Radon Mitigation: A New Business Opportunity

November 29, 2016
Winter 2016
A version of this article appears in the Winter 2016 issue of Home Energy Magazine.
Click here to read more articles about Healthy Homes

Of all the known dangers to residents associated with a home’s indoor air quality, the least recognized may be exposure to radon gas. Unless a building has been built to specifically prevent radon entry, there is no way to keep it out of the building’s interior if a radon source is present in the ground under the building. The good news is that it is easily measured and mitigated, which makes it a potential add-on service for a home performance contractor.

Radon is a radioactive element. It is a colorless, odorless, tasteless gas. It is chemically inert and heavier than air. It is generated by the spontaneous radioactive decay of uranium. All radioactive elements transform into other elements as they release radioactivity. The rate of change, called the radioactive half-life, is an inherent quality of the radioactive element and is the time required for one-half of a given amount of a radioactive element to transform into another element. The half-life of uranium-238 is about 4.5 billion years; of uranium-235 about 700 million years. The half-life of radon is 3.8 days. So the bad news is that the source of radon is, functionally, forever. The good news is that once you stop radon from entering a building, even if the building is hermetically sealed, the radon will go away.

EPA Map of Radon Zones
Figure 1. EPA’s radon risk map. (Environmental Protection Agency)

Continuous Radon Monitor Trace

Continuous Radon Monitor Trace
Figure 2. It is not useful as a measure of health risk to take radon measurements for periods of less than two days. (Dick Kornbluth)

A long-term charcoal alpha-track test kit. (National Cancer Institute)

Mechanisms of Radon Entry

Mechanisms of Radon Entry
Figure 3. Radon enters a building through cracks and holes in the building’s foundation and through other pathways. ©All Rights Reserved. How radon enters a house. Natural Resources Canada, 2008. Reproduced with the permission of Natural Resources Canada, courtesy of the Geological Survey of Canada, 2016. (Natural Resources Canada)

Subslab Depressurization

Subslab Depressurization
Figure 4. The most common mitigation method is subslab depressurization, which depressurizes the soil to permanently lower the radon driving force in the ground. (Dick Kornbluth)

Subslab Communication Test

Subslab Communication Test
Figure 5. The second step in establishing full extension of the subslab depressurizaion field is to conduct a subslab communication test. (Dick Kornbluth)

Elements of a Subslab Depressurization System

Elements of a Subslab Depressurization System
Figure 6. All work should be done in conformance with existing national radon mitigation standards (ASTM 2121). (Dick Kornbluth)

Where feasible (that is, where you have access to the entire surface), crawl spaces should be sealed with a membrane consisting of at least 6-mil reinforced polyethylene. (Dick Kornbluth)

Where Is Radon Found?

Uranium is found in certain geologic formations, including some shales and limestones. The shale formation most commonly associated with radon is the Marcellus shale.. EPA has been collecting data on radon distribution for many years and has developed a radon risk map (see Figure 1). The map is based on the measurement of indoor radon levels and associated health risks.

What Are the Health Risks of Exposure to Radon?

Radon is defined as a class 1 carcinogen. The only known disease associated with exposure to radon gas is lung cancer; it is the second-leading environmental cause of this disease after cigarette smoking. While there is debate as to what level of radon gas exposure is unsafe, there is no debate that some level of exposure to radon gas is associated with an increased risk of developing lung cancer. EPA has established an acceptable level of indoor radon gas of less than 4 picocuries per liter (pCi/l), and recommends that indoor radon gas levels be reduced to 2 pCi/l or less.

How Is Radon Measured?

There are two general methods for measuring radon gas: short-term and long-term. Short-term measurements are designed to run from two to seven days; long-term measurements from three months to one year. Because radon levels in a building fluctuate continuously, even changing by the minute, it is not useful as a measure of health risk to take radon measurements for periods of less than two days (see Figure 2).

Short-term testing devices are divided into two general categories: passive integrated devices and electronic continuous radon monitors. All short-term tests require that the house be placed in closed-house conditions (windows closed; doors closed except for entering and exiting) for at least 12 hours before the test begins and for the duration of the test. Passive integrated devices include charcoal test kits, liquid scintillation test kits, and E-perm tests. Charcoal and liquid scintillation test kits can be purchased as DIY tests; E-perm tests must be done by a radon professional. Short-term tests are useful as screening devices to determine if radon levels are elevated enough to recommend mitigation (see Figure 3).

Long-term testing is a more useful tool to establish the health risk in a building. The device used for this purpose is called an alpha-track test. It is a passive device and is simply placed in a test location and left for the test period. No specific building configuration is required. When the test is complete, the kit is mailed to the manufacturer and the average radon level over the test period is reported.

How Does Radon Enter a Building?

