Visual Guide · Step-by-Step
How Does a Radon Mitigation System Work?
A radon mitigation system creates negative pressure beneath your home's foundation, intercepting radon gas from the soil before it enters your living space. This animated cross-section diagram shows exactly how the system works — and how it reduces indoor radon by 50–99%.
The 5 Steps of Active Sub-Slab Depressurization
- Step 1 — Radon enters from soil. Radon gas is produced by natural uranium decay in Iowa soil. It rises from the ground and naturally enters homes through cracks and openings in the foundation. Iowa's uranium-rich glacial-till soils produce some of the highest radon emission rates in the US (8.5 pCi/L average indoor concentration — #1 in the nation).
- Step 2 — Suction point is cut through the slab. The contractor drills a 3-6 inch diameter hole through the basement concrete slab into the gravel layer beneath. A PVC riser is installed and sealed in place. This is the single point through which the entire system draws soil gas.
- Step 3 — PVC pipe routes from suction point through the house. Schedule 40 or 80 PVC piping (typically 3-4 inch diameter) routes from the suction point upward through the home — usually through an interior wall, closet, or utility chase. The pipe terminates in the attic where the fan is installed, then continues through the roof to discharge above the roofline.
- Step 4 — A continuous-duty fan creates negative pressure. A radon fan (most commonly the RadonAway GP301 in Iowa — 79 watts, 195 CFM) is installed inline on the pipe in the attic. The fan runs 24/7 and creates lower air pressure beneath the slab than above it. Because gas naturally flows from higher pressure to lower pressure, soil gas including radon is pulled OUT through the suction point rather than UPWARD into the home.
- Step 5 — Radon is vented safely above the roof. The exhaust pipe extends above the roofline — EPA placement requirements specify at least 10 feet above grade and at least 10 feet from any openable window, door, or air intake. At this height, the radon disperses harmlessly into the outdoor atmosphere where outdoor radon levels are negligible (national outdoor average ~0.4 pCi/L).
The Physics: Why Negative Pressure Works
Radon mitigation works because of a simple physical law: gas flows from areas of higher pressure to areas of lower pressure.
Without mitigation, soil gas (including radon) is at slightly higher pressure than basement interior air, due to:
- Stack effect: Warm indoor air rises in winter, creating slight negative pressure in lower floors that draws soil gas upward into the home.
- Wind effects: Wind blowing across the house creates pressure differentials.
- HVAC operation: Forced-air furnaces and exhaust fans can create negative interior pressure that pulls soil gas through cracks.
- Soil gas pressure: The gravel and soil beneath the slab contains gas at near-atmospheric pressure, which is slightly higher than typical indoor air during cold weather.
An active radon mitigation system reverses this gradient. By installing a fan that creates strong negative pressure beneath the slab (typically -0.5 to -1.5 inches of water column, as shown on the manometer), the pressure beneath the slab becomes LOWER than indoor air pressure. Now soil gas flows OUT through the system rather than UP into the home.
The pressure differential is small in absolute terms (less than 0.1 psi) but it's consistent and constant — running 24 hours a day. Over time, the negative pressure zone extends across the full footprint of the slab, intercepting essentially all soil gas before it can enter the living space.
How Effective Are Radon Mitigation Systems?
Real Iowa mitigation performance data based on pre/post verification testing across our partner network.
| System Type | Typical Reduction | Pre-Mitigation Avg | Post-Mitigation Avg | Success Rate |
|---|---|---|---|---|
| Active Sub-Slab Depressurization (ASD) | 70-99% | 8-15 pCi/L | 0.5-2.0 pCi/L | 99% |
| Sub-Membrane Depressurization (crawl space) | 70-95% | 6-12 pCi/L | 1.0-3.0 pCi/L | 95% |
| Block-Wall Depressurization | 60-90% | 10-20 pCi/L | 1.5-3.5 pCi/L | 90% |
| Drain-Tile Depressurization | 70-95% | 8-15 pCi/L | 0.8-2.5 pCi/L | 93% |
| Passive System Retrofit (activated) | 40-70% | 6-10 pCi/L | 2.0-4.0 pCi/L | 75% |
FAQ
How Radon Mitigation Works — Common Questions
Frequently Asked Questions
How does radon mitigation actually work?
Radon mitigation works by creating lower air pressure beneath the foundation slab than above it. Soil gas (containing radon) naturally flows from higher pressure to lower pressure — so creating a "negative pressure zone" under the slab causes radon-laden gas to be pulled out of the soil and vented away rather than entering the home through cracks. The system uses a continuous-duty fan, PVC piping from a sub-slab suction point up through the house, and an exhaust outlet above the roofline.
