Damp Under the Floor: Airbricks, Timber Floors and Musty Rooms

Rung 1: Find the vents and look (free)

GOV.UK retrofit guidance says air from outside can and should flow under suspended timber floors, typically through air bricks or grilles in the outer walls. Walk round the outside of the house and look.

• Can you see air bricks or grilles low on the outer walls? If not, ask why before you assume the floor is solid.
• Are they clear of soil, paving, decking, plants or debris? The guidance says ventilation must be effective and unobstructed by new materials or debris.
• Has the outside ground level risen against the wall? The guide says raised ground from soil or soft landscaping should be carefully reduced to restore full cross-flow of air.
• Has an extension been built since the house was new? Extensions can obstruct the cross-ventilation paths, according to both the GOV.UK guide and the Property Care Association.

Rung 2: Free up the air (free to cheap)

• Clear soil, leaves and debris away from every air brick, and keep plants and stored items off them.
• Do not block or cover vents to stop draughts. The Property Care Association says blocked air vents should be cleared, and that pockets of dead air favour fungal growth.
• Be careful with new floor coverings. The Association lists close-fitting linoleum, vinyl or laminate laid over unventilated or poorly ventilated timber floors as a cause of decay.
• Keep an eye on the ground floor: a persistent musty smell, a floor that feels cold and damp, or timber that looks dark or soft deserves a closer look.

Rung 3: Fix the cause (cheap to moderate)

The Property Care Association says finding the source of dampness and stopping moisture getting in, while promoting drying, is always necessary. Under a floor that usually means a leaking pipe or drain, a gutter or downpipe discharging against the wall, or ground that slopes towards the house, as well as blocked vents. Our guide to leaking gutters and downpipes covers what to check outside.

If you suspect rot, ask a Property Care Association member or a surveyor to look. Crawl spaces can be hazardous, so leave the inspection to someone equipped to do it. If you plan to insulate under the floor, the GOV.UK guide says the condition of the void and floor structure must be assessed first, including that the structure is free from rot, and that any reduction in cross-ventilation must be made up with equivalent ventilation.

Rung 4: When mechanical help makes sense

If the vents are clear, leaks are ruled out and the home still runs humid in the living space, controlling humidity across the home can help. A loft-mounted dehumidifier such as Dryzen is one option for owner-occupied homes with a loft. It is not suitable for rentals unless the landlord authorises changes, it needs a loft to mount in, and it will not dry a sub-floor void, cure rot or stop a leak. It helps stop the conditions mould and dust mites need but does not remove mould that is already there.

Mistakes that make under-floor damp worse

• Stuffing air bricks with newspaper or filler to stop draughts.
• Raising a patio, path or flower bed above the level of the vents.
• Laying a sealed floor covering over damp or poorly ventilated boards.
• Building an extension without keeping a through-flow of air to the old floor void.
• Treating the smell with air fresheners or a dehumidifier before finding the moisture source.

In Chatham and the Medway Towns

GOV.UK notes that many older houses have suspended timber floors. In Medway, our Brompton page describes terraces that are largely 17th to 19th century solid brick, so check the vents and ground levels if you live in one. Our Strood page covers a low-lying riverside area with chalk hills behind it, where ground moisture is a real question alongside condensation, and Gillingham has Victorian terraces that grew up beside the dockyard.

Questions we are asked

Related guides

• Why Victorian Terraces Get Damp Differently to New Builds
• Rising, Penetrating or Condensation Damp? How to Tell
• Damp on a Chimney Breast: Causes and Fixes
• The Healthy Humidity Level for Your Home
• Wet Rot or Dry Rot? Soft Timber and the Damp Behind It

Local help

• Damp, mould and condensation help in Brompton
• Damp, mould and condensation help in Strood
• Damp, mould and condensation help in Gillingham

Sources: GOV.UK: Guide to best practice retrofit floor insulation, suspended timber floors; Property Care Association: Fungal decay in buildings; GOV.UK: Understanding and addressing the health risks of damp and mould in the home.