(09-11-2011, 06:50 PM)Ernesto Wrote: I live in the desert and seen slabs so dry without a vapor retarder underneath it virtually sucks the moisture out of patch right before your eyes. I've seen them so wet you can smell it like a wet dungeon when you walk in.
You just need to know your concrete and know how to check it.
You can have a bone dry slab with no vapor retarder under it. BUT, you cannot place a moisture sensitive floor on that slab unless there is a proper functioning vapor retarder under the slab.
The concrete may have lived a life of extreme drying and could be rather dry on the surface even if there is high humidity in the soils below. But if you place a solid rubber floor on that slab the moisture will build up and there could be a catastrophic failure. This is what we see all the time with CaCl tests and bewildered installers wondering how a low MVER slab suddenly became very wet. They start blaming 'surface condensation at the time of installation' and all kinds of paranormal events when actually the problem was always right there.
Concrete Answers for Flooring Problems