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Would an exhaust fan in a basement reduce radon?

By Radon411 Editorial TeamUpdated How we research

Short answer

Probably not, and it can make things worse. An exhaust fan pushes basement air outdoors, and the make-up air that replaces it can pull more soil gas, and radon, in through the foundation. The fan that does work is a radon mitigation fan, which draws radon from under the slab and vents it above the roof.

Next step: a certified professional designs and installs the fix. Find a mitigation professional

A house usually sits at lower pressure than the soil around it, which is what draws radon in. Exhaust fans, dryers and other appliances that remove air add to that pull, so exhaust ventilation alone is a weak and uncertain way to lower radon.

A fan is connected to pipes in the soil beneath the slab, runs continuously, and creates a vacuum under the floor so soil gas is drawn away from the house. Subslab suction typically lowers radon by 50 to 99 percent.

What to do next: test with the house closed up; if the result is high, get estimates for a sub-slab system from certified mitigation professionals.

Why an exhaust fan can backfire

A house already runs at slightly lower air pressure than the soil around it, and that difference is what pulls radon in through cracks and openings in the foundation. An exhaust fan pushes indoor air out. The air that replaces it has to come from somewhere, and some of it comes from the soil. So a fan blowing basement air outdoors can drive more soil gas, and more radon, into the house.

How pressure inside a house draws in radon
EffectWhat happens
The house as a vacuumIndoor air pressure is usually lower than in the soil, so the house draws radon in through foundation openings
Exhaust fans and appliancesExhaust fans, dryers, water heaters and fireplaces remove air, which can drive soil gas in as make-up air enters the lower part of the house
Stack effectWarm air escapes high up, and make-up air, some of it from the soil, enters low down

That is why ventilation by itself is a weak and uncertain tool against radon, and why a do-it-yourself fix without the right knowledge can actually raise the level.

A radon fan is a different machine

A radon system also uses a fan, but it pulls from under the floor, not from the room. The fan is connected to suction pipes set into the soil below the slab; it draws radon from beneath the home, releases it outdoors above the roof, and at the same time creates a negative pressure under the slab, so soil gas is pulled away from the house instead of into it.

Cross-section of a house with a sub-slab depressurization system: soil gas drawn from beneath the slab into a suction point, up a vent pipe, through a fan outside the living space and out above the roofline.
Read this figure as text
Sub-slab depressurization system in a house cross-sectionGround levelLiving spaceBasement12345
  1. Soil gas drawn from beneath the slab
  2. Suction point through the slab
  3. Vent pipe
  4. Fan, outside the living space
  5. Discharge above the roofline
A sub-slab depressurization system: the fan draws from under the slab, not from the basement air, and vents above the roof.
Basement exhaust fan and radon fan, side by side
AspectBasement exhaust fanRadon mitigation fan
Pulls air fromThe basement roomThe soil under the slab, through sealed piping
Effect on pressureLowers indoor pressure, which can draw soil gas inCreates a vacuum under the slab, which keeps soil gas out
Where it sitsIn the living spaceNever in or below a livable area; usually an attic, garage or outside
How it runsOn and offContinuously; it must never be turned off
Typical reductionNo typical reduction is given for itSubslab suction typically lowers radon 50 to 99 percent

Ventilation approaches that can work

Some ventilation methods are recognized for radon, but they work differently from an exhaust fan, and all of them rank behind soil suction.

Typical radon reduction, EPA

Basement and slab-on-grade homes

  • Subslab suction (subslab depressurization)50 to 99 percentWorks best if air can move easily in material under slab.

Crawlspace homes

  • Natural ventilation in a crawlspace0 to 50 percentCosts variable.

Any type of home

  • House (basement) pressurization50 to 99 percentWorks best with tight basement isolated from outdoors and upper floors.
  • Natural ventilationVariable/TemporarySignificant heated or cooled air loss; operating costs depend on utility rates and amount of ventilation.
  • Heat recovery ventilation (HRV)VariableLimited use; effectiveness limited by radon concentration or the amount of ventilation air available for dilution by the HRV.
Typical radon reduction for soil suction and for ventilation approaches. Methods described only in words get no bar.
Ventilation methods for radon and their limits
MethodHow it differs from an exhaust fanLimits
House or basement pressurizationA fan blows air into the basement to raise its pressure and hold radon outLimited by construction, climate, appliances and lifestyle; lowest-level doors and windows must stay shut; moisture and energy penalties; only after common methods fall short
Heat recovery ventilatorBalanced: brings in outdoor air while exhausting indoor air, recovering heatVariable effect; best in the basement only; must run all the time; raises heating and cooling costs
Opening windowsMixes outdoor air with indoor air rather than pushing air outTemporary: levels most often return within about 12 hours of closing up

Exhaust fans during a radon test

During a the house is kept closed, and fans that bring in outside air are off. Small exhaust fans running only briefly, and a radon system's own fan, may run.

Fans during a short-term test
FanDuring the test
Radon system fanOn, for the whole test
Small exhaust fans for short periodsAllowed
Fans or machines that bring in outside airOff

More on what affects a reading is in what can affect a radon test.

Questions about fans and radon

Can an exhaust fan make radon worse?

It can. Exhaust fans remove air from the home, and the make-up air entering the lower part of the house can bring soil gas, and radon, in with it.

Would a fan blowing air into the basement help?

That is house pressurization, a recognized technique, but its effect is limited by construction, climate and how the home is used, and it is tried only after the common methods have not lowered radon enough.

Do ceiling fans or box fans reduce radon?

Fans and vents that circulate air are among the steps suggested for keeping radon low in a home below the action level, but this kind of ventilation is only temporary and is not considered radon mitigation.

Why does a radon fan go in the attic or outside?

Because the exhaust fan must not be located in or below a livable area. Common locations are unconditioned spaces such as attics and garages, or the outside of the home.

Can I install a radon fan myself?

A qualified contractor is recommended, because without the right equipment and knowledge you could increase the radon level. See do-it-yourself radon mitigation.

Will a radon system affect my furnace or water heater?

It is something contractors look at: a contractor may have concerns about backdrafting of combustion appliances when planning radon work, and may recommend having them checked by a qualified inspector.

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