Crawl space Venting

November 29, 2017

For new homes, vented crawl spaces should be illegal. That may seem harsh, considering the entrenchment of this common, code-compliant construction detail. I am on board for most local and common practices when they're done right, but it's tough denying that most building science research is condemning the classic vented crawl space in a humid climate. It's time for this building practice to be outlawed for new construction.

There are plenty of crawl spaces that seem to do fine, but even a dry crawl space can have negative effects on indoor air quality in the living spaces above. Most vented crawl spaces are not dry; they are building science disasters. Do you really think building a cave below your house is a good idea?

There is danger in breathing air that comes from open earth. RadonColorless, odorless, short-lived radioactive gas that can seep into homes and result in lung cancer risk. Radon and its decay products emit cancer-causing alpha, beta, and gamma particles. is one of the most alarming forms of indoor-air pollution. While radon can be an issue for all foundation types, homes on slabs (stem walls or basements) offer more control over water and soil gases like radon than houses built over a crawl space.

An unvented crawl space — or, better yet, no crawl space

In humid climates, vented crawl spaces don't make sense because they increase moisture problems.
In an unvented crawl space, plastic sheeting seals the ground and keeps most of the soil gases out of the air. But it's easier said than done.

The plastic should be completely sealed to the walls, piers, and any other penetrations from the ground. In areas prone to groundwater during heavy rains, the crawl space grade should be sloped to a daylight drain. Crawl space walls should be insulated, while allowing access to some portion of the wall for termite inspection.

Finally, unvented crawl spaces need some method of dehumidification or space conditioning, which adds to energy costs and maintenance needs.

One of our arguments against crawl spaces is that cramped conditions leads to inferior work from all parties involved with the home: carpenters, plumbers, electricians, HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. contractors, crawl space encapsulation workers, energy raters, the builder, and finally the homeowner. Show of hands: Who wants to go spelunking to ensure craftsmanship and maintenance?

Proper unvented crawl space encapsulation is not cheap. Moisture and ongoing energy loads have big lifecycle costs.

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