Short answer: new builds get mould for two main reasons. First, they are full of water from construction, in plaster, screed, concrete and timber, which takes many months to dry out into the air. Second, modern homes are built to be airtight, so if trickle vents are shut or the ventilation system is switched off or poorly installed, that moisture has nowhere to go. In the first two years, the builder is usually responsible for putting right defects that breach the warranty provider's standards. Condensation caused purely by how a home is used is generally not covered, so it matters which one you have.
Got mould or condensation now? Get up to 3 free quotes from verified specialists near you.
Get free quotesWhy new homes are damp
Construction moisture
Wet trades leave a lot of water behind. Plastered walls, concrete floors and sand-and-cement screeds all dry slowly, and much of that moisture comes out into the home during the first year, especially the first winter when windows stay shut. Developers often warn buyers about this in their handover packs.
Airtightness
Older homes leak air through gaps around windows, floors and chimneys. New homes are built to tight energy standards so that warm air doesn't escape. That's good for heating bills, but it means the home relies on its designed ventilation to get rid of moisture. If that ventilation is turned off, blocked or under-performing, humidity builds quickly.
Ventilation that isn't working as designed
Common problems in new builds include:
- trickle vents fitted but left closed, or painted over
- extractor fans that don't reach the required extract rate (Approved Document F expects 15 litres per second for an intermittent bathroom fan, and 30 or 60 for a kitchen depending on position)
- continuous mechanical extract (dMEV) or whole-house systems switched off because they're "noisy" or "costing money"
- MVHR systems with blocked filters, unbalanced airflow, or ducting that's crushed or disconnected in the loft
- extract ducts not connected to an outside terminal
Cold spots and defects
Mould in one specific spot, rather than everywhere, can point to a build defect: missing or slumped insulation, a cold bridge at a window or door head, a poorly sealed roof, or a leak. Damp that worsens after rain is likely to be water getting in, not condensation.
What to do in the first winter
- Open the trickle vents and leave them open. They're designed to be open all the time.
- Leave the ventilation system on. If you have dMEV or MVHR, it's meant to run continuously. Check the filters and ask the developer how to maintain it.
- Run kitchen and bathroom extractors during and after cooking and showering.
- Heat the home steadily rather than in short bursts, especially in the first year while it dries out.
- Avoid drying washing indoors in unventilated rooms. One load releases around two litres of water as it dries; our guide to drying clothes without mould shows how.
- Consider a dehumidifier for the first winter to help the building dry. See do dehumidifiers stop mould?
- Photograph and date any mould, damp patch or condensation problem, and keep notes.
Who pays: builder, warranty, or you?
Most new homes in the UK come with a ten-year structural warranty. NHBC's Buildmark is the best known; LABC Warranty, Premier Guarantee and others work in broadly similar ways, but read your own policy.
Years 0 to 2: the builder warranty period
Under NHBC Buildmark, for the first two years after completion the builder is responsible for putting right defects that breach NHBC's technical standards. Report problems to the builder first, in writing. If the builder disputes it or doesn't act, NHBC offers a resolution service that can investigate and, where it agrees there's a defect, require the builder to fix it.
Damp and condensation can be covered in this period, but generally only where they result from the builder failing to meet the warranty provider's standards: for example, a missing extract fan, a ventilation system that doesn't meet its design, missing insulation or a leaking roof.
Years 3 to 10: insurance period
After year two, NHBC cover narrows to certain structural and specified problems. NHBC's own literature lists damp, condensation and shrinkage that are not a result of the builder failing to meet its technical requirements as excluded. In practice, condensation issues reported for the first time after year two are hard to claim for.
The takeaway: if a new build has a mould problem, report it to the builder within the first two years, in writing, with photos, even if you hope it'll settle down.
Other routes
Builders registered with a consumer code, such as the Consumer Code for Home Builders, must offer a complaints process, and the code has its own dispute resolution scheme. If your developer is registered with the New Homes Quality Board, you may also be able to escalate to the New Homes Ombudsman Service. Check your purchase documents to see which applies.
How a specialist helps
An independent damp or ventilation specialist can give you evidence the builder and warranty provider will take seriously: measured fan extract rates, humidity readings, thermal images showing cold bridges or missing insulation, and moisture readings that distinguish condensation from a leak. That report is often what turns "it's just drying out" into an accepted defect. Our guide to damp surveys versus mould inspections explains which you need.
If you let a new build
Landlords of new-build properties have the same duties as any other landlord: keep the home free of serious hazards and fit to live in. Tell tenants how the ventilation system works and that trickle vents should stay open, and follow up mould reports quickly. If the builder is still within the two-year period, an early, documented report protects you as well as your tenant. See condensation control for landlords.
FAQs
Is it normal to get condensation in a new build?
Some condensation in the first winter is common while construction moisture dries out. Persistent mould, or mould in one spot, is not normal and should be reported to the builder.
How long does a new build take to dry out?
It varies with the construction, the season it was finished and how it's heated and ventilated. Many builders say to expect drying out across the first year, with the first winter the worst.
Will my warranty cover mould?
Only if the mould results from a defect, meaning the builder failed to meet the warranty standards. Report it in the first two years for the strongest position.
Should I turn off my MVHR to save money?
No. MVHR and continuous extract systems are designed to run all the time, and turning them off in an airtight home is a common cause of condensation mould.