Mould and damp in a conservatory

Mould spores are everywhere, and they grow wherever a surface stays damp for long enough. A conservatory gives them plenty of chances: cold glass, metal bars, rubber seals, blinds hanging close to the glazing, and corners where air hardly moves. Cleaning it off is the easy part. Keeping it off means finding where the water comes from.

Why does my conservatory get mouldy?

There are three sources of water, and it is worth working out which one you have before you spend anything.

• Condensation. Warm, moist air meets the cold roof and frames and turns back into water. This is the most common source in winter, and it shows up on cold mornings whether or not it has rained. Our page on conservatory condensation explains the physics and the fixes.
• Leaks. Rain getting in through gutters, seals, damaged panels or flashing. It shows up during or after rain, in one place. See leaking conservatory roof.
• Damp walls. Guardian’s installation guide advises checking the existing house wall for signs of damp before a new roof goes on. Water can also run down inside a cavity wall and come out where the conservatory roof meets it; our page on leaks where the roof joins the house covers that.

Two things make it worse. Cold bridges, such as aluminium bars or panel skins without a thermal break, stay colder than the surfaces around them, a risk one solid-roof installer highlights in some budget panels. And winter, when the windows stay shut and the roof is at its coldest, gives mould the long damp spells it needs.

How to get rid of mould in a conservatory

1. Find the water first. Note whether the damp appears after rain, on cold mornings, or all the time on one wall.
2. Clean it off hard surfaces with a mould remover made for that surface, following its label, with the windows open.
3. Wash or replace mouldy blinds and fabrics. Blinds that hang close to cold glass stay damp longest, so open them during the day to let air reach the glazing.
4. Air the room every day. Which? says ventilation is essential in a conservatory and recommends opening doors and using small vents at the top of window panes. SupaLite recommends trickle vents around the window frames when its roof goes on.
5. Cut moisture at source. Do not dry washing in the conservatory, and keep the kitchen door shut when cooking.
6. Run a dehumidifier and aim for relative humidity below 80%, the figure one LABC roof certificate uses. A cheap hygrometer shows you the reading.
7. Keep a low, steady heat on an independent heater with its own controls, which keeps surfaces warmer without affecting the building regulations exemption.
8. Fix any leak or damp wall before you redecorate, or the mould will return in the same place.
9. Deal with the cold surface if condensation keeps coming back despite all of the above (see below).

For the ventilation options in more detail, see our conservatory ventilation guide.

Black mould on seals and window frames

Seals, frame corners and the bottom edges of glazing are where condensation runs and collects, so they are often the first places black mould appears. Clean them regularly in winter and dry them off on bad mornings. Seals that have perished or cracked hold water and are hard to clean, so replacing them is worth considering. If the mould is only on one wall of the house, not the glazing, suspect damp in that wall rather than condensation.

Can a new solid roof get mould?

It can, and one certified system is unusually frank about it. The LABC Registered Detail certificate for Ultraframe’s LivinRoof says research suggests “plaster and paper surfaces should be avoided (including plasterboard) as there is a potential likelihood of mould growth - although the research suggests surface condensation should not occur”. It requires the homeowner to be told to keep up a thorough ceiling cleaning regime over the life of the roof, and to maintain ventilation to keep relative humidity below 80%.

Other makers rely on vapour control: foil-backed insulation with taped joints (Guardian), every joint sealed to create a vapour seal (Leka). Among the low-star themes on Guardian’s Trustpilot page is condensation. None of this means solid roofs are prone to mould; it means a warm roof still needs a ventilated room, and a badly built one can trap moisture.

When is the roof the real cause?

If you ventilate daily, keep humidity down and the mould keeps returning on or near the roof, the roof surface is too cold, or it is letting water in. That is when replacing it pays off.

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

A lower U-value means a warmer inside surface, so less condensation to feed mould. Insulated roof panels are the budget option; a tiled roof is the most thorough. Our conservatory roof costs page has prices by style and size. Installers usually quote including VAT, but check.

When you get quotes, ask each installer how the roof controls condensation (the vapour control layer, taped joints and thermal breaks), what ventilation they will fit, and for a copy of the maintenance guide.