Automatic vents for crawl space?

A homeowner building a small cabin in Maine asked about automatic foundation vents his builder wants to install in the crawl space. The vents open around 70°F and close around 40°F, and are marketed as saving energy and lowering the risk of frozen pipes. But the homeowner worried this timing works against moisture control: in summer, the vents let in warm, humid outside air that cools once inside the crawl space and can condense; in winter, the only air reaching the vents comes from the heated house above, which also cools and drops its moisture once it hits the space below.

Building science experts responding in the thread pointed out a bigger issue with the builder's plan. The cabin's foundation was going to have insulation on the slab and around the stem walls (concrete block foundation walls), which normally means the crawl space should be sealed off and treated as part of the heated interior of the house, not vented to the outside. Venting only makes sense for crawl spaces left outside the insulated envelope, where the real barrier is the floor above. If the perimeter is insulated, the space needs its humidity and temperature controlled directly rather than left to swing with outdoor air through vents.

The experts also flagged a separate concern: the slab was planned to sit on a bed of gravel and crushed stone rather than below the frost line or on a frost-protected shallow foundation, which carries some risk of frost heave unless water underneath has a clear way to drain.