Building radon resistance into a new house is far less costly than radon mitigation after construction. The passive system outlined below costs only about $300 for a modest house and has no operating costs. In contrast, an effective radon mitigation retrofit with sub-slab depressurization costs about $1,200 to install and can have significant operating costs.
Radon enters basements by pressure-driven flow through cracks and openings to the soil. It enters because basement air tends to be at lower pressure than the soil gasses with radon under the slab. This slightly lower pressure is due to winds and the buoyancy of warm indoor air in cold weather. To control radon effectively, sealing cracks and openings to soil and production of a greater depressurization under the basement floor are needed.
The following five features from the EPA are to be installed as new construction is built, to constitute a proper system for controlling radon. To meet the new NAHB National Green Building Standard for areas with high radon potential, the features of Appendix F in the ICC International Residential Code for One and Two-Family Dwellings are required, as summarized below. Appendix F is derived from EPA’s "Model Standards and Techniques for Control of Radon in New Residential Buildings," in which these key features are detailed in Section 9.
Where the gravel and plastic sheeting are standard, the only additional elements are the passive stack through a heated partition and the sealing of joints and other openings to soil.
Before the basement slab is poured, a three- or four-inch diameter PVC stack is placed vertically from under slab, to run up through an interior, heated partition of the house and through the roof. If the sub-slab region is well sealed from the basement, winds and (when outdoor air is cold) warmer air in the stack rising then create a slight depressurization under the slab. With the gravel, the depressurized region can extend completely under the slab. The bottom of the passive stack connects to a PVC tee with about 10 feet of perforated pipe on both ends in the gravel. Dirt-floor crawl spaces are treated similarly, with depressurization under a membrane adhered to the walls.
Studies across the country and in Wisconsin show that such passive stacks in properly built and sealed new construction typically reduce the radon in indoor air by 50%, compared to the radon measured with the stacks capped.
Since the radon in a home may still be elevated despite these precautions (although it is significantly less likely), a radon measurement is needed after the home is finished. If the radon is elevated, installing a fan in the stack, in space originally provided for it in the attic, ensures adequate control at a low additional cost. The fan must not be located in conditioned air; for example, not in the basement.
The trades persons doing this work must understand that the stack goes through a heated, interior partition, that space for installing a fan must be left in the attic, and that sealing of the floor-wall joints must be done before any basement finish walls are installed.
If indoor radon is elevated, a fan-powered mitigation system (the only thing that will work as a retrofit) can cause withdrawal of conditioned air via cracks and openings through the slab behind finish walls. The energy for the fan, plus heating of the cold outdoor air infiltrating to replace air withdrawn from the home, can cost $200 per year in northern climates.