How Radon Mitigation Works

Radon mitigation looks like a fan and a pipe. It isn't. A working mitigation system is a small piece of building science that depressurizes the soil beneath your home, captures radon-laden soil gas before it enters your living space, and exhausts it safely above the roofline — and then proves it worked with a written post-mitigation test.

This is the plain-language tour of how the system works, what the parts do, what's different about systems built for Colorado, and what to expect a quality install to look like.

Radon enters a home because the air pressure inside is slightly lower than the air pressure in the soil beneath the foundation. That pressure difference — called the stack effect in winter, when warm indoor air rises and creates suction at lower levels — pulls soil gas, and the radon dissolved in it, up through any small opening it can find. Hairline slab cracks, the floor-wall joint, sump pits, plumbing penetrations.

A mitigation system reverses that pressure difference. A fan applies suction to the soil under the slab (or under a sealed membrane in a crawlspace), so the pressure beneath the foundation is lower than the pressure inside the home. Soil gas now flows into the system instead of into your living space, and the system safely exhausts it above the roof.

This is why sealing cracks alone almost never works — and sometimes makes things worse. More on that here.

Sub-slab depressurization (SSD)

The most common method for basements and slab-on-grade homes. The contractor drills through the basement slab into the soil or gravel beneath, creates a small extraction pit, and connects it to a sealed PVC pipe that exits the home and exhausts above the roofline. A fan in the line pulls air from beneath the slab.

Full SSD walkthrough →

Sub-membrane depressurization (SMD)

The crawlspace equivalent. A heavy vapor barrier is laid across the entire crawlspace floor, sealed at the perimeter and at penetrations. The suction is applied beneath the membrane, so soil gas is pulled out before it can pass through.

Full SMD walkthrough →

An active system has a fan that runs 24/7 and reduces indoor radon by up to 99%. A passive system has the pipe in place but no fan — it relies on natural updraft and typically reduces radon by up to 50%.^[1]

Most homes that need mitigation in Colorado need active systems. Passive systems are common as a rough-in in newer Colorado homes (post-2009 builds often have one) but are converted to active when the post-construction test shows radon at or above 4.0 pCi/L.

Passive vs active full comparison →

Whether your install is sub-slab or sub-membrane, the working system has the same parts:

• Suction point through the slab (SSD) or beneath the membrane (SMD), connected to the rest of the system.
• Sealing of slab cracks, the floor-wall joint, sump covers, plumbing penetrations (and the perimeter membrane in crawlspaces). Sealing alone doesn't work — but it is essential alongside depressurization.
• Pipe — typically 3-inch or 4-inch Schedule 40 PVC, routed through the home or up an exterior wall.
• Fan — the inline fan that pulls air from the suction point. Located in unconditioned space (attic or exterior wall) so any leak in the positive-pressure side doesn't release radon back into the home.
• Exhaust point above the roofline, away from windows, chimneys, and adjacent buildings. AARST standards specify minimum distances.^[2]
• Manometer — a small U-tube or digital pressure gauge at the suction point. It's how you confirm the fan is running and the system is working.
• Post-mitigation test — within 30 days of install, a 2 to 7-day closed-house test that confirms indoor radon is below 4.0 pCi/L.^[3]

Equipment deep dive →

Altitude affects fan sizing

Radon fans lose roughly 4% of their airflow capacity for every 1,000 feet of elevation.^[4] Colorado Springs sits at about 6,000 feet. Denver at 5,280. A fan that's perfectly sized for a sea-level install is meaningfully underpowered in Colorado, which is why the right fan model matters more here than almost anywhere else. Full altitude correction explainer →

Front Range geology pushes baselines higher

Colorado's uranium-bearing granite and shale are the source rocks for indoor radon. Pikes Peak granite contains uranium-bearing accessory minerals; the Front Range as a whole averages much higher indoor radon than the U.S. national average. CDPHE estimates roughly half of Colorado homes test above the EPA action level.^[5]

State licensing is real

Colorado is one of the few states with state-level radon contractor licensing through the DORA Office of Radon Professionals. A correctly installed Colorado system is installed by someone who is both DORA-licensed and either NRPP or NRSB certified.^[6]

A working system runs quietly in the background. A few signals tell you everything's fine:

• The manometer shows a steady offset between the two columns of fluid (or a steady digital reading), indicating the fan is pulling vacuum.
• The fan motor is quiet from inside the home and barely audible outside near the exhaust.
• Your retest every two years confirms radon levels remain below 4.0 pCi/L.^[3]
• Your electricity bill increases by less than $10/month for the fan operation.^[3]

If any of those change — manometer columns equalize, fan starts to hum or rattle, retest shows levels rising — call the original installer. The fan itself is the most likely component to fail (typical life 5+ years), and replacement runs $150–$400 in parts plus labor.

Post-mitigation expectations →