Drying Out a Damp Bay Window

Why bays are worse than flat windows

A bay projects out of the building, so it has three or five exposed faces plus a roof and a floor, all of them thin, all of them cold. It has more glass per square metre of floor than any other part of the house. Its cills are frequently below the level of the internal floor structure and are cold bridged straight to outside. And it usually contains a radiator directly under the glass, pushing warm moist air onto the coldest surface in the room.

Diagnose before you spend

Condensation shows as even misting on the glass and water pooling on the cill, worst on cold mornings. Penetrating damp shows as a patch that darkens after rain, often around the cill ends or where pointing has failed. Rising damp is rare at bay cill level. Salts, blown plaster and a tide mark suggest something other than condensation. Treating one as the other wastes money.

• Even misting: condensation
• Wet after rain: penetrating damp
• Blown plaster and salts: investigate further
• Mould in corners: cold bridging plus humidity

Fix the building defects first

Check the bay roof covering and its flashing, the pointing between stone or brick and the frames, the cill throating and its drip, and whether the external ground level has risen against the bay. A bay with a leaking flat roof will keep being damp regardless of glazing. This is unglamorous work but it is the foundation.

Then treat the glazing

Secondary glazing across the full bay does two things for damp. The room side pane sits close to room temperature, so warm moist air no longer meets a cold surface and stops condensing there. And the continuous seal stops the cold draught that was chilling the whole bay area. Each facet gets its own panel, the corner posts seal against each other, and the head and cill details follow the actual geometry rather than a standard section.

Reconsider the radiator

A radiator under a bay window is an old convention from the days of uninsulated glass, intended to counter the downdraught. Once secondary glazing is fitted, the downdraught largely disappears and the radiator position matters less. If you are redecorating anyway, moving it, or at minimum fitting a reflective panel behind it and keeping curtains from covering it, improves both comfort and condensation.

Ventilate the bay area

Bays often sit behind heavy curtains that trap a pocket of cold still air. That pocket is where mould grows. Trickle vents in the secondary frames, curtains that stop at cill level rather than sealing the bay off, and a shelf that does not block air movement all help. Ventilation and warmth together beat either alone.

The order of work

Roof and pointing, then any plaster repairs, then glazing, then decoration, then furniture layout. Doing decoration before the glazing means doing it twice, and doing glazing before fixing a leaking bay roof means an expensive product sitting in front of an ongoing problem. We will tell you if we see building defects at survey rather than fitting over them.