Replies

1. GBA Editor
   Martin Holladay|Dec 28, 2012 04:09pm|#1

   Philip,
   First of all, a basement slab should always have a layer of polyethylene underneath it, whether or not there is also rigid foam. I hope your slab has a poly vapor barrier underneath it.

   If for some reason your contractor forgot to install the poly vapor barrier, that's unfortunate. However, the rigid foam will act as a capillary break and a vapor barrier, even without the polyethylene, as long as you don't have a high water table.

   Before you install the flooring, tape a square of clear polyethylene on the bare concrete floor and leave it there for 24 hours. Then inspect the poly to see if there are any water drops under the poly. If there are, that's a bad sign, and you should consult the manufacturer of the flooring before proceeding.

   In any case, be sure to follow the directions provided by the manufacturer of the flooring product you want to install. If your slab passes the test mentioned above, you'll probably be fine.

2. 
   Philip Thompson|Dec 28, 2012 04:24pm|#2

   Yes, there is a layer of polyethylene underneath the slab and the rigid foam was set on the polyethylene. I was concerned about trapping moisture. I have rigid foam and waterproofing on my outside basement walls, and I was told to use special insulation on the inside so the walls could breath.. I thought the same reasoning might apply to the floors. I will perform the test you suggested.

3. 
   David Meiland|Dec 29, 2012 10:41am|#3

   I would most likely install a VB under the floor regardless of what's under the slab. The concrete itself has moisture to give off. What do the flooring instructions call for?