No Plastic Under Concrete Slab

A concrete vapor barrier is simply a sheet of polyethylene plastic visqueen placed directly on top of the sub grade before the concrete floor or slab is poured to help keep moisture from the soil from passing up through the concrete.

No plastic under concrete slab.

Recently though research has shown that the old traditional layer of 6 mil visqueen polyethylene plastic under the slab is seldom effective for two main reasons. So why not just use plastic and not stones. Or just use the plastic sheet to do both be the vapor barrier and the capillary break. Although it may seem water tight this grade of material allows a lot of water vapor to pass through.

The structural why fil. One laboratory drying study indicated that 4 in. Why do concrete slabs need black plastic membranes under them. What is a vapour barrier under a concrete slab and why do you need one.

If the plastic is right under the concrete the bleed water is forced to rise to the surface. Almost everywhere except california that slabs are placed on the ground a plastic sheet a vapor barrier is located directly under the concrete slab and on top of the ground photograph 1. So it is common to use crushed stones with no fines as a capillary break and then add a plastic sheet over the top of the stones in contact with the concrete as a vapor barrier. It then reacts with the cement and soaks back into the concrete within an hour or two helping to complete the curing process.

One contractor says to put plastic under the concrete it prevents moisture from coming up into the concrete. In fact most professional shed slab builders will install a polyethylene membrane under a shed concrete slab even if it is not specified. This water can take literally years to dry out through the top of the concrete slab. Thick concrete samples having a w c 0 50 and sealed from below required 82 days for the vapor emission rate to reach 3 0.

This plastic sheet is in direct contact with the concrete as it should be in a rational world.