Crawlspace dehumidifier install basics: vapor barrier first, placement, drain, power and set point
A crawlspace dehumidifier only works on a space that is already covered and mostly sealed from the ground and outside air. The 2021 IRC (R408.3) lists dehumidification as one way to condition an unvented crawlspace, sized per the manufacturer's instructions. Lay or repair the vapor retarder first, set a crawlspace-rated unit level near the middle, give it a permanent drain (gravity or pump), power it from a proper grounded GFCI receptacle, and set about 45-50% RH. Fix standing water and plumbing leaks before any of this.
Before you start
- Crawlspaces are confined, hot or cold, and often have exposed wiring, pests and mold: tell someone where you are, use a headlamp and respirator, and see heat-illness-in-attics-and-crawlspaces.
- Do not run a dehumidifier into standing water or active seepage. Stop the water first.
- Receptacles for crawlspace dehumidifiers and fans should be GFCI protected; never run an extension cord across the crawlspace floor.
- Clean visible mold and remove soaked insulation before starting the unit, or it will spread spores and contaminate the dehumidifier.
Diagnostic steps
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