What humidity should my house be, and how do I measure it?

A hygrometer costs less than a takeaway and it turns damp from an argument into a measurement. It is the single most useful thing to own if you have mould, because it separates the two questions that get confused: is there too much water in the air, or is the wall too cold? Both produce mould, and the reading tells you which one you are fighting.

What you'll need

• A digital hygrometer, ideally two or three
• Infrared thermometer (optional)

Step by step

1. Buy more than one.They are cheap and the interesting information is the difference between rooms. A bedroom at 70 per cent and a living room at 50 tells you where the moisture is coming from.
2. Place them properly.About light-switch height, on an internal wall, away from radiators, windows, doorways and any source of steam. A meter on a windowsill reads the window, not the room.
3. Let them settle.Give a new meter a few hours in position before believing it. Moving one from a cold hall to a warm room and reading it immediately gives nonsense.
4. Read morning and night.Humidity swings enormously across a day. First thing in the morning in a bedroom that has been slept in with the door shut is the peak, and that is the number that matters for mould.
5. Interpret the bands honestly.Under 40 per cent is dry — static, shrinking timber, and not a damp problem. 40 to 60 is the target. 60 to 70 is a warning band where cold surfaces are already at risk. Sustained above 70 and mould on cold surfaces is reliable rather than possible.
6. Remember the reading is of the air, not the wall.Mould grows according to humidity at the surface, and at a cold corner that can be twenty points higher than the room figure. A room at 65 per cent has corners already in the mould band.
7. Use the reading to choose your action.High humidity and a warm house means too much moisture going in — fix the sources and the extraction. Moderate humidity and mould anyway means a cold surface — fix the insulation or the furniture placement.
8. Track it over a season.One reading is an anecdote. A note of morning and evening readings across a winter shows whether a change you made worked, which is otherwise almost impossible to tell.

Tips

• Cheap hygrometers vary in accuracy. Put two or three side by side for a day: if they agree within a few points, trust the pattern even if the absolute number is a little off.
• A bedroom door left ajar overnight, or a trickle vent left open, typically drops the morning bedroom reading noticeably. It is a free experiment.
• Absolute humidity matters as much as relative for understanding this. Cold outside air brought in and warmed has a low relative humidity, which is precisely why winter ventilation dries a house rather than wetting it.

Common mistakes

• Reading it once in the afternoon and concluding the house is fine — The peak is in the early morning after a night of occupation, and that peak is what the cold surfaces experience for hours at a time.
• Assuming that if the reading is under 60 there cannot be a mould problem — The surface humidity at a cold spot can be far higher than the room air. A moderate room reading with mould in the corner points at surface temperature rather than at moisture load.

Questions people ask