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About this tool
Find your target indoor humidity range, dew point, mould and condensation risk, and the water to add or remove.
The Indoor Humidity Comfort Guide builds a target relative humidity band from the EPA and ASHRAE baseline of 30% to 50%, tightened by whatever applies in your room — dust mite allergy, a history of damp, dry skin, wooden instruments. It then runs the psychrometrics: dew point from the Magnus equation, absolute humidity in grams per cubic metre, and the relative humidity right at your coldest surface, which is where mould actually starts once it stays above 80%.
Open Indoor Humidity Comfort Guide on AltFTool — it loads instantly in your browser.
Add your input to the workspace.
Adjust the options until the result looks right.
Copy or download the output and put it to work.
Calculates humidity at the cold surface, which is what decides mould, rather than the room average.
The band narrows as you tick the concerns that apply, each with the reason attached.
Converts the gap to target into grams of water to add or remove for your room volume.
The US EPA and ASHRAE both point to 30% to 50% relative humidity, with 60% as an upper bound for controlling mould and dust mites. If anyone in the house has a dust mite allergy, keeping it under 45% is better, because mites cannot maintain their water balance below roughly 50%.
Because mould responds to the humidity at the surface, not in the middle of the room. Air at 22 °C and 55% has a vapour pressure that corresponds to about 91% relative humidity against a 14 °C window reveal or cold external wall — comfortably past the 80% threshold used in EN ISO 13788 as the mould growth criterion.
The dew point is the temperature at which the air becomes saturated and water condenses out. If any indoor surface is at or below the room's dew point, water will bead on it — so a dew point of 12.5 °C means single glazing at 10 °C will stream while the room reading still looks reasonable.
Try heating less first. Warming outdoor air without adding water is what drives winter relative humidity down, so a degree less on the thermostat often lifts humidity by two or three points at no cost. If you still need a humidifier, keep the room under 50% and clean the unit — a dirty reservoir sprays contaminants into the room.