Q. We are building a coastal home on the eastern shore of the Chesapeake Bay and will have a crawl space under the house. The crawl space will be 28' by 36' and approximately 3-4' in height. We are weighing a traditional vented crawl space with vents al ong the cinder block exterior against a conditioned crawl space where heat and air are diverted into the contained crawl space while heating and conditioning the rest of the first floor living area from one heat pump. One reason for a conditioned crawl space is that humidity builds up in the summer and moisture forms on the pipes and could create mold and mildew in the ductwork over time.

There are pros and cons for both. The conditioned HVAC bid seems to be excessively high ($5000+) to just push a little air into a crawl space. Can you explain the technology of a conditioned crawl space versus the vented one and the probable costs differ ences?

A. I am still on record for detesting crawl spaces. I unfortunately find myself in them way too often, just yesterday in fact. The first time I wrote about a crawl space in this column what popped out of my head was the statement that “anyone who b uilds a basement so shallow that they have to rename it ”crawl” should be hit with a stick and not fed for week”. My sentiment hasn’t changed.

What happened in the evolution of homebuilding when it came to crawl spaces was builders knew they had to get the frame of the house up off the ground but had to get the foundation walls down to frost level, which around here is about 30 inches below gr ade. Once the builder ran the block walls or piers up to the level where they could start running the first floor framing they were happy and went to work. By the end of the first day the crawl space was covered over by floor framing and conveniently fo rgotten. The whole notion of venting a crawl space appeared in the building literature back in the late 1920's and 30's ending up in US government wood building guides and the prescribed amount of crawl space ventilation for a given area was cast in sto ne forever to be modeled and followed in the building codes since. And back in the days when we built houses out of wood and plaster only that made a whole lot of sense. And the idea of venting things in general is intuitively wise, so we want to do it.

But then we started to insulate houses and heat houses uniformly in winter and cool them in summer placing heating and air conditioning ducts and plumbing pipes in this space and the whole idea of what was a good idea relative to what kind of air was ven tilating the crawl space came into question for the very reasons you mention. It has become my understanding that ventilating crawl spaces the old-fashioned way and relying upon that manner of ventilation really isn’t a good idea in the long run. You o bviously have been exposed to some of this thinking if you are entertaining the notion of conditioning the air in your crawl and closing it off from the outside. In Maryland and especially along the water you really don’t want the ambient outside air to get under your house with the possible exception of during the months of October and May when the outside air temperature and humidity and the inside conditioned air are nearly equal.

Control of outside water intrusion is a must with any crawl space and some waterfront foundations are built designed to allow water to slosh through them under extreme conditions but that’s another storey.

What I have discovered while trying to nudge folks toward this notion of crawl space conditioning is the natural resistance of not only this isn’t the way we’ve always done things but, hey buddy, check the building codes. Convincing the local building inspector in some jurisdictions that what you are doing is a good idea can be frustrating. Your HVAC bid appears to me to possibly reflect fear in the bidder. If a contractor is afraid of a job or doesn’t want to do it or doesn’t understand it, one sure remedy is to bid high.

The guru of this sort of thinking is Dr. Joseph Lstiburek (pronouced STEE-brick) of Building Science Corp up in Massachusetts. I’ve known Joe for over twenty years and when all else fails they call him. His book “Building Guide for Mixed-Humid Climates” is a bargain at $45. and a must for anyone building in this environment. The website is www.buildingscience.com. Preventing problems is a whole lot smarter than fixing them later.