Unvented Crawlspaces
In most climate zones, conditioned crawlspaces perform better than vented crawlspaces in terms of safety, health, comfort, durability, and energy consumption. Research has demonstrated how these conditioned crawlspaces also do not cost more to construct than vented crawlspaces. Crawlspace venting is a widely accepted business practice across the U.S. However, in humid climates, the warm humid air entering the crawlspace is more likely to condense on crawlspace framing than it will to help dry out the crawlspace. This is because the outside air can often have a dew point that is higher than the interior crawlspace framing surface temperature. As evidence, existing vented crawlspaces have experienced serious moisture and mold problems, costing builders and homeowners significant resources to repair.
The housing industry has been reluctant to use unvented crawlspaces despite their compelling benefits and the history of problems with existing vented crawlspaces. One of the reasons commonly cited by builders and designers is 'the code does not allow me to build unvented crawlspaces.' This is generally correct but also misleading. The model codes do not allow the construction of “unvented” crawlspaces except in very limited circumstances, but they do allow the construction of “conditioned” crawlspaces. The distinction is important and necessary. Unvented, conditioned crawlspaces with insulation on the perimeter perform better in terms of safety and health (pest control), comfort (warm floors, uniform temperatures), durability (moisture), and energy consumption than passively vented crawlspaces with sub-floor insulation. This is because they are cooler and drier in the summer, which minimizes condensation on framing surfaces. In addition, there is less heat loss from the home during winter, which results in more comfortable floors and less risk of freezing pipes. Crawlspace temperature, dew point, and relative humidity should mirror those of the house's interior.
Crawlspaces should be designed and constructed as mini-basements – as part of the house within the conditioned space.
To meet code requirements, the crawlspace floor:
should be covered with a ground cover consisting of 6mm plastic that is overlapped and sealed at the edges and secured to the side walls;
perimeter walls should be insulated to code-specified levels (e.g., rigid foam on the exterior, or rigid fiberglass, spray foam, or rigid foam on the interior); and
perimeter drainage should be provided just like a basement when the crawlspace ground level is below the ground level of the surrounding grade.
The crawlspace can be conditioned in one of three ways:
supply air from the home to the crawlspace;
return air to the home via transfer grille, or to outside via an exhaust fan; or
connect the crawlspace to a conditioned basement.
A soil-gas venting system should be installed as part of a complete radon-resistant construction system.
Research encourages the use of slab-on-grade foundations rather than crawlspaces for locations that are not subject to frequent flooding.
On venting One ventilation opening should be installed within 3 feet of each corner of the crawlspace. The minimum net area of ventilation should be at least 1 square foot for each 150 square feet of under-floor space area, unless the ground surface of the crawlspace is covered by a vapor retarder.
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