What creates the negative pressure under the slab?
A continuous-duty radon fan installed in the system pipeline creates the negative pressure. The most common Iowa fan is the RadonAway GP301 — a 79-watt centrifugal fan that pulls approximately 195 CFM at zero static pressure. The fan runs 24/7 (typical lifespan 5-10 years) and produces enough suction beneath the slab to draw soil gas through the gravel layer under the foundation and into the PVC piping. A manometer (pressure gauge) on the pipe shows the system is actively pulling negative pressure.
Why does radon get pulled out instead of staying in the soil?
Physics — gas flows from higher pressure to lower pressure. Soil gas (including radon) is normally at slightly higher pressure than the basement interior, which is why it tends to enter homes through cracks and openings. When the mitigation system creates negative pressure beneath the slab (lower than both the soil AND the home interior), the pressure gradient reverses: soil gas is drawn out through the suction point rather than upward into the living space. This is called "Active Sub-Slab Depressurization" (ASD) — the dominant method in Iowa and across the US.
How is the system tested after installation?
Post-mitigation verification testing is conducted 24-96 hours after system activation. The contractor places a continuous radon monitor (CRM) or charcoal canister test in the lowest livable level of the home, under closed-house conditions, for 48-96 hours. After retrieval, results are compared to pre-mitigation levels to confirm the system has reduced radon below the EPA action level of 4 pCi/L. Iowa law and AARST-ANSI standards require this verification step before considering an install complete.
How long does the installation take?
A typical Iowa residential active sub-slab depressurization (ASD) installation takes 4-8 hours of on-site work. Sub-membrane (crawl space) systems take 1-2 days because of the additional vapor barrier installation. Post-mitigation testing requires another 48-96 hours of test time, so the full cycle from install to verified completion is approximately 3-5 days.
Why does mitigation reduce indoor radon so dramatically?
Properly installed ASD systems reduce indoor radon by 50-99% (most Iowa installs achieve 70-95%) because the system intercepts soil gas BEFORE it enters the home. Pre-mitigation, soil gas leaks into the basement through dozens of small cracks and openings, accumulating throughout the home. Post-mitigation, the negative pressure beneath the slab redirects essentially all the soil gas flow into the system piping and out the roof. The remaining radon in indoor air dissipates through normal air exchange within hours.
What if my home has no basement?
Homes with crawl spaces use sub-membrane depressurization instead of sub-slab. The process: install a heavy-gauge (20-mil typical) polyethylene vapor barrier across the crawl space floor, seal all seams and the perimeter with butyl tape and caulking, install a suction point and PVC piping connected to a radon fan, and vent above the roof. The principle is identical — create negative pressure beneath the barrier so soil gas is drawn outward. Slab-on-grade homes (rare in Iowa) use a different approach: typically interior drain-tile depressurization or block-wall depressurization depending on foundation design.
How do contractors decide which mitigation method to use?
Five factors determine the method: (1) Foundation type — poured-concrete slab gets ASD, crawl space gets sub-membrane, hollow block wall gets block-wall depressurization. (2) Radon source location — determined by diagnostic testing. (3) Existing infrastructure — homes with drain tile may use drain-tile depressurization. (4) Basement layout — finished basements may need exterior piping routes; unfinished basements allow interior routes. (5) Cost and homeowner preferences — multiple suction points or higher-CFM fans for difficult installs. An NRPP-certified Iowa mitigation specialist makes this determination during the initial assessment.
What are the common installation mistakes to avoid?
Six mistakes that reduce system effectiveness: (1) Suction point too far from radon source (diagnostic testing should identify the strongest entry point). (2) Pipe leaks at joints — should be sealed with PVC primer + cement, not just slipped together. (3) Fan too small for the home — under-sized fans cannot maintain negative pressure across the full slab. (4) Exhaust placement too close to openable windows — EPA requires 10 feet minimum. (5) Failure to seal slab cracks — undermines the negative pressure field. (6) Skipping post-mitigation verification testing — without it, you cannot confirm the system works. NRPP-certified Iowa contractors are trained to avoid all six.
How can I verify my contractor installed it correctly?
Five visual + documented checks: (1) Manometer is installed inline and shows unequal fluid levels (proving negative pressure). (2) Exhaust pipe extends at least 10 feet above grade and is at least 10 feet from any openable window or air intake. (3) Suction point and visible slab cracks are sealed (no open gaps around the pipe penetration). (4) Pipe routing is supported and secured (not just hanging). (5) Post-mitigation verification test report shows indoor radon below 4 pCi/L. Ask the contractor for written documentation of all five items at install completion. Iowa Radon Experts partner contractors provide this as standard practice.
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