Q: When I purchased my home, the home inspector’s report mentioned that the basement crawl space "soil is wet and includes a moisture barrier that should be removed."

The termite inspector’s report said the moisture barrier is necessary because the moisture in the soil could entice termites. What is your experience with moisture barriers? Is it best to leave it in place or should I remove it?

The term "vapor barrier" is a misnomer because it implies that all water vapor is stopped from migrating out of the soil. The term "vapor retarder" is more accurate because the diffusion of water vapor is slowed by the barrier, not stopped.

We’re not surprised that the soil beneath the plastic is damp. That’s supposed to happen. Even if the dirt in the crawl space appears dry, lurking beneath the surface is a large amount of water that’s constantly being pulled to the surface. This water turns into water vapor. Water vapor is attracted to warm air.

The air in the house is almost always warmer than the air in the crawl space. Floor insulation and carpet or wood flooring does little or nothing to stop the water vapor from migrating into the living space. If the vapor content is too high, you risk developing mold or mildew and the odors they cause.

The only effective way to impede the migration of water vapor is to properly install a vapor retarder, coupled with proper ventilation of the crawl space to control the moisture level. Installation of a crawl space vapor retarder is a relatively uncomplicated task.

But, to do it right calls for some attention to detail. The goal is to cover the entire crawl space and limit as much as possible water vapor migration from the soil.

We imagine you’ve got the standard 6-millimeter polyethylene sheeting acting as a vapor retarder. When properly done, all seams overlap a minimum of 12 inches and are sealed with pressure-sensitive tape. The vapor barrier should lap up onto the side walls of the crawl space and attach tightly to the foundation.

If yours doesn’t, a little retrofit is in order. Buy yourself some additional 6-millimeter plastic sheeting and cut it into strips wide enough to extend up the foundation walls and overlap the existing plastic by a couple of feet. Attach the new sheeting to the foundation wall by nailing it up using 3/4-inch pressure-treated furring strips and masonry nails.

Trim any excess plastic that projects beyond the wood strips. Apply a bead of acrylic caulk at the top of the wood strips to complete this part of the job. Next, tape all the seams where the new plastic marries to the old.

Finally, inspect all objects (pipes, columns, piers, etc.) that are sprouting from the soil. These require special attention. The vapor barrier must be carefully cut around these objects and sealed with pressure-sensitive tape. If it’s not, patch it with new material, sealing the seams with pressure-sensitive tape.

As long as you’ve gone this far, you should check that you have enough foundation vents to ventilate what moisture you do get in the crawl space. There should be at least 1 square foot of vent space for each 150 square feet of crawl space. If you’re short, consider installing extra vents.


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