What time of year is worst for mold?

Late summer — typically July through September — is the worst period for mold, when outdoor temperatures peak above 77°F and indoor humidity regularly exceeds the 60% threshold that drives explosive mold growth.

Mold spores are always present in the air, but growth requires two conditions to align: warmth and moisture. In humid climates like the Southeast, that window stretches from May through October. Attics and crawlspaces are hit hardest in summer because heat drives moisture vapor upward through the building envelope. A secondary spike happens in late winter and early spring, when cold surfaces collect condensation as temperatures fluctuate — a common trigger for basement and rim-joist mold. Airborne spore counts tracked by the EPA and AIHA consistently peak in August and September in most U.S. regions.

• Peak mold growth season: July–September, when indoor humidity most frequently exceeds 60% RH.
• Mold growth threshold: surface temperatures above 40°F combined with relative humidity at or above 60%.
• Secondary mold spike: late winter to early spring, driven by condensation on cold surfaces during temperature swings.
• Airborne mold spore counts peak in August and September according to EPA and AIHA seasonal tracking data.
• High-risk zones year-round: crawlspaces and attics, where heat and moisture vapor concentrate regardless of season.