The Right Dehumidifier Setting for a Boston Basement, and Why Most People Have It Wrong

A dehumidifier is the cheapest mold prevention a Boston basement can have, and most of the ones we see are set wrong, draining wrong, or shut off for the winter. Here is the setting, the setup, and what to do once the heat comes on.

The number

Set it to 50 percent relative humidity. Below 45 wastes electricity in a basement; above 55 lets mold establish on anything paper-faced or wood, which is most of what a basement is made of. If your unit has a dial instead of a number, buy a $10 hygrometer and set the dial until the hygrometer reads 50.

Drain it, do not bucket it

A bucket fills in a day in a damp basement and the unit shuts off when it is full. Then it sits off for a week while you forget. Run the drain hose to a floor drain, a utility sink, or the sump pit, or add a $40 condensate pump if there is nowhere lower to send it. A dehumidifier that runs continuously is doing its job. One that is shut off because the bucket is full is furniture.

Winter

People turn them off in November because the air feels dry. The upstairs air is dry. The basement is still getting moisture through the slab and the foundation walls all winter, and once the furnace runs, warm basement air holds more water, not less. Leave it on. Many units stop working below about 60 degrees, so if the basement gets cold, move it closer to the furnace or switch to a unit rated for low temperatures.

Placement

Middle of the room, not in a corner, with a foot of clearance around it. If the basement is divided, put it in the biggest open area and leave the doors open. A finished basement with closed bedrooms may need a second small unit.

When a dehumidifier is not enough

If it runs all day and the reading never drops below 60, water is coming in faster than the unit can pull it out. That means a grading or gutter problem outside, a crack in the foundation, or a slab with no vapor barrier. We can usually tell which within an hour, and the fix is almost always cheaper than the finished basement it is about to ruin.

After a flood

A household dehumidifier cannot dry a basement that has flooded. It is built to hold a dry room dry, not to pull water out of soaked drywall. That takes commercial equipment and a drying log. Call us for that, then put the household unit back to work once the room is dry.