Radon and Radon Mitigation

What is Radon?

Radon is a naturally-occurring, colorless, odorless gas. A product of the decay of uranium, it can be found in soil, water, and air. It accounts for about half of the radiation we receive from natural sources.

Radon is not a health problem outdoors. The health concern arises when radon is trapped in an enclosed space – a house – and does not get diluted. Long-term exposure to high levels of radon is linked to lung cancer.

Two things have to happen for radon to be a health concern: there has to be a source and a pressure difference. As the uranium decays, it produces radon, which gets trapped in the soil around the foundation. Infiltration, driven by the wind and stack effects, pulls air through cracks and openings in the foundation slab and walls. Radon enters the house in the air, and ‘pools’ in the basement.

How it Enters the Home

In any house, the pressure difference due to the stack effect is greatest in the below grade portion. However, the leakier the house, the stronger the stack effect. As a result, homes that are not built tight have a greater the possibility of high radon concentrations in lower areas of a house. In addition, poor air circulation in basement areas leads to higher radon concentration levels.

High radon concentrations are a health concern, but everything escalates when you have bedrooms in basements, as people are spending several hours every day in the part of the house with the greatest potential for high radon concentration levels.

Wind and stack effects are strongest during the winter. The result is that radon levels are typically higher during the colder months. This means, for accurate test results, at least a portion of long-term testing period should be in the winter.

Radon Mitigation

The first line of defense against radon is thorough air sealing. In new construction, a vapor barrier below the slab and good air sealing throughout the building envelope is crucial to slow the stack effect and the rate at which radon is pulled into the house.

Making new construction radon-resistant requires air sealing and a rough-in of an under-slab ventilation system to allow for what is known as active soil depressurization. This system includes the following components:

1. At least 4″ layer of clean coarse gravel below the slab
2. 6 mil polyethylene vapor retarder
3. A 3-4 inch solid PVC Vent Pipe extending from the gravel layer to the roof.
4. A junction box in the attic for use with a vent fan (if needed).

Radon Updated

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In new construction, radon-resistance measures can vary in cost depending on the radon risk potential in your area (see this map), however the cost to include radon-resistance during construction is far less than the cost to mitigate after the fact. Use this checklist to let home buyers know about your efforts.

In existing houses, where radon has been detected, a modified version of the radon-resistance measures can be installed. A hole is drilled in the foundation slab and a pipe is installed that goes up and out of the house at the roof level. A fan pulls the radon from under the house, through the pipe and to the exterior. Sealing and caulking the foundation and reducing the stack effect are important in this scenario as well.

Using the natural draught effect, a passive system will pull radon and other soil gasses out of the home even without a fan. If high levels of radon are detected, then a mechanical fan can be installed in the attic to help pull the gasses out of the home. The fan is also a useful addition in the event there is a leak in the pipe, the fan will help prevent the gasses from spilling into the home.

Here is a link to the EPA passive system:
For more detailed information about radon and housing, visit, http://www.epa.gov/radon/index.html


Radon levels in a house depend on:

  • Soil type
  • Condition of the foundation
  • Rate of air infiltration from the ground
  • Occupant lifestyle
  • Weather
2016-10-18T21:52:32-06:00 Technical Articles|