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.
Airtight homes need their ventilation for more than moisture. Read why ventilation matters for both mould and radon, and if you live in an area where radon is more likely, these are the radon questions to ask about a newly built home.
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)