Dear Inspector,

This month I will discuss some new thinking with regards to crawlspaces you may be seeing soon. This system seems to be most popular in the southern US, particularly hot and humid climates.

Most of us are very familiar with ventilated crawlspaces. This system allows exterior air, whatever its condition (hot and humid or freezing cold), into the space through the ventilation openings. New thinking is: why allow hot humid air into the crawlspace in summer to possibly condense on the cooler framing (occupants are typically operating air-conditioning all summer) and cause damage? Why allow that cold air into the space during winter to extract warmth from the house, and even freeze pipes?

Many new homes are being equipped with pressurized crawlspaces. Note I did not say conditioned (why heat and cool non-habitable space?). Here's how it looks; there are no ventilation openings (obviously). The vapor retarder (plastic sheeting) is carefully placed on the soil, the laps taped and the edges wrapped up the foundation stemwall and terminated with a bar or plastic barbed fasteners. Sometimes the foundation interior wall is insulated. The underfloor is insulated as usual. Somewhere in the supply ductwork there is a (sometimes more than one) small register (two or three inch, similar to those seen in high pressure forced air systems). There is no return. The output air from this small opening in the supply will pressurize the crawlspace just enough to keep out the exterior air, thus preventing the detrimental effects of that air. If a return is installed the space is no longer pressurized and the occupants are spending energy dollars to condition non-habitable space. The additional energy cost of a small supply opening is insignificant.

I personally think this system is great. Some of the added benefits are: less dust and general debris in the crawlspace due to no ventilation openings, fewer unwanted creatures due to less openings (there are always some openings), and of course less chance of frozen systems in cold weather and excess moisture in hot, humid weather.

Mike Casey
Kaplan Professional Schools
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