Mould around windows is usually caused by condensation. The glass, the frames and the walls immediately around a window are often the coldest surfaces in a room, so moisture in the air condenses there first, and mould follows where those surfaces stay damp. In most homes the remedy is a combination of reducing moisture, improving ventilation and keeping surfaces warmer. However, mould around windows can also point to a building defect, such as cold bridging at the lintel or reveals, failed seals or rainwater getting in around the frame. This guide explains why windows attract mould, the practical steps that help and the signs that a defect may be involved.
Why windows are the cold spots in a room
Condensation forms when air next to a surface cools to its dew point. Our guide to relative humidity and condensation explains the science; the practical point is that the coldest surfaces in a room are affected first, and around a window there are several.
Glass and frames
Single glazing and window frames are usually among the coldest surfaces in a home. The Glass and Glazing Federation notes that replacing single glazing with double or triple glazing helps to keep the room-side surface of the glass warmer. Older metal frames can be particularly prone to condensation, and the Federation stresses the importance of a thermal break in metal frames to prevent it.
Reveals, sills and lintels
The masonry around a window opening is frequently colder than the rest of the wall. Building standards guidance notes that thermal bridges commonly occur around openings, at lintels, jambs and sills, and that condensation may form on the inner surfaces of these areas. This is why mould often appears along the head of the window, down the reveals or on the sill board, even when the glass itself is dry. Where there are gaps in wall insulation, or insulation stops short of the opening, these areas may be colder still.
Where the moisture comes from
Condensation needs moisture as well as a cold surface. Government guidance explains that moisture from normal activities, such as showering, drying laundry and cooking, builds up in the air if it cannot be removed. Occupants also add moisture overnight by breathing, which is one reason why mould around windows is often worst in bedrooms. Drying clothes indoors without ventilation, bathing without extraction and cooking without lids can all raise humidity significantly.
Trickle vents and ventilation
Trickle vents are small, controllable openings in or above the window frame that provide background ventilation while the window is closed. Government guidance on damp and mould asks whether trickle vents are kept open and free from blockages, and notes that replacement windows are required under the Building Regulations to have them. The government’s guidance on ventilation (Approved Document F) confirms that a room already served by an adequate wall ventilator does not need further background ventilation, and that a window locked slightly ajar on a “night vent” is not an appropriate substitute.
Kitchens and bathrooms also need effective extraction. Fans that are broken, undersized or rarely used allow moisture to spread to cooler rooms, and the government guidance suggests that landlords consider humidity-controlled fans, which switch on automatically.
Window replacement and airtightness
Replacing old windows usually makes a home warmer and less draughty, but it also reduces the uncontrolled air leakage that previously carried some moisture away. Government guidance notes that improving the airtightness of a property may make it necessary to improve ventilation, to avoid unintended consequences. The Glass and Glazing Federation makes a similar point: without adequate ventilation after new windows are fitted, moisture may condense on the coldest surface, which may no longer be the window but an external wall.
From a surveying perspective, this explains a pattern that is often reported. After new double glazing is fitted, there is less condensation on the glass, but mould appears on the reveals, in the corners of the room or behind furniture. In older buildings, Historic England similarly advises striking a careful balance between reducing draughts and keeping the ventilation that helps to prevent dampness.
Curtains, blinds and furniture
Curtains and blinds help to keep heat in, but when drawn they separate the glass from the warmth of the room. Local authority guidance notes that curtains or internal blinds can increase condensation on the glass by lowering its surface temperature, and that pockets of trapped air can lead to condensation and mould. Sensible measures include:
- Opening curtains and blinds during the day to allow air to circulate around the window
- Avoiding curtains that cover radiators when drawn
- Leaving a gap between furniture and external walls
Practical steps to reduce mould around windows
Where condensation is the cause, the following measures commonly help:
- Wipe condensation from the glass, frames and sills each morning.
- Keep trickle vents open, particularly in bedrooms and the rooms used most.
- Use extractor fans during and after cooking and bathing, and keep kitchen and bathroom doors closed.
- Cover pans when cooking, and dry laundry outside where possible, or in a room with the window open and the door closed.
- Heat the home consistently and do not let rooms become very cold.
- Clean very small amounts of mould with a suitable mould and mildew cleaner, wearing a mask, gloves and goggles, as government guidance advises.
Cleaning deals with the symptom only. Our guide to black mould in the home explains the health risks and why mould returns if the cause remains.
When mould around windows signals a defect
If mould persists despite sensible heating, ventilation and use of the home, the building itself may be contributing, and government guidance makes clear that recurring mould should prompt further investigation.
Failed seals and leaks around the frame
Government guidance lists broken seals around windows, and windows that do not close fully, among the causes of penetrating damp. Rainwater can also enter through defective external sills, gaps between the frame and the masonry, or a missing or damaged cavity tray above the opening. Damp from these sources tends to be concentrated below the corners of the frame or sill, often with staining, and to worsen after rain, not in cold weather. Our guide to the types of damp explains how penetrating damp differs from condensation.
Misting between the panes
Condensation between the two panes of a double-glazed unit, which cannot be wiped away, is a different matter. The Glass and Glazing Federation lists misted glass panes among the common problems with existing windows, and misting of this kind usually indicates that the unit’s seal has failed and the unit may need replacing. It is not in itself the source of mould on the reveal.
Cold bridging at the lintel and reveals
Mould that forms in a consistent band along the head of the window or down the reveals, even in well-ventilated rooms, may indicate a significant thermal bridge. This can arise from the construction around the opening, from insulation that stops short of the frame or from gaps in wall insulation. Thermal imaging can show these cold areas clearly.
Persistent condensation
Condensation that continues despite reasonable measures may point to faulty extraction, blocked or missing trickle vents, inadequate heating, missing insulation or penetrating damp making the surrounding wall colder.
How an RICS Chartered Surveyor can help
An RICS Chartered Surveyor can establish whether mould around windows is the result of condensation, a cold bridge, a leak or a combination of these. Our damp and mould survey includes thermal imaging to identify cold spots and thermal bridging, readings of temperature, relative humidity and dew point, moisture readings to separate condensation from rain penetration, and an assessment of ventilation, insulation and heating. The written report includes photographs, a risk rating and prioritised recommendations. As Lansdown Associates does not sell windows, ventilation products or treatments, our advice is independent.
For a practical overview of the options for each cause, from short-term cleaning and ventilation to lasting building repairs, see our guide to damp and mould remedies.
Frequently asked questions
Is condensation on windows in winter normal?
Some condensation on the glass on cold mornings is common, particularly in bedrooms. It becomes a concern when it is heavy, runs onto sills and frames, lasts for much of the day or is accompanied by mould.
Why has mould appeared since new windows were fitted?
New windows are usually more airtight than the ones they replace. If trickle vents are closed or absent and extraction is limited, humidity can rise, and moisture may condense on the next coldest surface, such as the reveals or an external wall.
Should trickle vents be left open in winter?
Generally, yes. Trickle vents provide the background ventilation a room needs while the window is closed, and government guidance expects them to be kept open and free from blockages.
Does mould around windows mean the windows need replacing?
Not necessarily. In most cases the cause is condensation, which is better addressed through ventilation, heating and moisture control. Replacement may be appropriate where frames are decayed or sealed units have failed, but adequate ventilation should always be provided with new windows.
Speak to an RICS Chartered Surveyor
If mould keeps returning around your windows, our RICS Chartered Surveyors can identify the cause. Call 0151 374 0905 or email office@lansdownassociates.co.uk (Monday to Friday, 09:00 to 17:00).
