Relative Humidity 60%: The Threshold That Decides Mold Risk

Maintain indoor relative humidity below 60% to prevent mould growth. This single rule, repeated in every building science textbook for forty years, is the most actionable piece of guidance in residential moisture management. The science behind why 60% is the threshold is more nuanced than the rule suggests — but the practical answer is right.

What 60% RH actually means

Relative humidity is the ratio of actual water vapour pressure to saturation vapour pressure at a given temperature. At 60% RH and 22°C, the dew point is approximately 13.5°C. Any surface in the room cooler than 13.5°C will collect condensation. In a typical Israeli winter, exterior wall surfaces, single-glazed window frames, and cold supply-air ducts can easily run below 13.5°C — even when room-air RH is "only" 60%.

The mould-growth physics

Mould requires liquid water, not water vapour. The relevant variable is the surface water activity (a_w), not the bulk-air RH. Surface a_w equals the local vapour pressure at the surface divided by the saturation pressure at the surface temperature. A surface at the dew point has a_w = 1.0 (free water). A warmer surface in the same room has lower a_w.

The most xerotolerant indoor moulds (e.g. Aspergillus restrictus, Wallemia sebi) grow at a_w as low as 0.75. Common indoor moulds (Cladosporium, Penicillium) need a_w ≥ 0.78–0.85. Stachybotrys requires a_w ≥ 0.94. Translation: the 60% bulk-RH threshold targets the most-xerotolerant species, with margin.

Why "below 60%" is more conservative than necessary at 22°C — and not conservative enough at 18°C

The 60% rule was developed in temperate-climate, well-heated indoor environments. In a 22°C room, surface temperatures on uninsulated walls might run 18–20°C — generous margin against dew-point condensation. In the same room cooled to 18°C overnight (typical Israeli winter sleeping temperature), the same uninsulated wall might be at 14–16°C, putting it dangerously close to the 13.5°C dew point at 60% RH. Bedrooms in winter routinely violate the spirit of the rule even when the bulk-air number satisfies the letter.

The corrected guidance

Indoor temperature	Maximum safe RH	Resulting dew point
24°C	60%	15.7°C
22°C	55%	12.9°C
20°C	50%	9.3°C
18°C	45%	6.0°C

The principle: as you cool the room, you must dry it. The "60% rule" is shorthand for a 12–13°C dew-point ceiling, which corresponds to different RH values at different temperatures.

How to control RH

1. Source control — bathroom exhaust during and 30 min after showers; kitchen exhaust during cooking; cover pots on the stove; vent dryer to outside; no unvented combustion.
2. Mechanical ventilation — heat-recovery ventilator (HRV) maintains air change without losing heat in winter, drying interior air through dilution with drier outdoor air.
3. Dehumidification — refrigerant or desiccant dehumidifier sized to room volume. The 50-litre/day units common in retail are correctly sized for an 80 m² apartment in winter.
4. Building envelope — better window insulation raises interior surface temperatures and lifts the safe RH ceiling. Long-term, the cheapest fix.

The 60% rule is the most useful number in indoor moisture management — and the most poorly-understood. Treat it as a dew-point proxy, not as an absolute threshold.