Corners and the junction where external walls meet the ceiling are often the coldest surfaces in a room, so moist air condenses there first and mould follows. Missing loft insulation at the eaves, cold bridging and poor air movement make it worse.
Black mould in the top corners of a bedroom, along the line where the outside wall meets the ceiling, is the classic condensation pattern. I see it in every type of house, old and new.
Why corners?
An external corner has outside air on two sides, so it loses heat faster than a flat wall and runs colder. Where the external wall meets the ceiling, you also often have the edge of the loft insulation, the eaves and sometimes a lintel or concrete ring beam, all of which can create a cold bridge. Air also moves less in corners, so moist air sits there longer.
Put those together and the corner drops below the dew point before the rest of the room does, which is why mould starts there and spreads out.
Common contributing causes
- Missing or squashed loft insulation at the eaves. Insulation is often thin or missing right where the roof meets the wall, because it’s hard to reach or has been pulled back. That leaves a cold strip of ceiling along the external wall.
- Cold bridges, such as concrete lintels, steel beams and solid floor slabs.
- Bedrooms with the door shut overnight and no ventilation.
- Unheated rooms, such as spare bedrooms and box rooms on cold corners of the house.
- Wardrobes and furniture in corners, cutting off air movement.
What helps
- Check the loft. Make sure insulation runs right to the edge over the external walls without blocking the eaves ventilation. Eaves vents or rafter trays keep the airflow open.
- Improve ventilation. Trickle vents open, extractor fans working, and bedroom doors open when possible. Mechanical ventilation can help in persistent cases.
- Keep rooms heated. Even a low background heat in spare rooms helps.
- Keep furniture out of corners and off external walls.
- Clean the mould with a fungicidal wash, rather than bleach, and let it dry fully before redecorating.
Where the cold bridge is in the wall itself, such as solid walls or a concrete lintel, an internal thermal lining can raise the surface temperature.
If the mould is only in one corner and it’s brown and wet rather than black and spotty, check for a gutter or roof leak outside that corner.
When to call a specialist
If mould keeps returning in the same corners despite good ventilation, a condensation survey will pinpoint the cold spots and the right fix. Book a survey with All Dry on 01226 287181 or email info@alldrydampproofing.com.
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Frequently asked questions
Why does mould grow in ceiling corners?
Corners and wall-ceiling junctions are usually the coldest surfaces, so condensation forms there first.
Can missing loft insulation cause mould?
Yes, gaps at the eaves leave cold strips of ceiling where condensation forms.
How do I stop mould in corners?
Improve ventilation and heating, check loft insulation, and keep furniture out of corners.