There has been much debate over whether or not a crawlspace should be ventilated or encapsulated (unvented). Research by building scientists over the past couple of decades have determined that a properly encapsulated unventilated crawlspace is better for reducing humidity, preventing wood decay, improving indoor air quality, and reducing the potential for termite infestation. Our experience of inspecting thousands of crawlspaces would agree with these findings.
For new construction the building Code addresses closed crawlspace construction in section R409 of the Residential Building Code. For remediation of existing crawlspaces similar techniques are utilized.
In converting a ventilated crawlspace to an unventilated crawlspace, there are basically four techniques for controlling humidity in closed crawlspaces. In all cases the first approach is sealing the crawlspace completely. This is done by lining a minimum 6 mil thick vapor barrier on the entire crawl floor and taping all the seams. The vapor barrier should be wrapped around piers and run up the foundation wall within 3” from the top. The 3” “inspection gap” is a requirement by Code to inspect for termite tunnels. In our opinion, it is an unnecessary provision, but one we must follow nonetheless. In addition to sealing up everything, diverting water away from the house is a must. Once the vapor barrier has been installed and all of the vents and penetrations sealed, one of the following four humidity controlling techniques should be employed:
A permanent dehumidifier that is operated by a hygrometer/ humidistat is used to ensure the humidity stays at or below a desired setting. The only downside with dehumidifiers is that they use a lot of energy, especially in the wintertime when the equipment has to work much harder to remove moisture out of the air. Equipment cost is also higher compared to the other methods listed below.
- Supply Air:
A supply line is connected to the ductwork to pump at least 1cfm per 30 square feet of area to the crawl. This actively conditions the crawl to keep the air and humidity in the crawl similar to inside the house. A supply vent added would need to have a backflow damper that would seal the crawl air from drafting back into the house. Also, this modification would require a mechanical contractor to properly balance the system and install the supply line.
- Exhaust fan:
The crawlspace air is exhausted to the outside with a fan that supplies a constant volume of air equal to at least 1cfm per 50 sf. of area.
- House air:
House air is blown into the crawl at a rate of 1cfm per 50 sf. of area at a constant rate. This creates a positive pressure in the crawl and uses the least amount of energy as the air primarily stays within the thermal envelope.
Another system that has been successful in reducing humidity, though technically not an encapsulation system, is a controlled ventilation system known as an ATMOX system. This system uses sensors to determine when the outdoor temperature and humidity are favorable to bring in while flushing out the more humid crawlspace air.
Different conditions may determine which of the four methods are more preferable. In most cases, the fourth method of supplying house air is what we would recommend as the least expensive option for new construction, and the ATMOX system would be best for retrofit situations. In cases where an old basement exists and a vapor barrier over the slab may be undesirable or costly, a dehumidifier may be more appropriate. In cases where perhaps gasoline or other nasty chemicals are stored in the crawl, the exhaust fan may be a better approach.
We generally recommend closed crawlspaces when there has been a history of moisture or mold problems in the crawl, or if the owners are sensitive to mold spores. At the very least, we would recommend a vapor barrier be placed on the ground and cover 100% of the crawlspace floor.