Cellars and basements: radon, damp and mould

The short answer

Cellars and basements sit in direct contact with the ground, which makes them the part of a home most exposed to both radon and damp. UKHSA says every occupied basement, meaning one used for more than 50 hours a year, should be tested for radon whatever the area's radon potential.

Damp and mould need their own diagnosis. Treating one does not automatically deal with the other, so plan for both before you convert or start using a below-ground room.

Why are basements at higher risk of radon?

Radon comes from the ground, and UKHSA explains that buildings draw air in from the ground through cracks and gaps in their floors. A basement has floor and walls against the soil, so there are more routes in. That is why UKHSA's guidance on measuring radon asks for occupied basements to be tested even in areas where radon is usually low.

Why do basements get damp and mould?

The government's guidance on damp and mould in the home describes four kinds of damp. Three are common below ground: penetrating damp, where water gets in from outside through defects in walls or floors; rising damp, moisture from the ground through walls and floors in contact with it, often due to defective damp-proof courses and membranes; and condensation, where moist air settles on cold surfaces, which the guidance says is the most common form of damp.

Below-ground walls are often cold, so condensation can form when warm, moist air from the house reaches them. The guidance also says rising damp is often misdiagnosed, and that chemical testing is the most appropriate way to confirm it.

For a clear explanation of the different causes, see this guide to what causes damp and mould in a home.

Do radon and damp have the same fix?

Sometimes the work overlaps, but you can't assume one solves the other. UKHSA's cellar and basement guidance suggests, for a vented living space, increasing ventilation by clearing or adding vents or adding a fan to blow air in, or installing a sump with a fan. For a space without vents it suggests a sump with a fan or a positive ventilation system, sealing doors and other openings to the main house, and ventilating any other underfloor voids.

Extra ventilation may also help with condensation. But UKHSA's guidance on reducing radon is clear that sealing and extra ventilation do not reduce radon on their own, and a radon system won't cure water coming through a wall. A waterproofing or damp-proofing job should be checked against its effect on radon, and the reverse.

What should I do before converting a cellar?

In England, the Planning Portal's basement conversion guidance says building regulations apply to basement conversions, covering things like fire escape routes, ventilation and damp proofing. Converting an existing basement is unlikely to need planning permission, but excavating a new one probably will, so check with your council first. Wales, Scotland and Northern Ireland have their own rules.

Before the work is designed, get the space tested for radon and the damp diagnosed. Then ask the designer or contractor to show how the plan deals with both: the damp-proofing method, the ventilation, and any radon protection. Our guide to radon in new homes and building work covers the questions to ask about radon protection.

How do I test a basement for radon?

Follow the laboratory's instructions. UKHSA's standard home test runs for three months with two detectors, normally one in a living area and one in a bedroom; an occupied basement needs measuring in its own right. UKHSA recommends action where the annual average is above 200 Bq/m³. See how radon testing works for more.

What if there's already mould in the cellar?

Find and fix the source of moisture first, whether it's a leak, a defective damp-proof membrane or condensation. The government guidance warns that disturbing mould can release a lot of spores, so wear protective equipment and get professional help for larger areas. A damp survey or mould inspection can identify the cause before you pay for treatment.

Does a radon problem in the cellar affect the rest of the house?

It can. Air moves between floors, which is why UKHSA's cellar guidance suggests sealing doors and other openings between the basement and the main house, especially when a positive ventilation system is fitted. If the cellar has raised radon, test the main living areas too, and keep the cellar door closed while you get advice.

Which specialist do I need?

Usually two. A radon specialist can advise on a sump, ventilation or other radon protection. A damp surveyor or mould specialist can find where the moisture is coming from. If one company offers both, ask it to explain separately how each problem will be dealt with and how you'll know each fix has worked: a three-month radon retest for one, and a dry, mould-free space through the following winter for the other.

Keep the radon results, the damp survey and the details of any work together with your property records. They will be useful to a future buyer and to anyone designing later alterations.

Common questions

My cellar is only used for storage. Does it need a radon test?

UKHSA's basement advice is about occupied basements, used for more than 50 hours a year. A storage cellar doesn't fall into that, but radon from it can still reach the rooms above, so follow the testing advice for the rest of the house.

Will tanking a cellar stop radon?

Not reliably. Tanking is a damp-proofing method; ask the installer whether it also gives radon protection, and test afterwards.

Can a dehumidifier deal with cellar damp?

It can reduce condensation, but it won't stop water coming through a wall or floor. Find the source first.