Black speckles along the rubber seal or a grey bloom on the sill are common in Ashford homes, and the window is rarely the real culprit. Here is how to work out why it keeps coming back, cheapest step first.
Mould on double glazing grows where condensation keeps the frame, seal or sill damp. Windows are the coldest surfaces in the room, so they show the problem first. Clean the mould off, wipe the glass dry each morning, use your trickle vents and extractor fans, and only call a glazier if water is trapped between the panes or the seals have failed. A dehumidifier is the last rung, not the first.
Mould on windows usually appears in three places: the rubber gasket where the glass meets the frame, the bottom corners of the frame where water runs and pools, and the sill or the reveal (the side wall of the window opening). Black dots on a seal are surface mould feeding on damp and dirt. A darker patch spreading across plaster beside the window points to a cold, damp wall rather than a window fault.
Look at the glass too. Misting between the panes, or a permanent cloudy film that will not wipe off, means the sealed unit has failed. That is a glazing fault and not a ventilation one. For mould elsewhere on the wall see our guide to black mould in bedroom corners.
Work down the list and give each step a couple of weeks before moving on.
Ashford Borough Council's flood risk assessment describes the area's catchments as predominantly Chalk, with outcrops of Gault Clay, Lower Greensand and Weald Clay along the south-west margins, and notes that the East Stour rises on the Weald Clay. That is background, not a diagnosis of your windows.
What it does tell you is that a window problem here is best settled by measurement. BRE describes recording temperature and relative humidity in several rooms over time as the most effective way of diagnosing damp. A cheap thermometer and humidity meter beside the worst window, read each morning for a week, will show whether the room air is the issue.
If water is getting in around the frame, a seal has failed or a gutter is overflowing onto the wall below the window, drying the air will not fix it. Rising damp needs a specialist survey. Existing mould must be cleaned off, not left for a machine to deal with. A Dryzen system needs a loft to mount the unit and is not suitable for rented homes unless the landlord authorises the changes.
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, has an activated carbon filter and uses about 20 pounds a year in energy. The default target is 50 to 55 percent humidity, adjustable between 40 and 70 percent, and the smart app shows temperature and humidity (it needs 2.4GHz Wi-Fi and internet). A free survey will say honestly whether it suits your home or whether the fixes above come first.
Mould needs moisture and a surface that stays damp. Window frames, rubber seals and sills are among the coldest surfaces in a room, so moist indoor air condenses on them first and the water often sits there for hours. Wipe it off daily and cut the moisture at source.
Small patches on hard surfaces and seals can usually be cleaned with a suitable mould and mildew product, wearing gloves and a mask, then left to dry fully. GOV.UK guidance says larger areas of mould should be left to qualified professionals.
Not usually. Mould on the surface is normally a moisture and ventilation problem. New windows only make sense if the seals have failed (water or misting between the panes), frames are damaged or the windows no longer close properly.
Sources: GOV.UK: Understanding and addressing the health risks of damp and mould in the home; Stoke-on-Trent City Council: Damp, mould and condensation; BRE: Diagnosing the causes of dampness in buildings; Ashford Borough Council: Strategic Flood Risk Assessment (2014)
Last reviewed 2026-10-10. General guidance only; this is not a survey of your home.