Crawlspace vents must provide adequate circulation
By Tim Carter
Tribune Media Services
Q. Should I keep my crawlspace foundation vents open year-round? Are they really necessary? I live in Maryland and get all four seasons and temperature conditions.
Stephen King, Lexington Park, Md.
A. There are thousands of people who wonder what to do with crawlspace vents. In fact, years ago when I installed them in room-addition projects, I often thought about the science behind these smallish vents that didnt seem to let in much air at all into the crawlspaces. I would actually be in the crawlspaces on windy days and could barely feel a puff of air come through the vents.
Here is the reasoning behind crawlspace ventilation:
Take a clear piece of plastic and place it over what appears to be very dry ground in your yard. Put some boards around the edges to weight down the plastic so no air gets under the large plastic sheet. If you can do this on a sunny day, you will get almost instantaneous results. Ill bet within minutes you start to see a slight fog develop on the underside of the plastic. Wait longer and the fog will turn to water droplets. The soil around your house and inside of crawlspaces is constantly liberating water vapor. Sunlight, wind and natural evaporation pull this moisture from the ground.
The moisture content of the soil drives this water vapor engine. People who live in the extreme arid parts of the Southwest have little water vapor escaping from the soil for much of the year. But even they have a monsoon season and the soil does get wet at certain times of the year. People who live in the Northwest, Northeast, Midwest and Southeast have water vapor streaming from the soil year-round.
In the open parts of your yard, this water vapor readily escapes to the atmosphere. But under a crawlspace, this water vapor can collect and begin to condense on the foundation walls and the subfloor structure. If the subfloor is wood, the water can create mildew in short order, and eventually the wood will develop wood rot.
The theory for many years was to install the crawlspace vents so that outside air could get into the crawlspace and lower the relative humidity. But I am here to tell you that I have been in many crawlspaces with plenty of open vents and have felt as if I were in damp caves. The vents dont circulate enough outside air into the confined crawlspace.
The better option is to install a high-performance vapor retarder over the soil in the crawlspace with or without foundation vents, which your building code may require. This vapor retarder needs to lap up onto the sides of the foundation and be secured to the wall with treated lumber or rot-resistant wood strips. The best vapor retarders come with special tape that allows you to permanently seal any seams where the vapor retarder laps over itself or is cut around objects that stick up through the ground in the crawlspace.
These vapor retarders block the water vapor and effectively cancel out the need for the crawlspace vents. But, to reiterate, your local building code may require foundation vents. No matter what you do, always be sure to check with your local building department and obey the building code in your area.
If your local building department has adopted code modifications that allow you to install vapor retarders as I describe without any foundation vents, you must insulate the side walls of the crawlspace, and the heating contractor must pipe in a small amount of conditioned air into the space. Furthermore, the foundation insulation must be fireproof, or, if it is not, it must be covered with a fireproof material.
The bottom line is if you install the high-performance vapor retarders correctly, you can keep the foundation vents open or closed it will make little difference.
Crawlspace ventilation is another prime example of how the building code can be argued to be a set of minimum specifications. The building code is indeed a wonderful set of regulations, but it by no means offers a builder or a homeowner the best possible way to do a task. There are countless examples of where builders can go beyond the code and do extra work or use better materials that will produce a home that will last hundreds of years instead of perhaps 40 to 70 years.