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Poly under insultation, under vapour barrier?

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  • Poly under insultation, under vapour barrier?

    Is this a bad idea (In Canada)?

    I know the vapour barrier goes on the inside wall, just under the drywall; but is it okay for the insulation to be right up against the concrete? It was suggested to me that I might want to put a barrier between the concrete and the insulation, so I started to put down some polytarp over the 'crete, but then I started thinking; is this really necessary? Especially since I'll be putting polytarp on the other side of the insulation as well -- as a vapour barrier.


  • #2
    Re: Poly under insultation, under vapour barrier?

    Is this in a basement? Or is it on an outside wall? I would assume that would make the difference...

    Take into account that concrete soaks up moisture and holds it a lot longer than wood, for example. I would want something between the insulation and concrete, even if it was Typar.........Then again, all depends on the moisture conditions and where it's situated.
    Last edited by tinmack; 12-18-2008, 10:55 PM. Reason: clarity
    I'm on "The List" and I love it!!


    • #3
      Re: Poly under insultation, under vapour barrier?

      I'll assume concrete walls mean you're talking BASEMENT.
      I'm in central Wisconsin, which may be similar to your climate.
      The inspectors here don't want any vapor barrier because concrete is very porous, and as tinmack said, you're going to get a lot of moisture passing from the earth outside through the concrete. If you have a vapor barrier under your drywall, the water passing through the concrete gets trapped in the insulation and quickly rots it out, causing mold, etc.
      You are better off coating the inside of the concrete wall with a sealer/waterproofer.
      Drylok is a popular one.
      No matter how much advice you get here, construction practices change with every line of latitude, so call your city on this one.


      • #4
        Re: Poly under insultation, under vapour barrier?

        If this is a below grade basement in Ontario watch out what you do! I have seen people put Tar Paper on the block or concrete wall and plastic on the drywall side with insulation in between. What a mess this made in the spring, there was condensation (or WATER) dripping down the plastic with no place to go but down to the floor...mold everywhere. We had to rip out a 2 foot section of drywall at the bottom to tear out the tar paper (as much as we could) and remove some vapour barrier. DO NOT put anything on the block or concrete...nothing, not even liquid sealer. Anything you out on the block will not allow the block to breath. As you obviously can undertand, the plastic on one side and tar paper on the other will TRAP moisture and it wil not be allowed to get out. I will take it one step further, many contractors are insulating just down to 1 foot off the floor to allow the space to breathe. Also, some are not putting vapour barrier below grade in order to not trap moisture.


        • #5
          Re: Poly under insultation, under vapour barrier?

          Ok, I could give my 2 cent worth also, but Dairylander said it all with this statement,"construction practices change with every line of latitude". And thats the truth.

          I do know though, that I would put a channel around the whole perimeter to catch any water. Its not really about condensation, its about when your wall actually leaks. If you can't get at it, because its covered up, you cannot do any maintenance without a demolition job happening. So if you can remodel a basement to last at least 20 years, that should be your aim, and by god, don't cut corners if you do this, not in a basement....

          I will also space my interior wall away from the block/cement wall, of a space thats 1". This will allow for some air movement. I would also use rigid insulation and not blanket in the new walls.

          No matter what, basements make for a nice living/play area of your home, but they can be trouble after some years down the road. By channeling the perimeter, this will help with rot and maybe keep the mold from growing, down to a minimum.

          Heres a quick search on the channeling, you can do a more thorough research. I guess I did end up with giving you my 2 cents worth.

          Great Link for a Construction Owner/Tradesmen, and just say Garager sent you....

          A good climbing rope will last you 3 to 5 years, a bad climbing rope will last you a life time !!!


          • #6
            Re: Poly under insultation, under vapour barrier?

            I would do the appropriate external waterproofing. Put Tyvek (facing the correct way) against the block and then insulation touching the drywall. I would definitely leave out any plastic (polytarp?) as has been described by others. Do not cut corners on the Tyvek because it breathes. The cheaper brands do not and that is why they are cheaper.