Your summerhouse is a lovely thing in July, but winter is when trouble quietly moves in. Damp air, cold surfaces and closed-up spaces create the perfect conditions for mould, mildew and that unmistakable musty smell. The good news is that keeping a garden hut dry through the colder months is not complicated. It is mostly about airflow, a few well-placed damp traps and a bit of common sense about soft furnishings.

Why winter damp is different

In summer, warm air holds moisture easily. In winter, the same air cools down fast, especially overnight, and cool air cannot hold as much water vapour. That excess moisture has to go somewhere, so it settles on the coldest surfaces: window panes, metal fixings, wall corners and the underside of a wooden roof. This is condensation, and it happens even in a hut that feels dry to the touch.

There is a second source, too. If you use your summerhouse occasionally in winter, perhaps with a heater or a kettle, you add warm, moist air to a cold space. When you leave, that air cools and releases its moisture. Over weeks, that cycle feeds mould. The aim is not to seal the hut up tight, but to manage the moisture that inevitably gets in.

Ventilation: the simple fix most people skip

Air needs to move. A closed-up summerhouse with no ventilation is a damp trap waiting to happen. On a dry, breezy day, open the door and any windows for an hour or two. Even in January, that exchange of air can make a real difference. Avoid opening up on misty, drizzly days, as you will simply let more moisture in.

• Check for existing vents and keep them clear of cobwebs, boxes and clutter.
• If you have no vents, consider fitting a couple of low-level air bricks or slot vents, plus a high-level vent near the roof ridge, so air can flow through.
• Do not block the gap under the door with a draught excluder all winter. That gap is often the only ventilation a small hut gets.
• If your hut has windows that open, leave them on a latch rather than fully closed during dry spells.

Cross-flow matters more than volume. Two vents at opposite ends of the building will do more than one large vent in a single wall.

Damp traps and dehumidifiers: what actually works

Damp traps are cheap, silent and effective in small spaces. The crystal-filled tubs and rechargeable silica gel units are ideal for a summerhouse. A single 1kg crystal trap can last six to eight weeks in a small hut, depending on the weather. Place one in each corner and one near any damp-prone wall. Rechargeable units often change colour when saturated, then you dry them out on a radiator or in an airing cupboard.

• Electric dehumidifiers with a humidistat are excellent if you have power. Set it to 50–55% relative humidity and empty the tank regularly.
• Do not rely on bowls of salt or cat litter. They absorb very little and quickly become useless.
• Position damp traps away from direct contact with walls, so air can circulate around them.
• If you use a heater, choose a low-output tubular heater rather than a fan heater. It warms surfaces gently without blasting moist air around.

Cushions, curtains and soft furnishings

Soft furnishings are moisture magnets. Foam cushions, fabric curtains and rugs hold damp air and then grow mould on the underside. The simplest solution is to make them removable. Take cushions home at the end of the season, or at least bring them indoors during wet spells. Store them in breathable cotton bags or pillowcases, never in plastic bags, which trap condensation and encourage mildew.

• Vacuum cushions and curtains before storing to remove dust and spores.
• Wash removable covers according to their care label, then dry them thoroughly before packing away.
• Leave curtains open rather than bunched up. Folded fabric creates dark, damp pockets.
• If you must leave cushions in the hut, stand them on edge on a slatted shelf, not flat on a bench or floor.

Check the shell before the worst weather

A dry summerhouse starts with a sound structure. Before the first hard frost, spend half an hour checking the outside. Clear leaves from gutters and downpipes so water runs away from the base. Look for cracked or missing roof felt, loose shingles and failed sealant around windows and doors. Replace any cracked glazing putty and touch up bare patches of paint or wood stain, as exposed timber soaks up moisture like a sponge.

Inside, lift stored items off the floor onto slatted shelves or pallets. Move plant pots and compost bags away from the walls, as they release moisture as they dry. If your hut sits on a concrete base, check that the damp-proof course is intact and that soil or paving outside is not bridging it.

A simple winter routine that keeps mould away

You do not need to visit every day. A ten-minute check every fortnight will keep most problems at bay. Open up on a dry day, empty and refill damp traps, wipe any condensation from windows and feel the corners of the floor for dampness. If you spot a small patch of mould, wipe it with a diluted white vinegar solution and dry the area thoroughly. Then work out why it appeared: usually airflow, a leak or a forgotten cushion.

• Week 1: ventilate, check traps, wipe windows.
• Week 3: turn cushions, check under rugs, look at roof and gutters after storms.
• Monthly: recharge or replace damp traps, sweep the floor, check for musty smells.

With a little airflow, a few damp traps and a habit of taking soft furnishings home, your summerhouse will come through winter smelling fresh and ready for the first sunny morning of spring.