Radon reduction

Avoiding radon gas indoors

Q. How can I reduce radon levels?
A. Find the source and stop the pathways (intercept or block or reverse the main pathways), such as in summary:

  • Basement:  Seal openings in floor slab, and vent subslab to outdoors.
  • Crawl space:  Lay plastic vapor barrier over exposed soil, and keep vents open.
  • Exposed masonry:  Seal with polyurethane paint.
  • Well water:  Boil or aerate or store water outdoors.

For further advice on lowering radon levels, see below, or USEPA’s “Consumer’s Guide to Radon Reduction“.

Finding sources and pathways

  • Test again in more and different locations, on upper floors also, to find the source: soil gases? (most common) interior masonry, granite, or rocks?, well water?
  • Test also to find the entry pathways: basement? (most common) (more in one end or corner?), crawl space? well water?
  • Use your own direct reading continuous monitor Safety Siren SM-RAD-PRO3 for $130. from Safe Home Products, Inc.

Venting basement radon

  • Install active subslab suction (depressurization) to intercept soil gases under the floor slab with an exhaust fan to blow radon outdoors:
  • Use a 4-inch exhaust pipe through the slab (tied the into possible existing sump, drain tiles, perforated pipe, or hollow cores of concrete block walls).
  • Seal any cracks or gaps in the slab, such as the pipe penetration and at all joints.

Venting crawl space radon

  • Seal duct joints and penetrations through floors or walls, such as for air ducts, electrical wiring, outlets, plumbing (alone, or to assist the following methods).
  • Use passive ventilation: keep perimeter vents open; add more. Or better:
  • Install mechanical active ventilation using exhaust fans in one or more perimeter vents, with others blocked.  (Caution: Insulate water pipes against the cold, and avoid depressurizing and backdrafting any combustion appliances.)  Or best:
  • Install active submembrane suction with an exhaust fan to intercept soil gases (with perforated pipes in gravel under plastic sheets) and blow radon outdoors.

Avoiding room depressurization

  • Supply filtered outdoor makeup air into the lowest levels, best into the cold air return duct of the central heating system.
  • Supply outdoor air to near combustion appliances.


  • Use a direct reading continuous monitor after remediation to determine new levels; if not low enough, add another exhaust fan or suction pipe.

Get more info from the US EPA.

©2004-2016 Richard Knights, Blue Sky Testing Labs, Seattle, blueskylab@pobox.com