This map shows where winter outdoor air is damp enough and warm enough to support mold growth on cool surfaces in your home. Light yellow areas have the most mold-friendly winter conditions; dark purple areas have the least (driest).

Specifically, it shows the percent of winter days (December through February) where outdoor air is damp enough and warm enough to keep cool surfaces wet. The data comes from TMYx weather files at climate.onebuilding.org, which compile typical-year hourly weather observations from NOAA's Integrated Surface Database for nearly 200 stations across the US and Canada.

A day counts if its daily-average outdoor RH is at or above 80% AND its daily-average temperature is at or above 40°F. Both conditions have to hold for the same day.

80% RH is a standard building science benchmark where mold growth becomes possible on susceptible surfaces given enough time. Below that, outdoor air has enough drying capacity to pull moisture away from surfaces between exposure events. At or above it, any moisture that reaches a cold surface tends to stay.

40°F matters for two reasons. Mold growth essentially stops below that point because water on surfaces freezes or stays too cold for biological activity. And very cold air doesn’t hold much moisture, so even at 90% RH the absolute water content is small. A place like Bismarck or Fairbanks can spend most of winter at 90% RH and still rank low here. The air is technically saturated, but there's almost nothing in it, and the surfaces it touches are too cold for what little moisture is there to matter.

The mechanism is straightforward. In winter, outdoor air often sits near its dew point. Surfaces in contact with it, like rim joists, exterior walls, crawlspaces, single-pane windows, and unheated closets, can't dry out. Any moisture that reaches them stays. Add normal indoor moisture from breathing, cooking, and showering, and those already-marginal surfaces tip into condensation and mold growth.

Why this map doesn't use 1% dew point like the summer map

The summer map uses 1% dew point because summer mold problems are driven by absolute moisture load. Warm air holds a lot of water, and dew point measures exactly how much. Winter is different. Cold air barely holds any water in absolute terms, so dew point on its own misses the point. What matters in winter is how close the air is to saturation, which is what relative humidity measures directly.

The 1% framing also doesn't fit. Summer dew points across a region cluster tightly around their seasonal peak, so the top 1% represents conditions a home spends much of the season handling. Winter is more variable. A single percentile would either capture a brief extreme that doesn't reflect typical conditions or smooth over the very stretches that drive surface dampness. Counting days where both RH and temperature meet a meaningful threshold gives a more honest picture of how often the climate actively supports mold-friendly surface conditions.