By space

Basement dehumidifier sizing

Below grade on at least one side. Walls stay cool, so warm indoor air condenses on them and the room reads damp even when nothing is leaking.

Worked example: 1,500 sq ft, very damp

42 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 basement

Basement walls are cool because they are in contact with soil. Warm indoor air touching them condenses, so the room reads damp even with no leak anywhere. Sizing here is usually about holding a steady 50% rather than fighting a flood.

A sump pit is an open water surface. It gives off moisture continuously, which is why it carries a fixed addition in the calculator rather than being ignored. A sealed pit lid removes most of that load for the price of a lid.

If the basement is finished and heated, the house heating already does part of the drying, and you can often go a size down from the raw number.

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.