A tutorial on crawl spaces
23 July 2005
A. Iím not so sure that there is anything like a theory behind crawl spaces and I, for one, canít stand them and unfortunately find myself going in them a lot more than Iíd like. Crawl spaces seem to 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 donít dig down to a depth below the expected wintertime 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 bad here was in the 1970s. We had to jackhammer foote rs into the ground.
So builders who didnít want to excavate an entire basement under the house for whatever reasons, from site conditions such as water table depth to cost control, dug to frost depth then ran walls or piers up to just above grade and then built a house on t op 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 requireme nts were added to include vapor retarders-- all in response to the problems that crawl spaces create with reference 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 v ery construction a great big sump under a house.
Now, weíve all seen crawl spaces as dry as deserts and thatís a perfect world in which some lucky houses find themselves. Most others-- like yours-- have problems and the problems all have to do with water in its various forms.
If you are the unfortunate possessor of a house with a problematic crawl space there are a few simple 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 itís a grading issue. Get after that grading right away. Keep gutters clean, too.
The reason the insulation is falling down from the spaces in between the floor joists of your crawl space is because the water that has entered your crawl space has evaporated, traveled upward-- as vapor pressure-- penetrated the insulation and found a t emperature plane in the insulation below the dew point of that water vapor, condensed, made the insulation wet, making it heavy and causing it to fall down of its own weight. And it doesnít stop there. The moisture vapor continues up through building ma terials into the house and can cause wood floors to warp, mildew to form and Iíve even seen moisture problems in attics that were caused by a wet crawl space. Most builders and home owners donít want to think about crawl spaces. Builders donít make mon ey in them and home owners naturally donít want to go somewhere where they have to literally crawl in the dirt so they tend to get ignored.
What you have seen advertised are the efforts of a few who realize that crawl spaces are a problem and need to be dealt with in ways previously unheard of. The new building science thinking about crawl spaces is quite simply to get them dry and seal the m 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 plumbi ng etc. Retrofitting such a system into an existing crawl space isnít cheap. Itís not done so much around here but is a blossoming industry farther south in the Carolinas.
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 she ets of vapor retarder by eighteen inches and tape the joints. Use black plastic so any sunlight that filters into the crawl doesnít 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 itís hot and humid and the same when it cold and freezing.