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January 2013: Crawl Spaces

Addressing Crawl Spaces in the HPwES Guidelines

VENTED VERSUS UNVENTED CRAWL SPACES

“The principal perceived advantage of a vented crawl space over an unvented one is that venting may limit radon and moisture-related decay hazards by diluting the crawl space air. Additionally, providing a vented crawlspace may make sense in flood-prone areas such as coastal zones subject to hurricanes.  Venting can complement other moisture and radon control measures such as ground cover and proper drainage. However, although increased air flow in the crawl space may offer some dilution potential for ground source moisture and radon, it will not necessarily solve a serious problem. Vented crawl spaces are often provided with operable vents that can be closed to reduce winter heat losses, but also potentially increase radon infiltration. Although not their original purpose, the vents can also be closed in summer to keep out moist exterior air that can have a dew point above the crawl space temperature.  This approach, however, requires a high level of informed occupant participation to be successful”.(Source: Oak Ridge National Labratory http://www.ornl.gov/sci/buildingsfoundations/handbook/section3-1.shtml)

Considering that a majority of the housing stock in our region has vented crawl spaces, and because there is a huge advantage in conditioning or encapsulating them, the following requirements should be employed by LEAP contractors. Conditioning means closing venting, laying down a heavy vapor barrier and applying foam on the exterior walls. Encapsulating means making them dry and air tight. These recommendations are based upon feedback from LEAP’s Central Virginia Technical Advisory Committee in lieu of local current knowledge of building science/practices and what the 2009 IRC code is requiring:

• A continuous termite inspection strip should be present and located on the interior surface of the sill plate under the conditioning scenario.

• The gap between the sill plate and masonry surface below should be effectively air sealed with an appropriate product in both scenarios.

• A continuous class 1 vapor retarder should be used with a minimum of 6” overlap at the seams and 6” run up the stem walls and or piers.

• The vapor barrier must be sealed, taped or mechanically fastened along its entire length and seams to be effective.

• The crawlspace is allowed to be unvented by insuring the following: that most of the major connections to the space above have been sealed, the supply ducts and returns ducts have been sealed,(sealed and insulated in the encapsulation scenario) moisture issues have been addressed and humidity levels can be managed at or under the 60% humidity level threshold.

• It is recommended that humidity levels be monitored before any effort of dedicated dehumidification is in place.

• That it may be prudent to monitor humidity levels in any adjoining spaces with regards to the occupied spaces above, seasonal weather patterns and with respect to specific site considerations. (Oregon Scientific-$60)

• If levels are documented over 60% then dedicated dehumidification should be installed and set at a reasonable level to provide the intended benefit.

• Conditioning foam that is applied to the walls should be just below the sill plate and continue down to 18” below where the frost line exists.

Home Energy magazine published a good article for reference entitled “Fixing Wet Basements and Crawl Spaces” in their Nov/Dec 2012 issue.

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