When natural ventilation under a suspended floor is inadequate to reduce the radon level, active under-floor ventilation can be installed.

A fan is used to either continuously blow air into or extract air from the under-floor space. Both ways can be successful and it is difficult to know which will work best. It is usual to try one way and then retest. If radon levels are still high, the fan can be reversed.

The fan can be fitted outside, within a wall or under the floor, often using the opening of an existing airbrick.


  • When positioned outside, the fan can be placed in a ventilated housing.
  • When positioned under the floor, the fan will be hidden from view.
  • A silencer can be used to reduce noise if necessary.

Normally, one fan is used for an average sized home. Two smaller fans are sometimes used for more complex buildings.

Important points

  • Do not put a fan next to any combustion appliance such as an open fire, wood-burner, gas-fire or gas boiler because harmful gasses might be sucked into the room.
  • Do not put the fan within 1.5m of doors, windows, other vents or air-bricks.
  • Do insulate water pipes, including any central heating pipes in the under-floor space, to avoid the risk of freezing.

The typical cost is £700 but might be up to £1500: installation normally takes a day or less.

A list of fan manufacturers and more information is available on the BRE website

The animation shows an externally mounted fan. Fans can also be situated interally under the suspended floor as shown in the graphic below.





How to reduce radon levels overview


Disclaimer. It should be noted that the measures described on this sheet are not guaranteed to reduce the radon level in your home, however similar measures have been proven successful in many UK homes.