It surprises many people to find mould in a brand-new home. New homes are built to be warm and airtight, which is good for energy bills but means they depend on their ventilation working properly. There can also be a lot of water left in the building from construction.
Why new homes get mould
- Drying out. Concrete, plaster, screed and mortar all contain water that dries out into the home over the first months, adding to everyday moisture from cooking, bathing and drying clothes.
- Airtightness. A well-sealed home doesn't leak air the way an older house does. Moist air has to be removed by extractor fans, trickle vents or a whole-house ventilation system.
- Ventilation turned off or blocked. Trickle vents get closed to stop draughts, fans get switched off because of noise, and filters in whole-house systems get clogged.
- Cold spots. Gaps in insulation, for example around window frames or in the loft, can leave surfaces cold enough for condensation.
- Defects. Leaks from plumbing, roofs or poorly sealed windows.
What you can do
- Keep trickle vents open.
- Leave extractor fans on their normal setting and use boost mode when cooking or showering.
- If you have a whole-house ventilation system (such as MVHR), leave it running and clean or change the filters as the manual says.
- Dry laundry outdoors when you can, or in a room with the door shut and the window open.
- Keep furniture slightly away from outside walls.
- Clean small patches of mould early. See cleaning small areas yourself.
When to contact the builder
Mould that keeps coming back despite good ventilation habits may point to a defect: a fan that isn't powerful enough or vents into the loft, missing insulation, or a leak. Report it to your builder in writing, with photos and dates.
Most new homes come with a warranty. Check your warranty documents to see what's covered, for how long, and who to contact in the early years: often the builder first, then the warranty provider.
If you rent a new build, report it to your landlord. See who has to fix damp and mould in a rented home.
Sources
- Understanding and addressing the health risks of damp and mould in the home, GOV.UK, updated 1 April 2026 (ventilation, laundry and everyday moisture)