How to Install a Vapor Barrier Under a Concrete Floor

How to Install a Vapor Barrier Under a Concrete Floor. A vapor barrier is a plastic sheet that covers a construction site's pad to help keep moisture away from the bottom side of the concrete foundation. Moisture can weep through the slab and cause moisture damage in the home if the vapor barrier is installed correctly. The vapor barrier is put in place after installation of concrete footings and after utility rough-ins have been completed.

Things Needed

  • Bobcat skid loader
  • 6mm black plastic sheet
  • Duct tape
  • Gravel screenings
  • Compactor

Step 1

Scrape the pad level after the concrete footings have been poured. Be careful not to damage rebar coming out of the footings. A Bobcat skid loader works best for this step.

Step 2

Scrape the pad clean again after utilities have been stubbed out. Stub-outs are where the utilities come up through the concrete slab; examples are a toilet drain or water lines for a washer and dryer.

Step 3

Place a layer of 6mm black plastic over the pad. Tape all the seams to create a continuous vapor barrier. Duct tape works great for this. Hold the plastic down temporarily to keep the wind from blowing it away.

Step 4

Cut openings for the utility stub-outs to penetrate the vapor barrier. Tape neatly around the utilities, using duct tape to seal the vapor barrier.

Step 5

Spread a layer of gravel screenings over the plastic. This will not only hold the plastic down but also provided a gravel pad under the concrete slab. Use a compactor to make the screenings 6 inches thick.


  • Refer to the architectural plans for utility locations.


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