Damp Or Mould In A New Build

What's happening?

A new build contains a great deal of water in its bricks, blocks, plaster, screed and timber, and that water has to go somewhere. In the first year it dries out into the air, so humidity is high, windows run with condensation and walls can feel damp. Modern houses are also airtight and well insulated, so the moisture is slow to leave unless the ventilation works. Mould appears first on the coldest surfaces: window reveals, corners, the loft and behind furniture. All of that is normal in the first heating season if the house is heated and ventilated. What is not normal is a damp patch that stays after drying out, one wall that is wet, water coming in around windows, or a leak, which are defects. Most new builds carry a warranty and the builder has a duty to put defects right.

What usually causes it?

• Construction moisture from bricks, plaster, screed and timber drying out
• An airtight house with ventilation that is off, blocked or not working properly
• Condensation on cold surfaces such as window reveals, lintels and corners
• A cavity bridged by mortar or ground built up outside above the damp proof course
• A leak at a window, roof junction, pipe or gutter
• Not enough heat in the first winter to drive the moisture out

Is it dangerous?

Damp and mould are a health issue, particularly for children and anyone with breathing problems, so deal with mould promptly. Do not paint over damp walls with sealant or stain block before the cause is found.

Can I fix it myself?

Heat the house steadily rather than in bursts, open trickle vents, and run extractors in kitchens and bathrooms. Dry washing outside or in a vented room. Wipe condensation from windows daily and clean mould early. Photograph any damp, note the date and weather and report it to the builder or developer in writing. Keep the warranty paperwork to hand.

When should you call a damp proofer?

Report persistent damp, water entering or mould that returns to the builder or developer at once, in writing, and to the warranty provider if they do not respond. Call an independent damp surveyor if you want a diagnosis. NHBC and other warranty providers set out what the builder must put right.

What will the damp proofer actually do?

• Take moisture and humidity readings and compare them with the house's age and heating
• Check the ventilation: trickle vents, extractors, mechanical systems and the air path
• Look for bridged cavities, ground levels, leaks and window and roof junctions
• Say in writing whether it is drying out, condensation or a defect
• Treat mould and improve ventilation, or provide a report for the builder or warranty claim

Who fixes this

• Damp survey
• Condensation and mould treatment
• PIV installation

Questions people ask

Sources & References

1. NHBC: Buildmark cover in the first two years
2. GOV.UK: Approved Document F, ventilation
3. Property Care Association: condensation
4. GOV.UK: damp and mould, understanding the health risks for rented housing providers