Radon enters a building through cracks and holes in the building’s foundation and through other pathways (see Figure 3). The mere fact that a concrete basement floor shows no visible cracks is no guarantee that radon gas is not present in the building. My company has measured high radon levels in houses with no visible cracks in either the concrete floor or the foundation walls. Although diffusion may play a minor role in radon entry, the primary mechanism for radon gas entry into buildings is pressure—that is, the pressure of the radon gas in the ground relative to the living space adjacent to the ground. Any force that affects the pressure in the lowest level of the house can affect the rate of radon entry, including atmospheric pressure (which has only a small effect), stack effect, and mechanical ventilation. However, the largest determinants of radon entry rates are the soil concentration of radon gas and the porosity of the soil. So it’s not about the building so much as it is about what’s under the building.

When assessing the internal forces in a building that could contribute to increased radon levels, it’s important to understand the dynamics of air movement in the building. This was dramatically brought home to me several years ago when I was asked to do a radon mitigation estimate for a retired mechanical engineer. His initial basement radon level was in the range of 40 pCi/l. He attempted to lower his radon level by installing an elaborate duct system in the basement connected to an inline fan blowing out the basement rim joist. I looked at this enormous negative-pressure-inducing system and assumed that his basement radon level must have skyrocketed. What actually happened was that his basement radon level dropped from 40 pCi/l to 20 pCi/l. After thinking about this paradoxical result, I realized what had happened. The basement has several pressure boundaries (slab:ground, foundation:ground, foundation:outside, basement ceiling:first floor). If the pressure boundary between the slab and the ground was tighter than the pressure boundary between the basement and the first floor, the net direction of flow would be from the first floor to the basement, resulting in dilution of the basement air and reduction of the radon level. Unfortunately I didn’t get to complete his radon mitigation, so I don’t know how his project finally turned out.

(For an example of successful radon mitigation, see “Adventures in Radon and Moisture Mitigation,” HE July/Aug ’14, p. 20.)

Mitigating Radon

Since radon gas entry into buildings is driven by pressure forcing radon gas through holes in the foundation, there are several theoretical approaches to lowering building radon levels. These are

  1. sealing the foundation to keep radon out;
  2. pressurizing the foundation to overcome the negative pressure bringing radon into the building; and
  3. depressurizing the soil to permanently lower the radon driving force in the ground.

As a practical matter, sealing the foundation almost never works. It is extremely labor intensive and would require sealing the entire basement floor with an epoxy sealer with no guarantee of success.

Although, it is possible to pressurize the foundation to keep radon out, this is not practical either. To do a permanent fix would require isolating the basement from the rest of the house and either bringing in outside air or pressurizing from the living space above. Bringing in outside air would waste a lot of energy. Pressurizing from the living space above would significantly depressurize the living space and increase infiltration. And the fan would have to run continuously to be effective.

Subslab Depressurization

The most common mitigation method is subslab depressurization, which depressurizes the soil to permanently lower the radon driving force in the ground (see Figure 4).

A subslab depressurization system consists of an interior component that is one or more 3- or 4-inch PVC pipes vertically penetrating the concrete slab, and an exterior component consisting of an inline fan and 3- or 4-inch vertical PVC stack. When designing subslab depressurization systems you must consider both form and function. Function considerations require that the system must effectively depressurize the entire subslab surface area and also conform to the national radon mitigation standards (see Step 4). Form considerations require the system to have minimal impact on the utilization of the interior space; and the exterior components (fan and vent piping) must be as unobtrusive as possible. For example, on the interior there will be one or more vertical 3- or 4-inch PVC pipes penetrating the slab. They should not be in locations that will interfere with the use of the interior space, such as in the middle of the floor in a finished room. And it’s hard to imagine a homeowner who will be pleased to have a 3- or 4-inch white PVC pipe running up the exterior front wall of their house to the roofline.

The typical single suction point subslab depressurization radon system can be installed in a single day with a 2-person crew with an installed price range of $900-$1,200, depending on market competition. Additional suction points, French drain sealing and crawlspace sealing will increase both cost and installation time.

The basic system consists of five steps:

Step 1. Locate the position of the first subslab suction point, taking into consideration the location of the fan and vent stack on the outside of the house since the first subslab suction point will be connected to the exterior fan and stack.

Step 2. Evaluate the requirements necessary to establish full extension of the subslab depressurizaion field under the concrete slab. See Steps 3 and 4.

Eliminate large holes and cracks in the foundation, which could interfere with the establishment of a negative pressure field under the slab. This includes

  • covering sump crocks—we use ¼-inch Lexan and incorporate a check valve in the cover to allow surface water or condensate water to drain into the sump crock;
  • covering exposed perimeter drains;
  • installing check valves in floor drains connected to the ground;
  • sealing openings around plumbing pipes entering the ground; and
  • sealing large floor cracks.

Step 3.(Optional but recommended.) Conduct a subslab communication test (aka pressure field extension test) to determine the requirements for fully extending the subslab depressurization under the entire slab (see Figure 5).

