Trickle Vents and Mould on Double Glazing: What to Do

Rung 1: Read the pattern (free)

Look at the windows on a cold morning before you wipe anything. GOV.UK describes condensation damp as moisture that cools and condenses onto colder parts of the building, for example window frames and corners.

• Only bedroom windows wet, worst on waking: breathing and sleeping in a closed room.
• Only kitchen or bathroom windows: steam is not leaving the room.
• Wet glass and black spots on the seal and frame corners: condensation sitting on the coldest part of the window.
• Mist or droplets between the panes: the sealed unit has failed, which is a glazing fault and a job for the window company, not a damp problem.

Rung 2: Open, clear and wipe (free to cheap)

• Open the trickle vents. GOV.UK guidance asks whether windows have trickle vents and whether they are kept open and free from blockages. Check for paint, dust and insect screens. If you have no vents, GOV.UK guidance suggests the window can be left in a vent position, partly open but locked, as long as security is not compromised.
• Wipe the glass and sill each morning. Cambridge City Council advises wiping condensation off windows and windowsills with a sponge, and opening windows for around ten minutes, especially first thing, so the water does not soak into frames.
• Use the extractor fans. GOV.UK guidance asks whether kitchen and bathroom extractors are working and used, and suggests lids on pans and closing internal doors when cooking or showering.
• Clean the mould off hard surfaces. GOV.UK guidance says a mould and mildew product is preferred to bleach, and the surface should be left to dry completely. Wear a mask, gloves and goggles.

Rung 3: Fix the cause (cheap to moderate)

If the vents are open, the fans work and the mould keeps returning, check what changed. The Property Care Association says double or triple glazing and extra insulation can have the unintended effect of trapping moisture in the home, so ventilation has to keep up. GOV.UK guidance adds that some energy efficiency improvements reduce ventilation, and says replacement windows must have trickle vents under building regulations, controllable by the occupant rather than permanently open. Installing windows is controlled building work and needs a competent installer.

Press a timber frame gently with a screwdriver tip. The Property Care Association lists decay in window frames among the effects of ignored condensation, so soft wood is a job for a joiner as well as a moisture fix.

Rung 4: When mechanical help makes sense

If vents, habits and extraction are sorted and the home still runs wet, controlling humidity across the home can help. A Dryzen system is a loft-mounted dehumidifier with insulated ductwork to the central area or stairway. Each unit covers 150 square metres and extracts 12 litres a day, at about 20 pounds a year in energy, and the target humidity of around 50 to 55 percent can be adjusted in the app. It helps stop the conditions mould needs; it does not remove mould that is already there. It needs a loft and is not suitable for rented homes unless the landlord authorises the changes. It will not fix a failed sealed unit, a leak or rising damp.

Mistakes that keep window mould coming back

• Taping over or painting the trickle vents because they feel draughty.
• Drying washing on radiators in a closed room with the windows shut.
• Wiping the mould and never wiping the condensation, so the seal stays wet all winter.
• Assuming new windows will cure it. They reduce draughts, which can make ventilation worse if nothing replaces it.

In Chatham and the Medway Towns

Windows in the area range from older timber frames to newer uPVC units, and the order of checks is the same for all of them. If your home is listed or in a conservation area, ask Medway Council, the local planning authority, before replacing windows. For local pages see Rochester and Gillingham.

Questions we are asked

Related guides

• Condensation on Windows: What It Is Telling You
• Bedroom Condensation Overnight: Why Windows Run With Water by Morning
• Wiping Condensation and Cleaning Light Mould
• Bathroom and Kitchen Extraction That Actually Works

Local help

• Damp, mould and condensation help in Rochester
• Damp, mould and condensation help in Gillingham

Sources: GOV.UK: Understanding and addressing the health risks of damp and mould in the home; Property Care Association: Condensation; Cambridge City Council: A guide to managing condensation and mould in your home