Foundations

Introduction to Radon Abatement

The presence of radon is a function of local geology, and only testing in a completed home can determine if measures need to be taken to reduce indoor concentrations.

In regions where it is a known hazard, steps must be taken before pouring a structural or basement slab - steps that can be completed if testing confirms the problem.

Technically, the risk associated with radon in residences is labeled "uncertain." The Environmental Protection Agency recommends that every existing home be tested for radon and that action be taken if the test shows radon concentrations greater than 4.0 picocuries/liter of air.

Before acting, the best place to start is by looking at the EPA Map of Radon Zones. This is detailed down to the county level so you can determine with a relatively high degree of accuracy if the home in question is located over a geological zone where radon activity is high. If it is, don't hesitate to take mitigation steps. (If it isn't, well, the "uncertain" language applies. Take that as you will.)

Check Local Sources

The EPA recommends that all new homes be built with radon resistant elements (Radon Elimination Strategies, below). This recommendation has been adopted as a requirement by some codes. These precautions are, or course, more important in areas where higher concentrations of radioactive materials are present in local rock. Check with state public health and environmental agencies about testing and risks in your area.

Subslab Poly

The EPA-recommended details for radon abatement call for poly to be placed above the gravel subbase and below the slab. Placing concrete directly on plastic can contribute to slab cracking due to uneven shrinkage and curling. For that reason, consider putting a layer of damp compacted sand above the poly before placing the concrete see Subslab Vapor Barriers in Subgrade and Subbase).

Critical Construction Details

Figure: Radon Elimination Strategies
Different strategies for venting radon are required for different foundations: Houses with basements (left), houses with crawlspaces (center), and basements or slabs with sump chambers (right). The usual approach in new construction is to build-in, at a modest cost, the depressurization infrastructure. This typically needed only for homes in mapped zones with a risk for radon. Once the house is built, it can be tested, and only if radon concentrations are sufficiently high, do you need to install an inline fan to actively mitigate the risk.
Different strategies for venting radon are required for different foundations: Houses with basements (left), houses with crawlspaces (center), and basements or slabs with sump chambers (right).

The usual approach in new construction is to build-in, at a modest cost, the depressurization infrastructure. This typically needed only for homes in mapped zones with a risk for radon. Once the house is built, it can be tested, and only if radon concentrations are sufficiently high, do you need to install an inline fan to actively mitigate the risk.

The EPA recommends the following system to prevent radon from entering basements and crawlspaces, and to exhaust radon from below slabs to the outdoors:

  • Subslab: Place gas-permeable layer (4-in. layer of clean gravel).
  • In basements: Plastic sheeting should be placed on top of gravel sub-base and under slab.
  • In crawlspaces: The sheeting is placed over the crawlspace floor.
  • Seal and caulk all cracks, joints, or penetrations.
  • Use 3- or 4-in. PVC vent stack from sub-slab to above roof. Mark this clearly so plumbers won’t mistake it for a drain vent.
  • In attics: Supply a junction box for power in case homeowners choose to install a powered vent fan.