My daughter and future son in-law recently bought their first house together. They had the house inspected, and the inspector wanted some dirt moved away from the support posts in the crawl space to prevent rotting of the posts. (The house has no basement but is all crawl space under the house, with 4-mm Visqueen covering the dirt.) While we were moving the dirt, I noticed that the ground was very damp under the Visqueen; the house is in Seattle.

My question is: there are many crawl space ventilation grills going to the outside, but they are basically covered by the insulation between the floor joists. Would it be a good idea to pull the insulation away from the ventilation grills to achieve better air movement, or does enough air pass through the insulation to ventilate the crawl space? If we pull the insulation back, will the added air movement make the floor colder?

Your questions are both interesting and worthwhile. Proper preparation and maintenance of a crawl space are critical to the long-term success of the home. If crawl spaces are not maintained, they can cause poor indoor air quality and various moisture-related problems in the home such as mold, wood decay, pests, and bacteria.

Crawl spaces could add a considerable amount of moisture to a home. A vapor barrier of 4- or 6-mm polyethylene is necessary, and it should be laid with a minimum number of joints (at least make sure the joints overlap by a minimum of 24 inches). I would also check the insulation to determine if it has an asphalt-coated Kraft-paper or aluminum vapor retarder. The insulation should be installed with the vapor retarder facing the living space. This will also retard moisture infiltration into the home.

During summer months, the outside air will be warmer than the air in the crawl space, causing humidity to rise in the crawl space. Warm air, of course, holds moisture better than cool air. In drier climates this is not an issue, but in coastal regions and in northern climates, moisture may reach its dew point. The vapor barrier will prevent moisture from migrating out of the dirt floor. Proper ventilation will dilute the saturated air in the crawl space, keeping the relative humidity under control. Warmer air in the crawl space is the key.

So the insulation does need to be pulled back from the vents. Depending on the home's design, vents may be relocated below the insulation so they can be opened in the summer and closed in the winter from the outside. Operable louver vents would be easy to close in the fall and open in the spring. Alternatively, you could wedge a piece of cardboard or plywood between the joists at an approximate 45-degree angle down and away from the vent. This will allow air movement, preserve most of the insulation already, and save your future son-in-law the trouble of crawling around the crawl space twice a year.

Ventilating in colder months is OK, but the living space floors above the crawl space may be cold and uncomfortable and may waste energy, even if they are insulated. Your daughter and future son-in-law will want to make sure their Seattle-area home has a minimum R-19 level of insulation in the floor.

As you can see, proper preparation of the crawl space is pretty important to the home's long-term comfort and success. My guess is that you'll be elected to head up this project. Good luck!

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C.R.S., Inc. · Spokane, Washington · USA