[GSBN] Radon

[Derek Stearns Roff]

Robert Riversong offers the information below on Radon:

Derek,

The function of a passive radon vent, which is almost always dramatically effective at reducing indoor radon concentrations, is to depressurize - not evacuate - the ground under a slab or basement/crawlspace floor. Any inlet to this space undermines the function of a passive (or active) radon vent system.

A 1994 NAHB/EPA study in new MD homes found an average 89% reduction in winter radon concentrations from passive venting, and an average reduction in summer concentrations of 59%. Similar tests in other states (NM, ID, CO, NH, MA) found an average 50% indoor radon reduction in both seasons. All homes were built with EPA-recommended radon mitigation methods.

The Environment Protection Agency set recommendations:
4 pCi/L = the "action" limit (fix your home)
2 pCi/L = the "consider action" limit (consider fixing your home)

ASHRAE suggests a radon action level of 2 picocuries per liter or less for commercial buildings and residences.

The World Health Organization set a recommended radon reference level of 2.7 pCi/L for residential structures.

Outdoor radon levels in the U.S. range from 0.02 to 0.75 pCi/L (picoCuries per liter), averaging 0.4 pCi/L.

The U.S. Congress has set the natural radon concentration outdoors as the target level for homes, but approximately two thirds of homes exceed it. Half of American homes have a radon level above 0.67 pCi/L (the median level). The average (mean) radon level in US homes is 1.25 pCi/L, or three times higher than the average level outdoors.

If you build to IRC standards, then all subfloor areas must be air-sealed to everything but the radon stack vent.

2009 International Residential Code:

AF103.2 Subfloor preparation.
A layer of gas-permeable material shall be placed under all concrete slabs and other floor systems that directly contact the ground and are within the walls of the living spaces of the building, to facilitate future installation of a subslab depressurization system, if needed. The gas-permeable layer shall consist of one of the following:

1. A uniform layer of clean aggregate, a minimum of 4 inches (102 mm) thick. The aggregate shall consist of material that will pass through a 2-inch (51 mm) sieve and be retained by a 1/4-inch (6.4 mm) sieve.
2. A uniform layer of sand (native or fill), a minimum of 4 inches (102 mm) thick, overlain by a layer or strips of geotextile drainage matting designed to allow the lateral flow of soil gases.
3. Other materials, systems or floor designs with demonstrated capability to permit depressurization across the entire subfloor area.

AF103.3 Soil-gas-retarder.
A minimum 6-mil (0.15 mm) [or 3-mil (0.075 mm) cross-laminated] polyethylene or equivalent flexible sheeting material shall be placed on top of the gas-permeable layer prior to casting the slab or placing the floor assembly to serve as a soil-gas-retarder by bridging any cracks that develop in the slab or floor assembly, and to prevent concrete from entering the void spaces in the aggregate base material. The sheeting shall cover the entire floor area with separate sections of sheeting lapped at least 12 inches (305 mm). The sheeting shall fit closely around any pipe, wire or other penetrations of the material. All punctures or tears in the material shall be sealed or covered with additional sheeting.

AF103.6.1 Vent pipe.
A minimum 3-inch-diameter (76 mm) ABS, PVC or equivalent gas-tight pipe shall be embedded vertically into the subslab aggregate or other permeable material before the slab is cast. A "T” fitting or equivalent method shall be used to ensure that the pipe opening remains within the subslab permeable material. Alternatively, the 3-inch (76 mm) pipe shall be inserted directly into an interior perimeter drain tile loop or through a sealed sump cover where the sump is exposed to the subslab aggregate or connected to it through a drainage system.

The pipe shall be extended up through the building floors, and terminate at least 12 inches (305 mm) above the surface of the roof in a location at least 10 feet (3048 mm) away from any window or other opening into the conditioned spaces of the building that is less than 2 feet (610 mm) below the exhaust point, and 10 feet (3048 mm) from any window or other opening in adjoining or adjacent buildings.

Feel free to share this with the GSBN listserve.

- Robert

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