Soil/Concrete Barrier

[youngins]

I have read several references to using some sort of plastic or "poly" as a barrier between soil and concrete.  

If I was going to Home Depot and asking the Customer Service rep for this stuff - what, exactly, would I ask for?

Thanks

Chris

[John Raabe]

The most common product for a soil moisture barrier is a 6 mil black poly vapor barrier.

Probably the toughest and best for a long life is cross laminated Tu-Tuff.

[John_C]

Any 4 to 6 mil plastic would suffice. The primary purpose of the plastic is to keep the soil/sand/fill from drawing moisture out of the concrete, causing it to cure poorly.  We used a liquid plastic to spread over the top of a pour during the hot months in S. FL.  It would form a membrane that would keep the moisture from evaporating out of the slab.  Two or three weeks exposure to the sun and it would begin to peel off by itself.

[youngins]

If I am using Quick-Tube forms for piers, is it still beneficial to use poly around them?

[John Raabe]

In the ground? No - not on a pier foundation. Use the tube forms as they come.

I was assuming you were talking of a VB over the soil of the crawlspace. There it is valuable to reduce the moisture that the underfloor sees. The cross laminated poly is tougher if you will use the space for storage and getting around on to work on plumbing or wiring. The 4-6 mil standard poly is fine if you will put it under a slab. PS - one of the nice things for a usable crawlspace is a 2" thick concrete "dust slab" over the poly. Really cleans the place up.

[glenn kangiser]

I read - maybe in Oehler's or Kerns writings that humidity under the vapor barrier is nearly always 100%  - Seems most of the time there is enough moisture moving up through the soil that it makes mud or rains under the plastic.  

[John Raabe]

I have never seen poly in the crawlspace attract additional moisture. If you see a slurry or mud develop under poly that is an indication of serious water issues not a byproduct of the poly.

You can see condensation on the underside of the poly but this is the moisture in the soil going back and forth between vapor and liquid. Generally the soil drys out under the house as the water is diverted away on the perimeter.

Now, if you are building on top of a high water table or moving ground water that is being fed from an outside source then you can expect to have problems. A poly vapor barrier can mask the full effect of those problems but not forever... that type of problem needs to deal with the source water with gutters, downspouts, dry wells, French drains, under slab drainage, etc.

Houses should never be asked to be a watertight boat. Ultimately they will fail.

[Amanda_931]

John said:

Houses should never be asked to be a watertight boat. Ultimately they will fail.

Nice thought.