Open Up Crawlspace to Fight Moisture
QUESTION: Our house is a 700-square-foot bungalow built in 1973 in St. Vital on a number of concrete pillars. A crawlspace surrounds an approximately 7x7-foot "partial basement" with concrete block walls that rise about four to five feet high. The crawlspace begins at this height and rises another two feet higher or so.
There is also some wooden flat level space for storage above the block walls immediately before drywall, insulation and a vapour barrier separates the basement from the crawl space. Except for a small, open space where the furnace exhaust pipe exits into the crawl space, the rest of the crawlspace is sealed.
The crawlspace has only one vent in a concrete skirting around the foundation. A prominent basement company indicated that crawl space vents should be sealed because they let humid air into the house. Apparently, the only time that it does work to use a vent to dry a crawlspace may be in winter, or if the temperature is 21 degrees C and the relative humidity no more than 45 per cent year-round.
To begin a strategy to address a moisture issue and the possibility of radon in the exposed dirt floor, I've decided to separate the house from the dirt floor with a moisture barrier. I understand that, as part of this, I need to add soil next to the house to create a swale or a slope away from the foundation for precipitation run-off. However, this will cover up the crawlspace vent, because it is currently very low to the ground.
Should I keep the vent in the crawlspace but make a new one higher, or seal it, create the swale or downward slope with soil and dehumidify the crawl space, if necessary, as part of a moisture-reduction strategy?
Jonathan Neufeld and Brigitte Reimer
ANSWER: Crawlspaces are often a problem because of the high chance of moisture intrusion. This is due to the open soil and the location of the crawlspace, as you have indicated. You are being very pro-active in your attempts to keep this area dry and prevent radon intrusion, but you should also attempt to include other alterations in your plans.
Air movement is one of the main things that allows moisture to evaporate from damp air. Allowing an ongoing source of warm fresh air into the crawlspace, even if does have high relative humidity, will help prevent stale, damp air from creating a problem. Unfortunately, having a single vent may be next to useless, as this does not allow much air movement.
To answer your main question, you should definitely move the vent higher and install at least one more additional vent, or small window, at each side of the crawlspace. Adding these vents will help to allow wind-assisted air to enter and exit the confined space, removing damp, stale air that is likely to condense in the cool environment.
Building up the soil outside the concrete grade beam that encloses your crawlspace may help prevent excessive moisture intrusion, but that should be done in conjunction with interior grading as well. …