By space

Crawl space dehumidifier sizing

Low, closed, and usually over bare earth. The ground gives off water all year, so the load barely drops in winter.

Worked example: 1,500 sq ft, very damp

49 pints per day, which lands in the 50 pint class. Change the condition and the answer moves more than changing the area does, so measure the humidity before you shop.

What is different about a crawl space

The floor is usually earth, and earth is an unlimited water source. That is why crawl space sizing carries a fixed addition rather than a bigger area factor: the moisture is not proportional to the square footage, it is proportional to how much bare ground is exposed.

Temperature is the second trap. Crawl spaces sit near ground temperature all year, which in much of the country means below the range a standard room unit is rated for. A unit that ices up at 50 F is off for exactly the months the space needs it, and you will not notice because nobody goes down there.

Drainage is the third. There is rarely a floor drain, so either the unit gravity drains to the outside through a wall penetration, or it needs an internal condensate pump. Buying the unit first and working out the water afterwards is how units end up switched off.

See AlorAir crawl space and basement units

Before you size anything

• Fix the water you can see. A gutter dumping against the wall beats any unit you can buy.
• Lay or repair the vapour barrier if there is bare earth. It changes the load permanently, where the unit only treats it.
• Take readings for a week with a cheap hygrometer. Sizing off one bad day buys a unit that short cycles for the other 360.