Basement Moisture Calculators
Water and air quality come first. Size a dehumidifier here, or open one of the checkers below.
Your result
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Dehumidifier sizing charts
| Condition | 500 sq ft | 1,000 sq ft | 1,500 sq ft | 2,000 sq ft | 2,500 sq ft |
|---|---|---|---|---|---|
| Moderately damp | 10 pt | 14 pt | 18 pt | 22 pt | 26 pt |
| Very damp | 12 pt | 17 pt | 22 pt | 27 pt | 32 pt |
| Wet | 14 pt | 20 pt | 26 pt | 32 pt | 38 pt |
| Extremely wet | 16 pt | 23 pt | 30 pt | 37 pt | 44 pt |
AHAM sizing chart, pints per day, as published by Direct Energy.
| Condition | 22 pint | 35 pint | 50 pint |
|---|---|---|---|
| Mildly damp | 1,500 sq ft | 3,000 sq ft | 4,500 sq ft |
| Slightly damp | 1,000 sq ft | 2,000 sq ft | 3,000 sq ft |
| Very damp | 800 sq ft | 1,400 sq ft | 2,000 sq ft |
| Wet | 500 sq ft | 1,000 sq ft | 1,500 sq ft |
GE Appliances sizing chart. Assumes an 8-ft ceiling; add 25% to the area for 10-ft ceilings.
Which dampness level are you?
- Moderately damp: AHAM describes a space that feels damp and smells musty only in humid weather.
- Very damp: it always feels damp and smells musty, and there may be damp spots on walls or floor.
- Wet: walls sweat or water seeps in.
- Extremely wet: wet floors, standing water, or a high-load space such as a laundry area.
Measure with a hygrometer for a few days before you buy. The EPA advises keeping indoor humidity below 60%, ideally 30%–50%, to keep mold from growing.
Moisture comes first
Finishing materials hide water problems until they become mold problems. Before framing, tape a 2 × 2 ft sheet of plastic to the slab and to a wall for two days: water under it means moisture is coming through the concrete; water on top means humid air is condensing. The first calls for drainage or sealing, the second for dehumidification. Keep a hygrometer in the finished space afterward and aim for the EPA's 30%–50% range.
- Extend downspouts several feet from the foundation.
- Slope soil away from the house.
- Use materials that tolerate moisture below grade: foam board, treated bottom plates, vinyl plank.
- Leave access to the sump pit, the radon fan and the main water shutoff.
Where basement moisture comes from
| Source | Sign | Usual fix |
|---|---|---|
| Rain and snowmelt | Wet floor edges after storms | Gutters, downspout extensions, grading |
| Groundwater | Water through cracks or under walls | Interior drain and sump pump |
| Humid outdoor air | Musty smell, sweating pipes in summer | Dehumidifier, keep windows shut on humid days |
| Vapor through concrete | Damp under a plastic test sheet | Vapor barrier, foam board, sealer |
| Plumbing and appliances | Local stains | Repair leaks, vent the dryer outside |
Related calculators
Sources
- https://blog.directenergy.com/shopping-dehumidifier/
- https://products.geappliances.com/appliance/gea-support-search-content?contentId=34664
- https://www.epa.gov/node/78417