Guide

Vents or a dehumidifier: the two answers do not mix

Venting a crawl space assumes outside air is drier than inside air. For much of the year in a humid climate that is backwards.

The summer problem

Warm outside air at 80% relative humidity meets a crawl space at ground temperature. The air cools, its relative humidity climbs, and it deposits water on the coolest surfaces it can find, which are your joists and ducts. The vent did not dry the space, it fed it.

What sealing changes

Sealing the vents and laying a vapour barrier removes both the outside air and the ground as sources. In the calculator that is the difference between carrying the vented and bare dirt additions or not, which at 1,500 sq ft is 11 pints per day of load you no longer have to buy a machine to remove.

See AlorAir crawl space and basement units

Order of work

1. Fix bulk water first: gutters, grading, any active leak.
2. Lay the vapour barrier over the earth and up the walls.
3. Seal the vents.
4. Then size the dehumidifier for what is left.

Doing it in the other order buys a bigger unit than the finished space will ever need, and you pay for that every month it runs.