Radon and Buildings

Radon is a gas that is generated in the ground as a result of the decay of uranium. Radon behaves like all gasses and always moves toward an area of lower pressure.

Buildings are generally lower pressure than the outdoors, so radon is actually drawn into buildings from soil surrounding the building through any opening in the foundation.

Radon Entrypoints
How radon gas can enter your home.

These openings include:

  • sump pump pits
  • unplumbed floor drains
  • block walls
  • cracks in the concrete floor
  • joints where the concrete floor meets the basement wall or basement wall footing

Sealing these openings may reduce the building radon level; however, studies demonstrate that radon is pressure driven and will still find a way in the building.

As suggested in the above diagram, well water could contribute to the indoor air concentrations of radon; however, in the areas served by Highland Air, well water has NOT been found as a significant source contributing to the indoor air radon concentrations.

Once the radon is inside the building, the building itself prevents the radon from being diluted by the outdoor air. That is how radon concentrations become elevated in your home.

Active Soil Depressurization

To keep radon out, the area immediately around the building foundation must be depressurized. This method is called active soil depressurization or ASD. ASD uses plastic pipe and a fan to create a vacuum under the buildings foundation that reduces the pressure relative to the house.

These fans run continuously, 24 hours day, 365 days a year.

These systems require some home owner maintenance, are more expensive to operate (utility consumption), and are about three times more expensive to install than a typical ASD system.

HRVs should only be used for radon remediation when ASD cannot be employed.

What's Under Your Home?

The design of any ASD System is dependent upon what is beneath your home.

Builder Installed Stone

Prior to pouring the concrete floor, builders typically install a layer of stone.

This stone may allow the use of a low-power radon fan.

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Mixed stone – good for radon systems.
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Nice round stone, super for radon systems.
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Similarly sized stone – very good for radon systems.
Native Gravels

In some homes, no layer of stone was placed before the concrete floor was poured.

A layer of native gravel may run close to building foundations. Radon moves through these layers. The key to controlling radon gas movement is getting a vacuum into the gravel layer.

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Tight native gravel allows radon movement
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Average native gravel, good radon movement.
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Loose native gravel, excellent radon movement.
Tighter Soil Types

Radon can also move through tighter soil.

Tighter soil types may require a higher suction type fan.

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Very tight native gravel.
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Coarse sand and gravel.
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Native coarse sand.

Selecting The Right Radon Fan For an ASD System

Sub-slab (below the concrete) diagnostics are used to help determine which fan would be best for the mitigation based on soil gas availability. This is the first step Highland Air performs in designing a radon system. Highland Air cannot determine what fan would be used for the mitigation without first performing sub-slab diagnostics. For this reason, Highland Air cannot provide which radon fan will be used at the time of the mitigation proposal.

Fans used in radon mitigation can range in power consumption from 20 watts to 150 watts. These fans run constantly, therefore, operating cost can be as low as $36 per year for a minimal size fan; $130 for a medium-size fan; and about $260 annually for a high-suction or high-flow fan. These cost estimates are based on electricity cost of 21`cents per kilowatt hour.

Using the appropriate size fan will reduce the radon level to a low level (there is no safe level for radon) say, below 4 pCi/L. However, most ASDs will achieve a level below 2 pCi/L and often below 1 pCi/L. Installing an oversize fan will also reduce radon, but at a greater cost to the homeowner. 

Choosing a knowledgeable mitigator could save the homeowner money by having a system built with the right size fan to control the radon entry and minimize the loss of conditioned air (heated and or air conditioned). 

The factors that determine the appropriate fan for the job are the radon source, the house design, and construction details. There are over 30 different fan models to choose from the three main fan suppliers.

Examples of Commonly Used Fans

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Low suction, low flow fan.
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Moderate suction, moderate flow fan.
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High suction, low flow fan
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Moderate suction, very high flow fan.

Exterior Fan Enclosures

Exterior fans may be mounted directly on the outside of the house or secured under a “fan enclosure.” Highland Air uses “fan enclosures” for all exterior wall mounted systems, which provide protection from the elements and have a much better overall appearance.

In addition, Highland Air typically uses 3″ x 4″ rain gutter downspouts for the radon exhaust stack attached to the house. Often these stacks blend right in with the house’s trim or existing rain gutters.

Highland Air has four standard colors (White, Almond, Brown and Clay) for exterior systems however custom colored system can be made for an additional fee.

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White fan enclosure.
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Brown fan enclosure.
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Clay fan enclosure.
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Almond fan enclosure.

What's Under The Fan Enclosure

Some people ask, “Why are fan enclosures so big if they are just covering a fan?”

Under Fan
Under the fan enclosure.


The long black flat section attached to the house is called the “base plate” and it helps keep weather out by fitting inside the fan housing. A layer of foam insulation is placed between the base plate and the house to help eliminate any sound transfer to the house interior.

Between the fan and the stack is called the “transition box” and this helps to isolate the fan from the building to further reduce any fan noise transfer to the house. This is especially important when using high-suction fans to treat tight soils such as sands and tight gravels. 

For the most part radon mitigation systems cannot be heard, only tight gravels and sands may produce some low-level noise.

Interior Run Systems

Interior run systems will only have an exhaust stack showing on the outside of the house, much like a vent stack for a house plumbing system. They are left white or painted black to match existing plumbing vent stacks.

Interior run systems are run typically though a closest or through the garage. Garage wall mounted systems like the one shown are required to have “fire collars” mounted on the walls and ceiling to retain the fire rating of the wall and ceiling.

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Interior run system
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Interior system exhaust.