Homeowners · 4 min read · Updated 6 November 2026
Mould in a new build home: why it happens and what to do
Why new homes can be damp
Building a house uses a lot of water. Concrete floors, block walls, screed and wet plaster all hold moisture that takes many months to dry out after the keys are handed over. A new home can release a large volume of water into the air during its first year or so, on top of the moisture produced by the people living in it. Developers usually mention this drying-out period in their handover pack, and it is the reason small cracks and some condensation are normal early on.
Modern homes are also far more airtight than older ones. That is good for heating bills, but it means fresh air only arrives through the ventilation that has been designed in: trickle vents, extract fans, or a whole-house system such as mechanical ventilation with heat recovery (MVHR). If those are switched off, blocked or poorly installed, moisture builds up quickly.
Common mould spots in new builds
- Around window reveals where trickle vents have been closed
- Bathroom and en-suite ceilings where the extract fan is too weak, disconnected or ducted badly
- Inside fitted wardrobes on external walls
- Bedroom corners in the first winter, while blockwork is still drying
- Loft hatches and ceilings below poorly insulated or badly ventilated lofts
- Behind kitchen units where a leaking waste pipe or appliance connection has gone unnoticed
Ventilation rules for new homes
Approved Document F of the Building Regulations sets the ventilation that new homes in England must have. The 2021 edition, which applies to homes built under it from June 2022, tightened the requirements and expects extract in kitchens and bathrooms plus background ventilation, or a whole-house mechanical system. It also requires that the system is commissioned and that occupants are given information on how to use it.
| Ventilation type | What to check |
|---|---|
| Trickle vents | Open on every window, not painted shut or blocked by blinds |
| Intermittent extract fans | Run during and after showers and cooking, run-on timer working |
| Continuous extract (dMEV or MEV) | Left running all the time, boost setting used when cooking or bathing |
| MVHR | Never switched off, filters cleaned or replaced as the manual says, often every few months |
If an extract fan barely holds a sheet of toilet paper, or a bathroom mirror stays steamed up long after a shower, the fan may not be connected to its duct properly, which is a common snagging issue.
Snagging, the builder and the warranty
Most new homes come with a 10-year structural warranty from a provider such as NHBC, Premier Guarantee or LABC Warranty. Under the common arrangement, the builder is responsible for fixing most defects in the first two years, and the warranty provider covers specified structural problems for the rest of the term. Check your own policy, because cover varies.
- Report mould, leaks and damp patches to the developer in writing as soon as you notice them
- Take dated photos and note when it is worst
- Ask the developer to check fans, ducting, trickle vents and any MVHR commissioning
- Keep records of repair visits and outcomes
- If the developer does not resolve it, use the warranty provider's resolution service or the New Homes Quality Board process where your builder is registered
Condensation caused by the normal drying-out period is often excluded from warranty cover, which is why it matters to show that ventilation has been used properly.
How to help a new build dry out
Keep the heating on at a moderate level rather than very high, which can dry plaster too quickly and cause cracking. Leave trickle vents open, run fans as designed and use a dehumidifier in the first winter if condensation is heavy. Avoid drying washing on radiators. Leave a gap behind furniture on external walls and keep wardrobe doors slightly open or use ventilated doors where mould keeps appearing inside.
When it is more than condensation
Mould that sits in one patch regardless of weather, staining around ceiling lights, damp that spreads from a skirting board or a musty smell under the kitchen sink can point to a leak or a construction defect such as a missing cavity tray or poorly sealed window. Those are the builder's responsibility to investigate in the defects period, so raise them promptly rather than repainting over them.
Questions to ask at handover or soon after
- Which type of ventilation does the home have and how should it be run
- Where are the fan isolator switches and boost controls
- How often do MVHR filters need changing and where are they
- Has the ventilation been commissioned, and can I have the record
- How long does the developer expect the drying-out period to last
- Who do I report defects to and how quickly do they respond
Asking these early means you know how the home is meant to work before the first winter, and you have a clear point of contact if mould appears.
Frequently asked questions
Is it normal to have condensation in a new build?
Some condensation in the first year or two is common while construction moisture dries out, especially in the first winter. It should reduce noticeably after that. Heavy condensation every day, mould on several walls or a fan that does not clear steam are not things to put up with, and are worth reporting to the developer in writing.
Can I turn off my MVHR system to save electricity?
It is not a good idea. Homes with MVHR are designed to rely on it for fresh air and moisture removal, and switching it off can lead to condensation and mould quickly. Running costs for a well-installed system are usually modest. If it is noisy or seems to do nothing, ask the developer to check its commissioning and filters.
Will the developer pay to clean mould?
If the mould is caused by a defect, such as a disconnected fan duct, a leak or a faulty window seal, the builder should normally fix the cause and make good during the defects period. If it is put down to condensation from normal drying out, they may decline. Keep records showing ventilation has been used correctly to support your case.
new build ventilation warranty drying out
