Guide

What humidity level prevents mould?

Keep surfaces below about 80% relative humidity and room air below 60%, and — more importantly — keep any excursion above that short. Mould needs several continuous hours above the threshold to germinate, so brief peaks that clear are harmless while a persistent damp corner is not.

The targets

Room air: below 60% relative humidity, sustained.
Surfaces: below about 80%, which is the actual germination threshold.
Not below about 40%, because there is no further mould benefit and dry air has its own costs.

The 30–50% band that comfort guidance recommends sits comfortably inside all of that, which is convenient — the range that suits people also suppresses mould and dust mites.

Why duration beats the peak

This is the part that changes what you actually do.

Mould germination needs several continuous hours above the threshold, and visible growth needs that repeated over days or weeks. A single excursion does nothing.

Two situations with the same peak humidity and completely different outcomes.
	Bathroom after a shower	Cold north corner
Peak surface RH	~95%	~85%
Duration	20–30 minutes	10 hours a night, all winter
Repeats	Daily, but always clears	Daily, never clears
Mould?	No, if it dries	Yes

The bathroom has the higher peak by a wide margin and is not the problem. The corner is.

Which reframes prevention: the goal is not holding a number, it is making sure wet surfaces dry again quickly. Air movement does that without changing the room average at all, which is why an extractor fan run for ten minutes after a shower is worth more than a dehumidifier running all day in the hallway.

The two levers, and which to reach for

Same pair as everywhere else in this cluster, because it is the same physics:

Raise the surface temperature. Insulate the cold spot, heat the room, pull the furniture off the wall so air can reach it, open the curtains. This attacks the problem where it actually is.

Lower the dew point of the air. Extract at the source, dry laundry elsewhere, ventilate when the outdoor air is drier, or run a dehumidifier.

For a specific cold corner, the first lever is almost always better — the room is fine and one surface is not. For a generally damp building, the second.

Ventilating: check first

Opening windows only dries a space if the incoming air carries less water than the air it replaces.

In winter that is nearly always true — cold air holds very little water whatever its percentage says, so bringing it in and warming it is powerfully drying.

In a mild, damp spell it can be false. Outdoor air at a 60°F dew point will not dry a thing, and ventilating with it raises the indoor dew point instead.

The outdoor dew point settles it, and it is what this site shows — the half of the comparison most people never have.

Removing what is already there

Lowering the humidity stops growth but does not kill it. Mould goes dormant in dry conditions and resumes when moisture returns, so existing growth has to be physically removed as well as the conditions corrected.

If it keeps coming back after the surface has been dried and kept dry, the moisture is arriving from somewhere else — a leak, a bridged damp course, a failed seal — and that is a building problem rather than a humidity one.

What this site measures

Outdoor air. That tells you whether ventilating will help, and it is genuinely useful. It cannot tell you the humidity at a surface in your building — that needs an instrument against that surface, and no phone has a humidity sensor.

Common questions

What humidity should I keep my house at to prevent mould?

Below 60% in the room air, which for most buildings keeps cold surfaces under the 80% surface threshold. If you have a specific problem corner, the surface there is what actually matters and it will be considerably damper than the room average.

Is 65% humidity too high?

Sustained, yes — it is above the usual guidance and will put cold surfaces past the mould threshold. Briefly, no. A bathroom at 90% for twenty minutes after a shower is completely normal and does not cause mould if it clears.

Does a dehumidifier prevent mould?

It can, by lowering the dew point of the air so that surfaces stay below the threshold. It does nothing about why a surface is cold, so insulation or heating is often the more effective intervention for a specific cold spot.

How dry is too dry?

Below about 30% most people get dry skin, irritated airways and static, and timber and glued furniture can crack. There is no mould benefit to going below 40%, so drying a space out aggressively trades one problem for another.

Will mould die if I lower the humidity?

It stops growing and goes dormant rather than dying. Spores survive dry conditions indefinitely and resume when moisture returns, which is why existing growth has to be removed as well as the conditions corrected.

Sources

Published 12 August 2026