. The other half will be wood framed with ply subfloor, hung from ledgers on the 2×6 wall framing. The two dissimilar subfloors will meet on the centerline of the room. I’m thinking that continuing the ply subfloor from the wood framed portion onto the concrete would help with the “feel” of the floor and avoid telegraphing the seam through the finished, carpeted floor.

Even though the concrete pour would include a vapor barrier below, I assume another layer under the ply wouldn’t hurt. The ply would be fastened with powder act. gun nails to the concrete slab (or other suggested method).

My question may be a matter of preference rather than right or wrong. But I’d like to know what you would do if the added cost were not an issue.

Thanks!
Dave

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  1. designbing | May 27, 2009 01:17am | #1

    If you have enough room pour concrete low enough to install pt sleepers on top of concrete and then run ply over both.  It will feel more comfortable and moisture problem will be minimized.

    Bing

    1. daveyg | May 27, 2009 02:53am | #2

      I have only enough room for the 4" slab before ceiling height becomes less than 8 feet.
      Dave

      1. designbing | May 27, 2009 08:00pm | #3

        I would sacrifice 2" of headroom to get a comfortable floor that can be properly waterproofed and seamless.

        Bing

        1. daveyg | May 27, 2009 08:56pm | #4

          You guys are right. 1-1/2" sleeper,plus 3/4" ply is not much loss of head room compared to the gain in comfort and piece of mind.
          Thanks your help!
          Dave

  2. cliffy | May 27, 2009 09:49pm | #5

    Don't put a second vapour barrier in. One is all you need and does not trap moisture the way two would.

    Have a good day

    CLiffy

    1. daveyg | May 28, 2009 04:24am | #6

      I was wondering how the new concrete would dry when sandwiched between two vapor barriers. I suppose one sheet on top of the pour, under the sleepers would be best. But then it will be full of holes when the sleepers get nailed down. What do you think?Also I was thinking of laying foil faced rigid foam between the sleepers to get a little insulation. Come to think of it...why not forget the sleepers, lay down continuous rigid insulation, cover with ply and fasten the ply through the insulation into the concrete with powder gun nails?Dave

      1. cliffy | May 28, 2009 10:09pm | #7

        There is a basement article about  two years back by Andy Engel where he puts down rigid foam on concrete then plywwood on top of the foam. Quiet and dry and warm feet. I seem to remember in a post or an email that he got the technical advice from Joe L at Building Science Corporation to aid in his design.  The foam is impermeable. I poured concrete over 2 inches of styrofoam a few times and it worked out well. Over the concrete you could use Dricore 2x2 panels if you were worried about putting carpet right over the concrete. 

        Have a good day

        Cliffy

        1. daveyg | May 29, 2009 08:13am | #8

          Thanks for the reference to Building Science! That was the most detailed explanation of concrete slabs I've ever read. It seems we've got it all wrong here in California. Now I only need to convince the building department. After all the reading it seems the best way to go is to put the vapor barrier between the top of the slab and the sleepers/ unfaced rigid insulation and subfloor. That way the slab can cure slowly in contact with the moist soil/gravel fill below, without affecting the materials above.Dave

          1. cliffy | May 30, 2009 04:34am | #9

            This would be a great time to proudly say Joseph L is a Canuck!

            Have a good day

            Cliffy

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