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PVC sheet under foundation overhang or stick with plywood?

CRHawk | Posted in General Questions on

I have a 1978 contemporary in Massachusetts.

There are 2 foot overhangs beyond the edge of the foundation on both back and front.  Both of them have had mold problems, partly from a gutter issue I have fixed and I *think* partly due to air movement from inside to the outside as there was no blocking between the joists.  I will be sealing the joist bays to stop air movement.

I’m leaning towards replacing the plywood bottom plates with PVC sheet as I want to remove as much wood as possible from the outside.  I figure the joist bays can dry upwards through the floor and outwards through the rim joist.  

Is this a reasonable choice?

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  1. Expert Member
    BILL WICHERS | | #1

    PVC here is a good idea in that it is relatively mold proof (but mildew can still grow on it when it gets dirty). PVC is waterproof. PVC is a vapor barrier though, so you're correct that you need to make sure your assembly can dry to the interior. My guess is that there probably isn't a secondary vapor barrier in a floor, so you're probably OK.

    Note that PVC sheet is "floppier" than similarly dimensioned plywood, so you might need to add a bit of support structure to keep it flat. Whether or not this is necassary depends on how far apart the supports are that the PVC will be attached to, so just keep it in mind when you're assembling things and add supports if the PVC sheet sags more than you'd like. I would probably just add 2x2 or 2x3 pieces to use as nailers to screw into through the PVC sheet if I had to add additional support.


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