I wish vented crawl spaces would just go away.
We could then turn the entire conversation into a discussion about conditioned crawl spaces. It is still the norm to have vented crawl spaces in the Northwest. New vented crawl spaces are being built every day, in numbers much greater than conditioned ones.
It is important to understand how crawl spaces work because we are going to continue seeing them in older construction, as well as in newer construction.
If you live, or have lived, in other parts of the country, some of this may seem counter intuitive. One could create problems in the crawl spaces of Minnesota or Georgia if one did some of the things we have to do in the Northwest.
The numbers one puts in the equations are different, but the science is the same.
Getting just a few things right will allow a crawl space to behave itself and manage moisture conditions adequately.
The crawl space should be constructed such that vents can be installed on at least 3 sides. A good vapor retarder on the crawl space floor is essential to the success of most crawl spaces.
It would be nice if all the seams of the plastic were welded together and all the edges were caulked/sealed to the foundation. However this degree of tightness is not necessary. I have seen crawl spaces with floating vapor barriers and normal wood moisture content levels. Adequate passive ventilation will remove the amount of vapor that finds its way around the seams of the vapor retarder. The building codes dictate how much ventilation is to be installed around the perimeter of the crawl space.
Crawl spaces that only allow for the installation of vents on one side, or two sides, may require a means of moving air through areas of poor circulation. This can be accomplished with vents at the open side that are ducted to the poorly vented areas. We then install a power vent fan in the duct to move the air mechanically.
It is unusual to see crawl spaces that need mechanical ventilation. It may be warranted in instances where enough passive ventilation cannot be installed. You certainly cannot fix a moisture issue in a normally vented crawl space with power ventilation. You first must address the moisture issue.
The purpose of crawl space vents is not to lower moisture levels created by flooding and plumbing leaks.
The purpose of venting is to deal with minor amounts of soil moisture vapor and to lower humidity that builds up seasonally.
We must understand the science of Northwest crawl space moisture.
On a recent inspection there was a power vent installed. Operation of the fan was based on crawl space temperature. It was set to run at 50°F. It was running at the time of inspection. The unit’s built-in sensor shuts the unit off at 40°F. This particular system operated under the assumption that the higher the temperature, the more the fan needed to run.
This is exactly opposite the science.
In the summer we have moderate relative humidity and higher temperature than we do in winter. In winter, we have very high humidity with lower temperature.
70°F at 65% humidity would be normal in summer, while 37°F at 80% humidity would be normal in winter.
While humidity in summer is lower than in winter, the “actual amount” of moisture in the air is much higher in summer. Warm air can contain more moisture.
In summer, when that warm moist air enters the crawl space, it mixes with the warm wet crawl space air to effectively raise humidity levels in the space as it passes through. This elevated humidity in turn raises wood moisture content of the crawl space framing.
In winter, the air outside the crawl space has very high humidity but at low temperature. When we bring that wet cold air into the crawl space, it mixes with the warm moist crawl space air and effectively lowers the humidity as the air passes out of the space. Wood moisture content of the crawl space framing lowers as humidity drops.
Crawl space wood moisture content goes up and down with the seasons.
If we do not recognize this, plan for it and build for it–bad things are likely to happen. A few of those “bad things” might be mold, rot and wood destroying insects.
Some climates that are dry in the winter close their crawl space vents in the winter. Closing vents in winter in the Northwest would result in increasing moisture levels in the space year round.
Wood moisture content will increase if the power vent is allowed to run all summer. In the winter, when it is colder, the fan will shut down. We will not lower the levels that built up in summer–levels that increased more than normal because of the fan. If you have a situation where power venting is necessary, the fan should run in winter and not in summer.
A good vapor retarder on the crawl space floor and repairs to all bulk water issues is assumed.
There are vendors in the Northwest that would have consumers believe that no crawl space can be successfully vented. This likely has more to support product sales than science.
Charles Buell, Real Estate Inspections in Seattle and the Great Northwest
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WA State, Home Inspector Advisory Licensing Board