Musty smells and mould in a garden building: finding the moisture source

Three sources, three different fixes

Moisture in a garden building comes from outside the fabric, from the ground below it, or from the air inside it. Each leaves a different signature.

Ingress from outside is localised and weather driven. Ground moisture affects the lower part of the room, is worse after wet spells and often shows first on skirtings, low shelves and anything stored on the floor. Air borne moisture is general, worst on cold surfaces and cold corners, and is closely tied to how the room is used and heated.

Read where the mould has appeared

Mould does not grow at random. It settles where a surface stays damp longest, so its location is a diagnostic.

Cold external corners and behind furniture pushed against a wall: still air and a cold surface · Around window reveals and frames: a thermal weak point, or a failing seal · Along the base of walls and on floor level belongings: moisture coming up or in low down · A single patch on a ceiling or high on one wall: look for ingress above it · On leather, fabric and paper before it appears on walls: a room wide humidity problem

Rooms that are shut up for weeks

A garden building used every day usually smells fine. One that is locked from November to March very often does not. Without heating, the fabric drops to outside temperature. Without ventilation, whatever moisture is in there condenses and stays.

This is the most common cause of a musty room that has no leak at all. Even minimal background heating and a small amount of continuous ventilation keeps the internal surfaces above the point where moisture settles, which prevents the smell rather than masking it.

What is being kept in there

Storage matters more than people expect. Cardboard boxes on the floor, garden cushions put away slightly damp, sports kit, tools brought in wet and firewood all release moisture and give mould something absorbent to grow on.

If a room doubles as an office and a store, the store side will usually be where the smell begins. Lifting everything off the floor and leaving an air gap behind stacked items makes a measurable difference on its own.

Clearing it properly

Deal with the source first, then the surface. Treating mould while the cause is still active is the definition of temporary. Once moisture is under control, affected surfaces are cleaned back, allowed to dry fully and redecorated with a suitable coating.

Where mould has been present a long time, absorbent materials that have held moisture may need to come out rather than be cleaned. Plasterboard, insulation and softwood that have stayed wet do not recover simply because the room has dried.

Keeping it away

The lasting answer is a room that stays a little warmer than the outside air and that always has a small amount of air movement. Continuous low level ventilation costs very little to run and is far more effective than opening a door for an hour once a week.

Outside the building, the same rules apply as anywhere else: keep soil and planting away from the cladding, keep the ground below floor level, make sure rainwater is taken away from the base, and cut back anything that keeps a wall permanently in shade and permanently damp.

Frequently asked questions

Is a musty smell always mould?

It usually indicates microbial growth somewhere, though it can be in carpet, stored items or under a floor rather than on a visible wall.

Will a dehumidifier solve it?

It manages symptoms well and is genuinely useful in a room shut up over winter, but it does not stop water arriving from outside or from the ground.

Why is mould only in the corners?

Corners are colder than the middle of a wall and air barely moves there, so moisture settles and stays on those surfaces longest.

Should I leave the heating on in winter when the room is not used?

A low background level does more good than occasional bursts. Keeping surfaces above the temperature where moisture settles is what prevents growth.

Can mould appear in a brand new garden room?

Yes, if the room is closed up before it has dried out or if it is used for damp storage from the start. It is not proof of a fault, but it is worth investigating.