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Why a summerhouse goes cold in winter — and what insulation solves

Most garden buildings are sold as fair-weather rooms. The walls are a thin skin of timber, the floor is a single sheet of boarding over joists, and the roof is felt over board. None of that holds warmth, so by October the summerhouse becomes a store for deckchairs rather than somewhere you actually want to sit.

The good news is that you don't need a builder's budget to change that. A sensible layer of insulation in the floor, walls and ceiling, plus a bit of attention to draughts, will keep a summerhouse comfortable from early spring right through to late autumn — and perfectly usable in winter with a modest heater ticking over. The trick is to insulate thoroughly while still letting the structure breathe.

Two principles run through everything below. First, insulation slows heat loss. Second, ventilation carries moisture away. Get one without the other and you'll either stay cold or create a damp problem that rots the very timber you're trying to protect.

Work with breathable materials, not against them

Timber garden buildings move with the seasons. If you seal a vapour-tight material on both sides of a wall, any moisture that gets in — from cooking, breathing, wet coats, or simply damp air — has nowhere to go. It condenses inside the cavity, and rot follows.

So favour vapour-open insulation: mineral wool, wood fibre, sheep's wool or hemp. These allow moisture to pass through and dry out. Rigid foam boards work well too, but they're best used where you can be confident about the ventilation path behind them.

Whatever you choose, leave a clear air gap of at least 25mm between the back of the external cladding and the insulation. Don't cram insulation right up against the boards. A typical wall build-up looks like this: external cladding, 25mm ventilated cavity, breathable membrane, insulation between battens, then your internal lining.

• Keep the cavity open at the bottom and top of each wall so air can move through it.
• Never block existing vents, soffit gaps or air bricks — they're doing a job.
• Allow a small gap around window and door frames rather than sealing everything solid.
• Use a breathable membrane, not plastic sheeting, on the cold side of the insulation.

Start with the floor — it's the cheapest win

Cold feet make a room feel cold long before the thermometer agrees. If you can get underneath the building, push 50–100mm of mineral wool or rigid board between the joists and hold it in place with netting or battens. Make sure the underside stays ventilated so air can move beneath.

If there's no access, lay rigid insulation boards directly on top of the existing floor and put a floating deck over them — tongue-and-groove boards or 18mm ply. Even 25mm makes a noticeable difference; 50mm transforms it. Use a breathable underlay beneath any carpet or mat so moisture can escape rather than sitting against the timber.

Walls: battens, insulation, lining

Screw vertical battens to the inside of the frame at around 400mm centres, fill the gaps between them with insulation, then line over the top with ply, MDF or matchboard. Between 50mm and 75mm of mineral wool is a good sweet spot for a garden building — enough to take the edge off, without eating too much floor space.

Where space is tight, 25mm rigid board plus a 25mm air gap behind it does a reasonable job. Don't forget the door: a panel door can be insulated by cutting rigid board to fit inside the frame, and a heavy lined curtain across the doorway makes a surprising difference on a cold evening.

The roof is where most heat escapes

Warm air rises, so an uninsulated roof will quietly drain everything you've gained below. Unless your summerhouse already has a lined ceiling, the simplest approach is to add one rather than trying to insulate between sloping rafters.

Fix ceiling battens, fill with 100mm of mineral wool, then line with plasterboard or ply. Above the insulation, keep a ventilated void that connects to the eaves so moisture can escape. If you are insulating between rafters, maintain a 50mm air gap above the insulation and never block the flow of air at the eaves or the ridge.

Draughts, ventilation and a little gentle heat

Once the structure is insulated, draughts become the main enemy. Fit brush or rubber seals around doors and windows, and run a bead of sealant into obvious gaps at the corners. Secondary glazing — a clear plastic sheet or a second pane fixed inside the frame — is cheap and effective.

But don't seal the building airtight. Leave a trickle vent open, or air the room for ten minutes each day. If you notice condensation on the windows, that's your cue to ventilate more, not less. A small dehumidifier is a useful backup during damp spells.

With the shell sorted, heating becomes a minor matter. A thermostatically controlled electric heater, an oil-filled radiator or a tubular greenhouse heater of around 1kW will hold a comfortable temperature rather than fighting the weather. Insulation doesn't just keep you warm — it makes every unit of heat you pay for go much further.