New Build Damp in Your First Winter — What’s Normal and What’s Not

Why New Builds Get Damp in Winter

A newly built house contains an enormous amount of trapped moisture. The concrete, plaster, screed, and timber used in construction hold roughly 1,500 litres of water in a typical 3-bed semi — some estimates put it higher depending on the construction method.

That moisture needs to evaporate somewhere. During your first autumn and winter, when windows are closed and the heating is on, the moisture migrates out of building materials and into the air. The result: condensation streaming down windows, damp patches on walls, and sometimes visible mould in corners and behind furniture.

This doesn’t mean your house is defective. In most cases, it’s a normal part of the drying-out process.

How Long Does the Drying-Out Period Last?

Most new builds take 9-18 months to fully dry out, depending on construction type, ventilation, occupancy, and season. The first winter is almost always the worst — you’re heating the building for the first time, driving moisture out of materials and straight onto cold surfaces.

By the second winter, the building has lost most of its construction moisture and condensation typically reduces significantly.

What’s Normal and What Isn’t

Normal in a new build first winter:

• Condensation on windows in the morning, especially in bedrooms and bathrooms
• Slight dampness around window reveals
• Hairline cracks in plaster (shrinkage as the building dries)
• Humidity that feels higher than your previous home
• Occasional musty smell in rooms that aren’t well ventilated

Not normal — investigate further:

• Water running down internal walls (not just condensation on windows)
• Damp patches that persist even in warm, dry weather
• Mould growth that returns repeatedly after cleaning
• Damp in areas away from windows — behind kitchen units, under flooring, on internal walls
• Wet spots on ceilings below bathrooms (possible plumbing issue)

If you’re seeing the second list, report it to your builder under your NHBC or equivalent warranty. These symptoms suggest a construction defect — not normal drying.

What You Can Do About It

Daily Habits That Help

Ventilate properly. Run bathroom and kitchen extractor fans during and after use. If your home has trickle vents on windows, keep them open. If it has an MVHR system (mechanical ventilation with heat recovery), make sure it’s running — some new homeowners turn it off thinking it wastes energy.

Don’t dry clothes indoors without extraction. A single drying rack of wet laundry releases over 2 litres of moisture into the air. If you have to dry clothes indoors, do it in a closed room with a window open or a dehumidifier running.

Keep consistent heating. A constant low background heat (18-20C) works better than blasting the heating for a few hours and letting the house go cold overnight. Consistent warmth drives moisture out of materials steadily; temperature swings cause condensation bursts.

Keep furniture away from external walls. Leave a 5-10cm gap between wardrobes, sofas, and beds and the walls behind them. Trapped air behind furniture doesn’t circulate, creating cold spots where condensation and mould thrive.

When to Hire a Dehumidifier

If daily ventilation isn’t keeping up — windows dripping every morning, humidity that won’t drop below 65-70%, or mould starting to appear — a dehumidifier gives the building the push it needs.

A compact dehumidifier (19-20L) handles a single problem room. For a whole floor or a home where every room feels damp, a 38L commercial unit provides much faster extraction.

How long to hire for: A 2-4 week hire through the worst of winter typically makes a significant difference. Move the dehumidifier between rooms daily, or hire a second unit if multiple areas need simultaneous treatment.

The goal isn’t to dry the house completely in one go — that’s the building’s 12-18 month job. The dehumidifier prevents moisture from causing visible damage (mould, staining, swollen timber) while the building dries naturally underneath.

If you’re not sure how much extraction a damp home needs, our guide to how many dehumidifiers you need breaks it down by room size and how wet the space is.

Should Your Builder Be Dealing With This?

If the issue is construction moisture drying out naturally, it’s not a defect — it’s a normal process, and your builder isn’t obligated to fix it. Most builders will tell you this, and they’re generally right.

However, if the problem is caused by a construction defect — a missing or poorly installed DPC, inadequate ventilation provision, plumbing leaks, or damaged vapour barriers — that’s a warranty claim. NHBC, LABC, or your warranty provider typically covers defects in the early years and structural issues for ten years. Check the specific terms of your own warranty.

The test: If the damp is worse than expected for a new build, concentrated in specific areas, or persists well beyond the first winter, push for an inspection. Normal construction drying is diffuse and gradually improves. Defect-caused damp is localised and persistent.

Frequently Asked Questions