Mould Near a Bedroom Window

Waking up to streaming windows and black spots creeping across the window reveal is a very common winter problem in UK bedrooms. It is almost always caused by condensation, and it is worth dealing with properly because bedrooms are where we spend a third of our time breathing the air.

Why bedrooms are prone to it

A sleeping adult releases a surprising amount of moisture overnight through breathing. With the door and window shut and the heating turned down, that moisture has nowhere to go. The window glass, frame and the wall around it are the coldest surfaces in the room, so that is where the water condenses and mould starts to grow.

Cold bridges around the window

The reveals, sill and head of a window are often poorly insulated, especially where old windows have been replaced. These cold bridges sit below the dew point on cold nights, which is why mould typically forms in a band around the frame and in the top corners of the room.

What helps

• Trickle vents: if your windows have them, keep them open. If they do not, they can often be fitted to provide constant background ventilation without opening the window.

• Positive Input Ventilation: a PIV unit gently supplies filtered fresh air to the home, diluting moist air and lowering humidity, which is very effective against bedroom condensation.

• Anti-condensation or graphite thermal liners: applied to cold external walls and reveals, they raise the surface temperature so condensation is far less likely to form.

• Furniture placement: leave a gap between wardrobes or beds and external walls so air can circulate.

Daily habits

Wipe condensation off the glass and sill each morning, open the window for a short while to air the room, avoid drying clothes in bedrooms, and try to keep the room at a steady low heat rather than letting it get very cold overnight.

When to get help

If mould keeps returning despite these steps, or appears alongside damp patches that worsen after rain, there may be water getting in around the window frame or through the wall. A survey with temperature, humidity and dew-point readings will show exactly what is happening, so the right combination of treatment and ventilation can be put in place.

Related help

If this sounds like your home, our mould removal and remediation service explains how we diagnose and treat it in practice. You may also find Why Does Mould Keep Coming Back and Black Mould on a Bathroom Wall useful.