Condensation in a Garden Room

A missing or punctured vapour control layer

Warm internal air carries moisture, and in a timber-framed building it will move into the wall build-up unless a continuous vapour control layer stops it on the warm side of the insulation. Once that air reaches the cold outer sheathing, the moisture condenses inside the structure — you see it as damp patches at the base of walls, or smell it before you see it.

Failures usually come from the layer being taped at the sheets but cut through afterwards for sockets, lights and cable runs without being resealed. On existing buildings this is difficult to correct without opening up, which is why it belongs in the specification rather than the snag list.

No ventilation path

A modern garden room can be more airtight than the house it sits behind. Two people working in it produce a surprising amount of water vapour, and with the door shut and no trickle vents there is nowhere for it to go.

• Trickle vents in the window and door frames as a minimum
• A humidistat-controlled extract fan where there is a shower, kitchenette or heavy occupancy
• A clear ventilation gap behind the cladding so the outer skin can dry
• Ventilation to the sub-floor void where the building sits on a raised deck

Intermittent heating

Heating a garden room hard for two hours a day and letting it drop to outside temperature overnight guarantees condensation. Every surface is cold when the warm, humid air arrives, and the glass streams within minutes.

Low, continuous background heat costs less than most people expect in a properly insulated building and keeps surface temperatures above the dew point. It is the single most effective change for a building that is already built.

Cold bridging at the junctions

Insulation is easy in the middle of a panel and difficult at the corners, at the floor-to-wall junction, around door and window frames and where the roof meets the walls. Those junctions are where mould appears first if the insulation was not continuous.

Damp only in corners, or a stripe of mould along the base of one wall, points to a cold bridge rather than a general ventilation problem, and can often be improved with internal insulated lining to that junction alone.