How to stop condensation in a conservatory

Condensation in a conservatory is water that was in the air, not water that came through the roof. It forms when warm, moist air touches a surface colder than its dew point, and in winter a glass or polycarbonate roof is about as cold a surface as your home has. The fixes are about three things: less moisture, more air movement and warmer surfaces.

If water only appears when it rains, you have a leak rather than condensation; see our page on a leaking conservatory roof.

Why does my conservatory get condensation?

Air always carries some water vapour, and warm air can carry more than cold air. When air cools, the vapour it holds gets closer to its limit. The temperature at which it can hold no more is the dew point, and any surface colder than that collects water from the air touching it.

A conservatory roof hits that point easily, for three reasons:

• It loses heat fast. Glass and polycarbonate roofs have U-values of about 1.0 to 2.8 W/m²K, against 0.15 for an insulated roof, so their inside surface sits much closer to the outside temperature.
• It faces the night sky. On clear nights a roof loses heat by radiation to the sky and can end up colder than the outside air, which is why the worst condensation often follows a clear, cold night.
• Metal bars conduct heat. Aluminium glazing bars and panel skins carry cold through to the inside unless they have a thermal break, a point one solid-roof installer makes about some budget panels.

The moisture itself comes from everyday life: breathing, plants, cooking steam drifting in from the house, washing hung up to dry, and heaters that burn gas or paraffin inside the room, which give off water vapour as they burn.

How to stop condensation in a conservatory

1. Air the room every day. Which? says ventilation is essential, recommending open doors and small vents at the top of window panes. A short burst of fresh air swaps damp air for drier air, even in winter.
2. Use trickle vents and roof vents. SupaLite “strongly” recommends trickle vents around the window frames, and air bricks in the dwarf walls, when its solid roof goes on. Ultraframe fits trickle ventilation in the ridge of its duo-pitch glass roofs as standard. Conservatories up to 30m² are exempt from the ventilation rules in Part F, so nothing forces a builder to fit them. Our ventilation guide covers the options.
3. Cut moisture at source. Do not dry washing in the conservatory, keep the door to the kitchen shut when cooking, and avoid flueless gas or paraffin heaters.
4. Run a dehumidifier. It takes water out of the air so less condenses on the roof. We found no independent test of how well they work in conservatories, and running costs vary by model.
5. Keep a low, steady heat. Warmer air and warmer surfaces both help. Use an independent heater with its own thermostat, since extending the house heating ends the conservatory’s building regulations exemption.
6. Wipe the roof and sills on bad mornings. It stops drips soaking into floors, blinds and seals, where they feed mould.
7. Check polycarbonate sheet ends. Sheets should have solid tape at the top and breather tape at the bottom so condensation inside the flutes can escape. Misting inside the sheet means that has failed.
8. Make the roof warmer. The Glass and Glazing Federation says energy-efficient glazing reduces the risk of condensation, and an insulated roof reduces it much further.

Condensation between the panes of a glass roof

Mist trapped between the two panes of a sealed glass unit is different. It means the seal around the unit has failed, so moist air has got inside. Ventilating the room will not clear it, and the unit needs replacing. Our page on a cracked or broken roof panel explains how single units are swapped.

Will a solid roof stop condensation?

It makes it much less likely on the ceiling, because an insulated roof at around 0.15 keeps its inside surface close to room temperature. But solid roofs depend on keeping moist air out of the insulation:

• Guardian insists on foil-backed insulation with foil-taped joints.
• Leka seals every insulation joint “to create a solid vapour seal”.
• Ultraframe’s LivinRoof uses plasterboard with a metalised film “to act as a vapour control membrane”.

The LABC certificate for LivinRoof goes further. It requires ventilation to be maintained “to keep the relative humidity of the space/room below 80%”, warns that plaster surfaces carry a potential likelihood of mould growth, and says the homeowner must keep up a ceiling cleaning regime over the life of the roof. In other words, a solid roof reduces condensation but does not let you stop ventilating.

Watch out for roofs built over the old one. PanelX says wrap-over roofs can bring condensation and heat loss problems, and Bryant & Cairns warns about cold bridging through aluminium panel skins without thermal breaks. Neither comes with test data, but they are the right questions to ask.

When is a new roof the answer?

If you ventilate daily, keep the moisture down and the roof still streams, the roof is the problem. That is especially true if it is also too cold in winter or leaking.

New roof	Roof U-value (W/m²K)	Typical cost, medium conservatory
Performance glass	1.0 to 1.2	£3,500 to £7,000
Insulated panels	0.29 at the centre of the panel	£1,800 to £4,600
Tiled roof	0.15 (makers quote 0.12 to 0.18)	£7,400 to £12,100

Insulated roof panels are the budget route and a tiled roof the most thorough. See conservatory roof costs for prices by style and size.

When you get quotes, ask each installer how the roof controls condensation: the vapour control layer, how joints are taped, whether bars have thermal breaks, and what ventilation they will fit.