How a Radon Mitigation System Works — Explained for Homeowners

March 2, 2026

A radon mitigation system sounds technical, but the concept behind it is straightforward: pull the radon gas out of the soil before it enters your home. The standard method — sub-slab depressurization — is proven, reliable, and used in hundreds of thousands of homes across the Midwest. Here’s exactly how it works.

The Problem: How Radon Enters Your Home

Radon is produced continuously in the soil and bedrock beneath your home. Soil acts as a permeable medium — radon migrates through it in all directions. The path of least resistance is usually upward, through cracks in your foundation slab, gaps around utility penetrations, sump openings, and block wall cores.

Your home also creates a stack effect: warm interior air rises and escapes through upper levels, creating slight negative pressure at ground level. This negative pressure literally draws soil gas — including radon — upward through any available opening in the foundation. It’s the same principle that makes a chimney draw.

The result: without intervention, radon accumulates in your basement and lower levels, sometimes reaching levels many times the outdoor concentration.

How Sub-Slab Depressurization Works

Sub-slab depressurization (SSD) reverses this process. Instead of letting your home pull radon upward, the system pulls it sideways and out before it reaches floor level.

The components:

1. Suction pit The contractor drills a hole (typically 4–5 inches in diameter) through the concrete slab. They excavate a small pocket in the aggregate beneath — enough to create a suction point that communicates with the gravel or soil under the entire slab.

Before drilling, a good contractor performs a diagnostic test — drilling a small hole and measuring pressure differentials to confirm the aggregate communicates across the foundation. This determines how many suction points the system needs. One point handles most Indiana homes with a single basement; two or three may be needed for complex foundations.

2. PVC pipe A 3- or 4-inch PVC pipe runs from the suction pit, through or around the home, to the exterior above the roofline (or in some installations, out the side wall at least 12 inches above grade). The pipe is the radon’s exit path.

3. The fan An in-line radon fan — mounted in the attic, exterior wall, or garage — is the heart of the system. It creates continuous negative pressure in the sub-slab space. This negative pressure is greater than the stack effect your home creates, so instead of your home pulling radon in, the system pulls it down and out through the pipe.

The fan runs 24 hours a day, 7 days a week. This is intentional — radon is produced continuously, so the system must run continuously.

4. The discharge point The pipe terminates outside the home, above the roofline or at least 10 feet from any window or air intake. Radon is discharged directly into outdoor air, where it disperses immediately to safe concentrations.

5. The manometer A U-tube manometer — a small plastic tube containing colored water or oil — is installed on the pipe. When the system is running, the fluid in the tube is uneven, showing the negative pressure the fan is creating. When the fluid sits level, the fan has stopped running. This is your early warning indicator.

What Happens to Radon Levels

After installation and a 24-hour settling period, a properly designed system will typically reduce radon levels by 50–99%. Most Indiana homes that start at 8–12 pCi/L achieve post-mitigation levels of 1–2 pCi/L — well below the EPA action level.

The exact post-mitigation level depends on:

• Starting concentration (higher starting levels may need more suction points)
• Foundation type and integrity (major cracks may need sealing)
• Sub-slab aggregate permeability
• Fan sizing

This is why a post-mitigation test (performed 24–48 hours after installation) is mandatory. It confirms the system achieved adequate reduction. A legitimate contractor always provides this.

Foundation Type Variations

Basements: Standard SSD as described above. One or two suction points through the slab handles most cases.

Crawl spaces: A different approach. The contractor installs a continuous polyethylene membrane over the crawl space floor (if not already present) to create an air barrier, then runs the fan system to depressurize beneath the membrane. Without the membrane, the open soil would allow too much air infiltration for the system to maintain negative pressure.

Slab-on-grade: SSD works the same way — drill through the slab, create a suction point. Access for pipe routing may be more challenging since there’s no basement.

Block foundations: Block cores can be entry points for radon. SSD is often combined with sealing the top course of block and caulking obvious penetrations.

Maintaining Your System

Radon mitigation systems require minimal maintenance:

Check the manometer monthly. If the fluid is level, check the fan — it may have failed or the circuit may have tripped.

Listen for the fan. A functioning fan produces a low hum. Silence from a system that was previously humming means the fan has stopped.

Retest every two years. Even with a properly functioning system, testing confirms that levels remain below 4 pCi/L. Homes settle, new cracks appear, and soil conditions can change. A $100–200 test every two years is inexpensive insurance.

Fan replacement. Fans typically last 5–15 years. When a fan fails, a licensed contractor can replace it in an hour. The cost is usually $200–$400 including the new fan.

Finding a Qualified Installer

Sub-slab depressurization requires proper diagnostic evaluation, correct fan sizing, and a post-mitigation test. In Indiana, only licensed IPLA radon mitigators are authorized to perform this work for compensation.

Browse Indiana-licensed mitigators by county on this site — every profile shows a verified IPLA license number so you can confirm credentials before calling.