Damp Basement: Humidity Targets, Dehumidifier Size and Mould

Basements are cool, often in contact with damp ground, and frequently the most humid space in a house. Targets are the same as elsewhere: EPA recommends below 60% RH, ideally 30–50%^[1].

Step 1 — stop water getting in

• Make sure the ground slopes away from the foundation so water does not collect around it^[1].
• Clean and repair gutters^[1].
• Fix plumbing leaks; dry wet materials within 24–48 hours^[1].
• Insulate cold water pipes that sweat^[1].

Step 2 — size the dehumidifier

ENERGY STAR's chart for spaces under 2,000 sq ft: 25–40 pints/day if very damp (damp spots on walls and floors, 75–90% RH), 30–50 if wet (walls/floor sweat or seepage)^[2]. Use the sizing calculator.

Step 3 — check cold surfaces

Basement walls and floors are often below the air's dew point in summer, when warm humid air enters. Measure the wall surface and room air and run the condensation risk calculator.

Mould already there?

Under ~10 sq ft you can usually clean it yourself; above that, or after a lot of water damage, follow EPA's professional guidance^[1]. See mould on walls.

Sources

1. US EPA – A Brief Guide to Mold, Moisture and Your Home
2. ENERGY STAR – Dehumidifiers (sizing chart)