  1. Locate the place in the ground level floor where the subslab system primary suction point will be installed.
  2. Drill a hole in the slab at that location the same diameter as a Shop-Vac hose (the suction hole).
  3. Drill small test holes (3/8-inch–½-inch in diameter) through the slab at the corners of the basement away from the subslab system suction hole.
  4. Measure the pressure in each test hole relative to the basement using a micromanometer. You will normally get measurements in the range of +1to +5 Pa.
  5. Insert the Shop-Vac hose in the suction hole and measure the change in pressure at the test holes. A change from a positive to a negative pressure, indicates communication between the suction hole and the test hole.
  6. Using a scale drawing of the basement floor, map the results of the subslab communication test, drawing lines from the suction hole to the test holes where communication was established.
  7. Where communication was not established, incorporate additional subslab suction points teeing off the main subslab system pipe to ensure 100% extension of the subslab suction field.

Step 4. Drill the primary and any additional required 3- or 4-inch suction points through the slab. We use a Bosch RH540s rotary hammer (approx. $400-$500 from various vendors) to drill the suction holes. Masonry bits may add an additional $100-$150 to the investment.

Step 5. Install interior piping.

Step 6. Install exterior piping to include an inline fan and an electrical connection with a disconnect. A pressure gauge or pressure alarm is installed on the interior piping to indicate fan performance. All work should be done in conformance with existing national radon mitigation standards (ASTM 2121). [Fig 7: Elements of a Subslab Depressurization System]

The main requirements are as follows:

  • Radon fans used in ASD radon mitigation systems shall be installed either outside the building or inside the building, outside of occupiable space, and above the conditioned (heated/cooled) spaces of a building” (ASTM E2121-11, section
  • It is acceptable to install the fan and vent in an attached garage as long as there is no living space above the garage.
  • The radon vent pipe must be rigid and have a diameter of at least 3 inches. Radon pipes are usually PVC or ABS, and Schedule 40 or Schedule 20 for interior piping.
  • The radon vent pipe must extend above the edge of the roof.
  • The radon vent pipe must terminate at least 10 feet away from any window or door open to occupiable areas of the building unless it terminates at least 2 feet higher than the nearest opening.

Crawl Spaces

Where feasible (that is, where you have access to the entire surface), crawl spaces should be sealed with a membrane consisting of at least 6-mil reinforced polyethylene. This membrane should be secured to the walls of the crawl space foundation using pressure-treated furring strips mechanically fastened to the foundation walls (see Figure 8).

Crawl spaces are partly or wholly inaccessible can be problematic. Before dealing with the crawl space as a radon source, all the other mitigation measures for the house should be completed, and the basement and living space above the crawl space tested for radon. If the radon problem persists, the crawl space should be isolated from the basement, from the exterior, and if possible, from the living space above. The crawl space can then be depressurized with a separate fan system.

Foundation Walls

Foundation wall construction is concrete block, poured concrete, rubble stone, or wood. We have rarely had to deal with concrete block foundation walls as a source of radon. I suspect that the waterproofing routinely used on the exterior of concrete block walls is an effective barrier to radon entry. Where we have had to deal with a concrete block wall, it has been the wall between the house and the attached garage, which I suspect was not waterproofed, since it was an interior wall. While it would be reasonable to assume that rubble stone walls would be extremely porous, we have never had to resort to sealing one to solve a radon problem. I’m not sure what prevents rubble stone walls from being a source of radon, and I cross my fingers every time we encounter one. Wood walls behave like concrete block walls, and poured concrete is not porous.


During the early days of radon mitigation, EPA incorporated a backdraft test into its radon mitigation standard. This made sense. Although in theory the ground is completely isolated from the basement during the operation of a Subslab depressurization system, it is certainly possible that there would be sufficient connection between the ground and the basement for the radon fan to depressurize the basement. And in my area of New York, it is not uncommon for homeowners to replace atmospheric heaters with sealed-combustion units, leaving orphan water heaters venting into oversized flues..

The current national radon mitigation standards have eliminated the requirement for backdraft testing of atmospheric combustion appliances. The only language referencing backdrafting in the standard says that radon mitigation system should not cause backdrafting.

We have routinely incorporated backdraft testing in our projects ever since we caused a backdraft problem with a radon mitigation system that worked as designed to eliminate a radon problem. The customer called us several weeks after the system was installed to complain about a smell in his basement. I went to his house and discovered that his water heater was backdrafting. The source turned out to be the open tops of his concrete block walls. Even though the radon mitigation system was working, the radon fan was drawing enough basement air into the block walls to depressurize the basement.

A Business Opportunity

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If you are a home performance contractor, adding radon mitigation to your suite of services can be a good business opportunity. Radon exposure is a health hazard in significant areas of the United States. Testing for radon is easy and mitigating radon does not require a large additional investment in equipment for an existing home performance contractor (mainly a rotary hammer and bits).

Richard Kornbluth is principal at Dick Kornbluth, LLC, a home performance consulting company. He is a board member of Building Performance Contractors Association of New York and the Building Performance Institute, Incorporated.

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