Jim Rooney On The Level

Q.I need help with my crawl space. We live in a house over a crawl space that was built around 1985. The insulation in the crawl space under the house is in terrible shape. Most of it has fallen to the ground or is in the process of doing so. The moisture barrier on the ground is torn or in disarray, leaving portions of the ground exposed. There are a couple of wet corners. The water source just seems to be from improper drainage of the landscaping. There are three standard sized vents and the access to the crawl space but there doesn’t seem to be any air circulation under there. What is the theory behind a crawl space? Lately I’ve seen TV commercials for the complete encapsulation of the crawl space, saying that the idea of a crawl space doesn’t work. Should I seal up the vents and access? Do I use a window or a screen on the access? What are the steps I need to take to put this space back in tip-top shape?

A. Sometimes I think the only theory behind crawl spaces is to make me miserable. I can’t stand them and unfortunately find myself going in them a lot more than I’d like. They exist for a reason. Crawl spaces and basement depths follow frost lines. In order to build a house upon a foundation you have to put the very base of the structure on solid ground that will not move.

If you don’t excavate down to a depth below the expected winter frost depth then the foundation will be in danger of heaving out of position in response to a deep freeze. Around here that depth is institutionalized into the building code to a depth of just under three feet below the top of the dirt next to the foundation. The last time I saw a winter that cold here was in the mid to late 1970s.

Builders who didn’t want to excavate an entire basement under a house for whatever reason, from site conditions such as water table depth to cost control, dug a trench to frost depth then ran walls or piers up to just above grade and then built a house on top of that. The space left underneath became what we know as the crawl space. Sometime in the 1920s or 30s when building techniques became codified by government agencies, requirements for venting were added and in more recent times additional requirements were added to include vapor retarders-- all in response to the problems that crawl spaces present relative to water. After all, the definition of a sump is a receptacle for water at the lowest level of a building and crawl spaces are by their very construction a great big sump under a house.

Now, we’ve all seen crawl spaces as dry as deserts and that’s a perfect world in which some lucky houses find themselves. Most others-- like yours-- have problems and those problems all have to do with water in its various forms.

If you are the unfortunate owner of a house with a problematic crawl space there are a few simple things to keep in mind. Simple to think about but not always easy to accomplish. The rule you want to live by is to keep water out of the crawl space by any way that you can. You mention the wet corners in your crawl space and you know that it’s a grading issue. Get after that grading right away. Keep gutters clean, too. I wrote about that last week but it bears repeating.

The reason insulation is falling from the spaces between floor joists in your crawl space is because water that has entered your crawl space has evaporated, traveled upward-- as vapor pressure-- penetrated the insulation and found a temperature plane in the insulation below the dew point of that water vapor, condensed, made the insulation wet, making it heavy, causing it to fall of its own weight. And it doesn’t stop there. Moisture vapor continues up through building materials into the house and can cause wood floors to warp, mildew to form and I’ve even seen moisture and mildew problems in attics caused by a wet crawl space. Most builders and home owners don’t want to think about crawl spaces. Builders don’t make money in them and home owners naturally don’t want to go somewhere where they have to literally crawl in the dirt so they get ignored.

What you have seen advertised are the efforts of a nationwide franchise who realizes that crawl spaces are a problem and need to be dealt with in ways previously unheard of. What looks like a local ad can be seen in every TV market in the land where wet crawl spaces abound. I’ll bet you don’t see those TV ads in the southwest but I’ve seesn them in Oregon.

Modern building science attitude torward crawl spaces is quite simply to get them dry and seal them up against outside, unconditioned air and to introduce a small amount of conditioned air-- heated or cooled-- to them. Access must still be maintained for inspection and ability to service any other systems that pass through the crawl such as a plumbing, electric wiring, HVAC etc. Retrofitting an encapsulation system into an existing crawl space isn’t cheap. I wish it were part fo the building code to do it at initial construction.

What I recommend you do is aggressively control water from getting into your crawl space. Place a plastic vapor retarder over the earth floor of your crawl space and run it up the perimeter foundation walls about a foot to a foot and a half. Lap the sheets of vapor retarder by eighteen inches and tape the joints. Use black plastic so any sunlight that filters into the crawl doesn’t foster growth under the plastic. And think dry.

Only open the vents when the outside air temperature and humidity almost equals the air inside of your house. Close the vents when it’s hot and humid like it is now and the same when it cold and freezing.

Keep the mail coming. If you've got a question, tip, or comment let me know. Write "On The Level," c/o The Capital, P.O. Box 3407, Annapolis, MD 21403 or e-mail me at jimrooney@jimrooneyonthelevel.com or inspektor@aol.com.