The short answer
Condensation forms when warm, moist air inside the conservatory touches a roof surface that is colder than the air's dew point. Glass and thin polycarbonate are poor insulators, so they are often the coldest surface in the room. More ventilation, less moisture indoors, and a better-insulated roof all reduce it. If it persists through dry, mild weather, look to the roof itself rather than the day's weather.
Where the water comes from
Air always carries water vapour, and warm air carries more than cold. When that air touches something cold, it cannot hold on to the moisture and it turns to droplets. Breathing, cooking, drying washing, houseplants, a hot tub or an unflued gas heater all add moisture to the room.
A conservatory sits at the edge of the house, so it has big areas of thin material between warm interior and cold exterior. Single-layer glass and twin-wall polycarbonate are poor insulators compared with a solid wall or a tiled roof, and on a cold night the inside face of the roof drops below the dew point easily. Water collects, runs to the lowest point and drips onto the frame, the floor or the furniture.
The pattern gives clues. A film that appears on cold mornings and clears by noon is normal for a room like this. Moisture that lingers all day, forms inside twin-wall sheets, or leaves black mould along the bars points to something else: poor ventilation, failed seals or a roof that no longer insulates at all.
It is worth separating condensation from a true leak before spending anything. A leak drips from one spot, usually near a joint, and gets worse in rain. Condensation covers the whole surface, gets worse when the room is cold and humid, and often drips from the lowest edge of every sheet at once. Wipe a patch dry and see how fast it returns: a leak keeps arriving while the rain does, and condensation reappears as the air changes.
Cheap steps that reduce it
Before spending on the roof, tackle the moisture and the airflow. These measures cost little and make a visible difference.
- Ventilate. Open roof vents or windows for a short period each morning, even in cold weather. Moving air carries moisture out before it settles.
- Use a dehumidifier in a room that is closed off for long periods, and keep drying clothes out of it.
- Keep the room warm enough. A slightly heated conservatory raises the surface temperature of the roof and lowers the risk.
- Wipe and dry. A squeegee or a cloth removes the water before it stains the frame.
- Check seals. Failed seals on double-glazed units let moisture inside the unit, where you cannot wipe it away.
Our page on conservatory roof insulation explains what happens when the surface temperature is addressed directly.
Why polycarbonate struggles
Twin-wall polycarbonate looks insulating because of its air channels, but it lets heat through quickly compared with a proper roof. Its surface cools fast at night, and the channels can also take in moisture through unsealed ends. When tape at the ends fails, dust and water get in, and the sheet goes cloudy and green inside. That is a sign of age, not a fault in your housekeeping.
HomeBuilding and The Eco Experts both suggest roughly 10 to 15 years for a polycarbonate roof, and sheets near that age lose their seals and clarity. Our page on polycarbonate roof lifespan explains the signs to look for. Replacing a sheet may help a little, but the room will still be cold, so the better question is whether the whole roof should change.
Blinds, vents and what makes it worse
Some well-meant fixes do the opposite of what you want. A blind pulled across the inside of the roof traps a layer of cold air against the glass, and moisture settles behind it. Our guide to conservatory roof blinds covers the pros and cons. Closing the door to the house keeps the heat in the house, so the conservatory cools further.
Roof vents are the opposite. They give moist air a route out and are usually worth fitting where the design allows. Read conservatory roof vents for how they work and where they go.
Do not use cling film, sealant or thin foil over the sheet to cure condensation. They trap moisture and can cause damage to the frame behind them.
What a new roof changes
A well-insulated roof keeps its underside warm, so nothing is cold enough for moisture to settle on. A solid tiled roof does this best, with insulation, a moisture barrier on the warm side and a plasterboard ceiling. A modern glazed roof with a low-emissivity coating gets a good way towards it. A replacement also brings planned ventilation, and it lets you pick a colour and finish to suit the house. See how to match a conservatory roof to the house for the options, or start from the main conservatory roof replacement page.
Changing to a solid roof is a material alteration under the Building Regulations, so a building control application has to show the structure, energy performance and ventilation are right. We explain that process and look after the paperwork on notifiable jobs. Anyone unsure about their own case can put the question to the building control body before work begins.
How we can help
The survey checks roof and frame, sheets and seals, and photographs the evidence for you before a written quote follows. If better airflow and one repaired seal would cure the damp, the quote reflects that and goes no further. If the roof is the real cause and replacement is justified, you will see the options for covering and finish side by side, with reasons.
A 10-year written workmanship guarantee stands behind roof work, and tidy-up is part of every visit. Phone 01793 626096, or write through WhatsApp or the form below the page.