Even in a down economy, new home construction in the U.S. this year should reach approximately 200,000 new units. Of those, 20% will be constructed on vented crawlspace foundations, bringing the total number of homes in the U.S. built on vented crawlspace foundations to an estimated 26 million.

For all intents and purposes, that means that by the end of this year, there will be 26 million homes in the U.S. that are subject to moisture, mold, rot, insect infestation and higher energy bills, simply because they are constructed with vented crawlspaces. Those of you who are familiar with some portion of building codes may immediately disagree with part or all of the last statement, because common thinking and most building codes require that crawlspaces be vented.

Most codes, which are based on the theory that vents will allow moisture to escape from a crawlspace read something like this, "The under floor space between the bottom of the floor joists and the earth under any building (except space occupied by a basement or cellar) shall be provided with ventilation openings through foundation walls or exterior walls. The minimum net area of ventilation openings shall not be less than 1 square foot for each 150 square feet of under floor space area. One such ventilating opening shall be within 3 feet of each corner of said building."

While building codes read like that, numerous new studies and a growing number of contractors and restoration companies, including RestorePro, believe that crawlspaces should not be vented. Research indicates that rather than removing crawl space moisture, venting exacerbates the problem. Researchers, as well as contractors and restoration companies, have found consistently that when warm, moist outside air enters a crawlspace, it instantly cools and drastically increases the relative humidity in that area. When the relative humidity goes over 100 percent, the moisture is released into the crawl space atmosphere, where condensation accumulates on the walls, floors and building components.

Think of a crawlspace as a smaller version of a full basement. It’s located under your living space; it has supply lines for water, heating and air conditioning duct work and can be opened into your living space. A simple question can be asked, do you leave the windows in your basement open to vent the moisture from that space? Obviously the answer is no, then why leave open a crawlspace?

The following study gives a good overview of the new thinking behind closing the vents in crawlspaces.

Closed crawl space performance in cold and hot-humid climates Project Summary