Q: I would appreciate your opinion on whether I should hire a company to line the crawl space of my house. It’s about 2,400 square feet, and I suspect the work would be pretty pricey. Is it worth it?

Although we installed French drains around the house and sump pumps in our crawl space when we remodeled about four years ago, we still get some dampness under the house. I don’t know if it’s because we are at the bottom of a hill or because of the low water table, but I worry about it, especially when I go on a trip and the house has been closed up for a while. I can smell moisture in the air when I return.

Do you think I could handle this as a do-it-yourself job? I would need to know what materials to buy and if it is really necessary to line the foundation walls. How do you tack down the liner to the ground? Do you really need to level the dirt?

A: Dampness under the house is not necessarily a bad thing, but it must be controlled. Controlled, not eliminated, is the operative word.

Our boyhood home in San Leandro, Calif., was at the bottom of a hill just above Interstate 580. When Dad built the house, he was aware of the possible effect of water on the foundation. So he installed a French drain directing groundwater from the uphill side of the house through drainpipes to the street below. We remember water gushing out of those drainpipes and the crawl space being bone dry.

Unfortunately, after many years, most of the water was wrung out of the clay soil; the ground began to shrink; and the foundation sagged. That drainage system was too much of a good thing.

In our opinion, taking one more step in an effort to control the moisture in your crawl space is the right thing to do.

A vapor barrier blocking evaporation of ground moisture will lower the relative humidity in the crawl space and hopefully the mustiness you complain of.

Research shows that humidity levels in crawl spaces more or less follow the relative humidity of the outside air. Most soils contain a significant amount of clay. Clay acts as a desiccant.

It readily absorbs moisture, but gives it up grudgingly. At different times a crawl-space vapor barrier does double duty, preventing the soil from absorbing moisture from the air and blocking the release of ground moisture into the crawl space. For a more detailed discussion about crawl space moisture and vapor barriers, see www.smartvent.net/docs/crawlspacestudy.pdf and www.fpl.fs.fed.us/documnts/techline/crawl-space-ventilation.pdf.

We suggest you install the vapor barrier and that you do it yourself. It’s not complicated and requires only a utility knife, polyethylene sheeting, rigid insulation, duct tape, construction adhesive and a willingness to get dirty.

Crawl-space vapor barriers are constructed from 6-mil or thicker polyethylene sheeting. The material is available in rolls at lumberyards and home centers. Installation is pretty straightforward. First, remove all debris, wood and otherwise, from the crawl space. The ground doesn’t have to be level, but it should be free from rocks or clumps of soil that might puncture the plastic.

Your vapor barrier will probably be made up of several sections of plastic sheeting. Tape each seam together with duct tape. The goal is to construct a monolithic sheet that prevents water vapor from escaping into the crawl space. To hold the sheet in place, we suggest you use sand. This prevents the penetration of the sheeting. If you do decide to use some kind of mechanical fastener (landscape staples, for example) reinforce the area with a piece of duct tape, run the staple through the plastic then cover the staple head with another piece of duct tape. Either way, the goal is to have an airtight seal free from any penetrations.

Running the vapor barrier up the foundation walls is recommended. But in locations where termites are a problem, the height to which the vapor barrier can be run will be limited. If the vapor barrier is run up the crawl-space wall to the level of the floor joists, termites could build mud tunnels on the foundation wall that would be hidden from view.

If you decide to treat the foundation walls, we suggest that you use rigid insulation. Run the polyethylene sheet about 6 inches up the foundation walls and apply the rigid insulation over the sheeting and glue the insulation to the concrete using a suitable construction adhesive.

This should help control the humidity in the crawl space. As a final measure, try to ventilate the house when you are away. Leave a window open, or if you are not comfortable with that, turn on the bathroom fan to get at least a little air movement inside the house.


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