Many homes and cottages have crawl space foundations. These foundation types are equipped with operable vents designed to provide ventilation for moisture control. Unfortunately, venting a crawl space during air conditioning season allows hot, humid outside air into the cooler environment inside the crawl space.

Instead of drying the crawl space, outside ventilation actually increases the moisture load. Once there, moisture gets into the colder floor framing surfaces where the moisture content of the wood can go up to a point that is ideal for fungal growth and wood damage. Temperature stability makes crawl spaces an excellent location for air conditioning equipment, ductwork and plumbing, however; high crawl space humidity during summer months may lead to condensation on ductwork and equipment damage. Additionally, high humidity provides an ideal environment for insect growth.

The long-term solutions for these problems must focus on eliminating the source, not treating the symptoms (e.g., spraying insecticides and re-insulating ductwork). Research and experience has shown that unvented and conditioned crawl spaces provide better moisture control than vented crawl spaces and they behave similarly to basements. Outlined below are techniques for unvented and conditioned crawl space construction.

Unvented crawl space retrofits are a solution to high moisture conditions in homes. This technique is not in full compliance with local and national building codes and should not be specified for new construction. However, extensive field experience indicates that it provides exceptional environmental control for a safe and healthy crawl space environment. Before any efforts are made to control ground or air born moisture, all bulk moisture entering the crawlspace (e.g., rainwater, underground springs) must be eliminated. The following steps must then be followed in their entirety or the desired results might not be achieved.

1. Install gutters and slope grade away from the foundation at least 5% (6 inches per 10 feet) to direct rain and surface water away from the house. Empty downspouts 8 to 10 feet from the foundation

2. Cover the crawl space with a 20 mil polyethylene vapour barrier called CleanSpace. Overlap all seams and seal with CleanSpace vapour bond tape. Extend the outside edge of the CleanSpace up the foundation wall to a few inches below the top plate. Secure the CleanSpace to the foundation wall by using construction adhesive. The goal of this step is to provide 100% ground coverage and eliminate as much outside air as possible from coming through the block walls.

3. Close and seal all foundation vents with CleanSpace vent covers or spray foam the cavity to eliminate warm, moist outdoor air from entering the crawl space. Cut blocks of ½” to 2” rigid polystyrene and pressure fit into the backside of vent openings to further reduce outside air.

4. Seal forced air ductwork to reduce or eliminate leakage. Supply air leaks can lower crawl space temperature leading to higher relative humidity. Return air leaks cause negative crawl space pressure, increasing the rate of soil moisture evaporation and soil gas migration.

If the crawl space is saturated when corrective measures are taken, run a dehumidifier for several weeks to bring the moisture content under control. From that point forward, the space should operate safely. With outside ventilation eliminated, under no circumstances should combustion products, such as lawnmowers, gasoline, paints or solvents be stored in the crawl space.

If a combustion appliance such as a gas or oil furnace is present, a dedicated combustion air supply may be necessary during the heating season.

With new construction, conditioned crawl spaces can be created and they can be specified for retrofit applications when existing insulation is damaged and must be removed. The conditioned crawlspace varies slightly from the unvented crawl space in that it treats the crawl space area as part of the living space, much like a basement.

This construction technique also meets local and national building codes due to the insulation of the exterior foundation wall and the introduction of conditioned/house air into the crawl space.

Because this foundation technique is used in a relatively small number of new homes, you should talk to your local, county or city code official before construction. Following several construction guidelines can ensure that an unvented and conditioned crawl space will get the desired results.

1. Gutters and downspouts to control rainwater and a 5% grade sloping away from building. Downspouts should empty 8 to 10 feet from the foundation or be tied to an underground drain system.

2. A foundation drain system to control groundwater. If the outside grade is higher than the inside grade, a damp-proof coating on the exterior foundation wall is recommended. If the interior grade is higher than the exterior grade, damp proofing and foundation drains may be eliminated.

3. The durable CleanSpace vapour barrier covers the foundation wall and terminates under the sill plate (or lower if retrofitting).

4. Outdoor air is kept out by sealing all openings and installing gaskets around access doors.

5. Outside foundation walls are insulated using code-approved rigid insulation with flame spread less than 25. Rigid insulation extends down to the top of the footing with the polyethylene ground cover sealed to its face at the bottom.

Batt insulation extends down the wall and terminates 2 to 3 feet into the crawlspace with seams continuously taped. In either case, the band joist is insulated using either rigid or batt insulation. Access doors are insulated using 2 inches of rigid foam board or equivalent. CleanSpace crawl space access doors can also be used.

6. A dampered duct attached to the forced air heating/air conditioning system or a small exhaust fan transferring air from the house provides conditioned house air to the crawl space. The code also allows for the installation of an exhaust fan to vent crawl space air to the outside. This method is not recommended because venting can create negative pressure in the crawlspace.