Radon

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Sealing. Although not a final solution, sealing entries is still the first step in radon mitigation, and if your home is only marginally above four pCi/L, this may be sufficient to bring the concentration below the threshold. Look for cracks in masonry floors and walls, gaps around plumbing runs that penetrate the walls or floor (including shower and toilet drains) and cold joints between slabs and footings. Enlarge the openings and clean them out with a wire brush and vacuum cleaner. Then fill them progressively with a flowable, gas-proof, nonshrinking silicone caulk.

Application of an epoxy sealant or waterproof coating to the inside of block walls will reduce the permeability of the material, lessening radon concentrations slightly. Sound poured concrete is too impermeable to bother with.

Sump pump ventilation. The sump pump is the single greatest offender when it comes to throwing the door wide open for radon. Sumps with underfooting connections to foundation drains are particularly good at collecting radon. An accompanying illustration shows a technique for sealing a sump pump housing and venting it to the outdoors by means of a fan. The fan for a sump-alone system should have a capacity of about 50 cubic feet per minute (cfm); for footing drain systems, 250 cfm or more may be needed. Because the fan operates in suction, it's very important that it and its plumbing be well sealed. The demands on the fan will be high, so be sure to get one that's capable. One source of appropriate fans is Kanalflakt, 1121 Lewis Ave., Sarasota, Fl 33577.

A well-sealed sump pump ventilation scheme can reduce even very high radon levels by better than 90%.

Block wall ventilation. Researchers have found that radon readily makes its way through the cold joint between a footing and a concrete block foundation. A hollow-block wall then offers a perfect path upward, and, in some older homes, the top of the block wall isn't capped. Radon then can slip past a poorly sealed sill and band joist and into the house. The tops of all block foundation walls (interior or exterior) should be sealed, but even after this is done, radon may make its way into the basement through cracks or mortar joints.

Two approaches to ventilating a block wall are shown in the illustrations. The simpler of the two has an individual pipe penetrating each section of wall (one four-inch plastic pipe every 24 feet). These runs are joined and feed to a single suction fan that pulls the radon outdoors. The other technique uses a plenum that's carefully sealed all along the base of the wall. The second system is thought to be somewhat more effective at creating a negative pressure under the slab (thereby preventing radon from entering the house), but both have shown greater than 90% effectiveness in overall radon reduction.

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