Knowing how to install a vapor barrier, before the concrete crew shows up to pour the concrete, could save you from having problems with moisture vapor transmission later on.
The first thing to consider before installing the vapor barrier is the condition of the sub-grade. The sub-grade should be leveled and compacted without using any large sharp rocks that could tear the vapor barrier. Having a smooth, level sub-grade will also help speed up the installation of the vapor barrier.
Another thing to consider is using fibermesh for reinforcement in the concrete instead of wire mesh. Wire mesh will easily damage the vapor barrier with its sharp edges.
Tips for concrete floor vapor barrier installation:
1. Unroll 1st piece of vapor barrier along one edge of floor.
2. Using a utility knife, cut the length long enough to wrap edges up the wall 3 - 4 inches.
3. Wrap vapor barrier up edge of wall 3-4 inches and tape to wall. (may be higher, check job specifications or building codes)
4. Unroll 2nd piece overlapping 1st piece by 6 inches and tape seam.
5. Continue to unroll overlapping seams 6 inches, taping edges up the wall, until entire floor area is covered.
Note: The areas to be taped should be dirt and dust free to allow for maximum adhesion of the vapor barrier tape.
How to deal with protrusions in the floor.
Cut a slit around pipes, drains, conduit, rebar, and other penetrations sticking up through the floor to install the initial layer of vapor barrier.
Cut any excess vapor barrier away and seal the slit with some tape. To seal around pipes, cut a piece of vapor barrier in a rectangular shape, long enough and wide enough to wrap completely around the bottom of the pipe. Cut slits on one side of the rectangular piece to be able to wrap it around and partially up the pipe. Completely tape to pipe and vapor barrier.
If the vapor barrier gets damaged before the concrete pour it must be repaired. Cut another piece of vapor barrier material large enough to cover the damaged area by a minimum of 6 inches on all sides. Tape all the seams to adhere the repair piece to the existing vapor barrier.
Those are the basic steps on how to install a vapor barrier under a concrete floor or slab. Installing the vapor barrier is really quite easy as long as you take your time and follow the guide. Installing concrete vapor barriers isn't something you want to rush, you won't be able to go back and fix it after the floor is done if you develop moisture problems.