Why the underside of my glass roof drips on cold mornings

The three kinds, and telling them apart

Wipe the glass. If the moisture comes away from the inside face, that is interior condensation caused by humid air. If the surface you can reach is dry but there is dew on the outside, that is exterior condensation, which happens because the glass has stayed cold overnight rather than bleeding heat out of the room. If you cannot wipe it away from either side, the moisture is inside the sealed unit and the unit has failed.

Interior

wipes off inside, caused by warm damp air meeting cold glass

Exterior

dew on the outer face at dawn, harmless and clears with the sun

Interstitial

trapped between panes and cannot be wiped, unit replacement

Why glass roofs collect it more than walls

A roof faces the night sky, which is far colder than the air around it. On a clear still night a horizontal or shallow pitched pane radiates heat upwards and can drop several degrees below the ambient temperature, so it is the coldest surface in the room by a margin. Warm air rises, hits it, and the moisture it carries has nowhere to go. That is why the drips appear at the roof rather than on the vertical glazing, and why they are worst on the first clear night after mild wet weather.

Where the moisture is coming from

A household produces a startling amount of water vapour and most of it has to leave through the air. Drying clothes indoors, an unvented tumble dryer, cooking without an extractor, a hot tub or spa near the doors and even a lot of houseplants all raise indoor humidity. Where a glass room adjoins a kitchen or utility area with the doors left open, it will always be the room that shows the moisture first, because it holds the coldest surface.

• Washing dried on airers inside the glass room
• A hot tub or plunge pool immediately outside open doors
• Cooking steam drifting through from an adjoining kitchen
• Heating the space in short bursts rather than gently and steadily

What actually reduces it

Ventilation and gentle background heat beat every gadget. A trickle vent or a roof vent left slightly open lets moist air out before it condenses, and steady low level warmth keeps the glass closer to the room temperature. Where a glass room is used year round, upgrading the glazing specification changes the sums, because a warmer inner pane simply does not reach dew point as often. Dehumidifiers work, but they treat the symptom while the vent treats the cause.

• Leave a vent or window slightly open, even in winter
• Use low steady heat rather than a blast of hot air
• Move drying laundry out of the room entirely
• Consider a roof vent if the space has none
• Wipe the frames dry so water does not sit on seals

When it is not condensation at all

There is one honest exception. If the water appears in the same spot every time, runs along a glazing bar, or arrives during rain rather than after a cold night, that is a leak and needs looking at as one. Condensation is spread across a surface, appears in still cold conditions, and clears through the morning. A leak is localised, arrives with weather, and gets worse rather than better as the rain continues.

Condensation is general and clears by mid morning. A leak is local and follows the rain. If in doubt, note the weather each time it appears.