Q:

I’m building a new home soon, and my builder wants to try an unconventional type of heating system. The crawlspace would be insulated and heated by small openings cut into each duct boot. The theory is that the system would be more efficient because the ducts would be running in a conditioned space. Do you have any information on this technique?

Christophe Allen, Olympia, WA

A:

William B. Rose, a research architect at the Building Research Council at the University of Illinois, replies: The Small Homes Council of the University of Illinois has recommended conditioning the air in below-grade crawlspaces since 1959. In short, the crawlspace is treated like a scaled-down version of a basement. But the process is subject to some strict conditions.

Next, the floor of the crawlspace should probably be concrete poured over a well-draining sand or gravel base. The crawlspace needs to be visited frequently. Given the possibility of dust accumulation and its consequent health effects, we’re moving toward a recommendation of regular cleaning and vacuuming in crawlspaces. So I suggest keeping kneepads and a mechanic’s creeper inside the entry to help you maneuver in tight quarters. The crawlspace should be well lighted and be a comfortable height.

In your moderate climate, I would not insulate under the slab. But in cold or mixed climates (such as yours), I recommend installing exterior insulation on the walls of the crawlspace, above and below grade if possible.

If there are ducts or mechanical equipment in the crawlspace, I would consider venting the crawlspace to be a bad idea. But you can expect conflict with building-code officials and others on this subject. If vents are installed to appease code officials or to set up a pump or a fan to exhaust odors, then think of the vents as basement windows. Make sure that the bottoms of the vents are at least 8 in. above grade to keep water from coming in.Vents closer to the ground, especially vents at grade level, just end up becoming sluices for rainwater.

Although the crawlspace should be considered conditioned, I am not sure that supply boots would be necessary. Crawlspace temperature will be conditioned by vagrant loss from the uninsulated ductwork. And the crawlspace humidity will be conditioned by good rainwater management and by having no openings for airflow to the outside.

Also, I’d recommend setting up the sump for subslab air evacuation. That would tend to induce a mild flow of air from the house to the crawlspace.To set up this sump system, the sump should have a tight-fitting lid. A vertical riser from the lid up through the house and the roof provides natural exhaust of soil gas and radon.

Researchers are conducting studies about health and crawlspaces, and preliminary findings indicate some serious concerns. So think of this as interim guidance. The work described above sounds expensive and is. But given the rising importance of indoor environments and their affect on your health, proper detailing of your crawlspace will be money well spent.

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