Just what is a conditioned crawl space, and why would you want one? Very good questions. Let’s start with what they are.
Crawl spaces have been constructed basically the same way for years. The floor is generally dirt with hopefully some sort of vapor barrier covering the entire floor. The walls are typically un-insulated concrete block, and there are many small fresh air vents that can be opened or closed to allow in fresh air. The area is very cold in the winter and is very moist in the summertime. Conventional wisdom has always been to open the crawl space vents in the summertime to keep the moisture levels down, however, doing this simply allows the moist outside air to completely fill your crawlspace and start to condense on things that are cool like under insulated cooling ductwork and floor joists cooled by being in contact with the cooler conditioned air. The condensation that occurs causes damage to the ductwork and equipment that it comes in contact with, and it ruins batt type insulation, growing mold and mildew. This type of moist environment also creates a great habitat for insects, which you probably aren’t interested in hosting in your house.
It is very important that you seal all outside moisture from entering your crawlspace. The first step is to make sure that the ground around your house is actually sloping away from your house. This can be a difficult task in many areas near the water, where things can be terribly flat to begin with. Gutters or some way to channel the water away from the house are a must.
Once you have eliminated all sources of water intrusion, the next step is to seal things up. A heavy plastic vapor barrier can serve as a complete barrier to moisture on the ground. It is important to run the barrier one third of the way up the wall and up all of the internal block piers. All vents are then covered with a piece of rigid foam to help seal them from allowing the outside air in. Once this is done, we get to the spray foam insulation part.
All of the exterior walls and all of the piers are then sprayed with a layer of foam insulation. This insulation not only insulates the crawl space from the varying temperatures outside, but it also completely seals the crawl space from any water or moisture intrusion. This type of spray foam insulation is rigid and absolutely waterproof.
If the crawlspace was moist to begin with, then a dehumidifier would be a good idea for a couple of weeks, until the air could be completely dried out. You can then alter your existing ventilation ductwork to allow some of the conditioned air into the crawlspace. Since there is no return in the crawlspace, the supply of conditioned air creates a positive pressure in the crawl area driving any moist air attempting to enter the crawl area back out.
What you end up with is a completely dry, mold free, bug free environment that will remain roughly the same temperature as the rest of the house. By allowing a small amount of the conditioned air into the crawlspace, your ductwork that is more than likely under your house is now running through conditioned space, causing your heat pump to work less, extending its life. You have basically turned your crawlspace into a very short conditioned basement which is a whole lot better than sitting your house on a giant ice cube!