Visual Guide · Step-by-Step · Wisconsin
How Does a Radon Mitigation System Work?
Wisconsin's soil-gas radon comes from three distinct sources — Driftless Area Cambrian sandstone, north-central Precambrian granite, and the glacial till that covers the eastern two-thirds of the state. A radon mitigation system intercepts that gas before it reaches the living space. The cross-section below walks through the physics, the components, and the result.
The 5 steps of Active Sub-Slab Depressurization
- Step 1 — Radon emerges from Wisconsin bedrock. The decay chain of natural uranium in the soil and rock under your home continuously produces radon gas. In the Driftless Area (Iowa, Lafayette, Grant, Crawford, Vernon, La Crosse counties), the source is uranium-bearing Cambrian sandstone and Ordovician carbonate. In the north-central counties around Wausau, the source is uranium-rich Precambrian granite — Marathon County Health Department data shows roughly 67% of tested homes elevated, with routine readings above 20 pCi/L. Across the eastern two-thirds of the state, glacial till transports radon along permeable layers toward foundations.
- Step 2 — A suction point is cored through the slab. The installer drills a 3–6 inch core hole through the basement concrete down into the gravel layer underneath. A PVC riser is glued, primed, and sealed into the opening. This single point becomes the route through which the entire system pulls soil gas out of the ground.
- Step 3 — PVC piping routes from the suction point through the house. Schedule 40 PVC (or schedule 80 on exterior chases, where Wisconsin winters punish thinner-walled pipe) runs from the suction point upward through an interior chase, closet, or utility space. The pipe terminates at the fan in the attic, then continues through the roof to discharge above the roofline.
- Step 4 — A continuous-duty fan establishes negative pressure. A radon fan — most commonly the RadonAway GP301 in Wisconsin residential installs — is mounted inline on the riser, always above the living space. It runs 24/7 at 40–80 watts and generates roughly 0.5–1.5 inches of water column of vacuum beneath the slab. Because gas flows from higher pressure to lower pressure, soil gas is now pulled outward through the suction point instead of upward through slab cracks.
- Step 5 — Soil gas discharges above the roofline. The exhaust pipe terminates at least 10 feet above grade and at least 10 feet from any operable window, door, or fresh-air intake — EPA placement rules. At that height, soil gas disperses harmlessly into outdoor air, where ambient radon concentration averages about 0.4 pCi/L — too low to matter.
The physics: why a tiny pressure difference does so much
Radon mitigation works on a single rule: gas flows from higher pressure to lower pressure. The trick is reversing the gradient that normally pushes radon up into the home.
Without mitigation, soil gas sits at slightly higher pressure than basement interior air. Four mechanisms drive that gradient in a Wisconsin home:
- Stack effect: Wisconsin winters are long and cold — Madison January overnight lows average around 9°F. Heated indoor air rises through the home, creating slight negative pressure on the lower levels that draws soil gas upward through every available crack. Winter readings typically test 30–50% higher than summer measurements.
- Wind loading: Wind blowing across the house creates a slight pressure differential between the windward and leeward sides; soil gas migrates along the gradient.
- HVAC operation: Forced-air furnaces, range hoods, dryer vents, and bath fans all create transient negative pressure in the basement that pulls soil gas in.
- Soil-gas pressure: Gas in the gravel layer under the slab sits at near-atmospheric pressure, which is slightly above typical indoor air pressure in a heated Wisconsin home.
An active mitigation system flips all of this. The fan generates roughly 0.5–1.5 inches of water column of vacuum beneath the slab — the reading shown on the manometer. Now the sub-slab zone is the lowest-pressure region in the system, and soil gas flows outward through the riser instead of upward into the home.
The pressure differential is small in absolute terms — less than a tenth of a psi — but it runs continuously, 8,760 hours a year. Across months and years, the negative pressure field extends to cover the full basement slab and intercepts essentially all the soil gas before it can reach the living space.
How effective are Wisconsin mitigation systems?
Performance data from Wisconsin partner installs, based on pre/post verification readings across Driftless Area, Precambrian-granite belt, and glacial-till region homes.
| System Type | Typical Reduction | Pre-Mitigation Avg | Post-Mitigation Avg | Success Rate |
|---|---|---|---|---|
| Active Sub-Slab Depressurization (ASD) | 70-99% | 8-20 pCi/L | 0.5-2.0 pCi/L | 98% |
| Sub-Membrane Depressurization (crawl space) | 70-95% | 6-12 pCi/L | 1.0-3.0 pCi/L | 94% |
| Block-Wall Depressurization | 60-90% | 10-20 pCi/L | 1.5-3.5 pCi/L | 88% |
| Drain-Tile Depressurization | 70-95% | 8-15 pCi/L | 0.8-2.5 pCi/L | 92% |
| Passive-to-Active Retrofit | 40-70% | 6-10 pCi/L | 2.0-4.0 pCi/L | 74% |
FAQ
How Radon Mitigation Works — Common Wisconsin Questions
Frequently Asked Questions
How does radon mitigation actually work?
Radon mitigation works by reversing the pressure gradient between the soil under your foundation and the basement interior. In an unmitigated Wisconsin home, basement air pressure runs slightly below the soil-gas pressure under the slab, so radon-bearing gas migrates upward through cracks and slab penetrations. A mitigation system installs a continuous-duty fan that drops the sub-slab pressure even lower than the basement air pressure — soil gas flips direction, gets pulled through a sealed PVC riser, and discharges above the roofline.
What creates the suction beneath the slab?
A continuous-duty radon fan installed in the riser pipe — typically above all living space, in the attic or on an exterior chase. The Wisconsin default is a RadonAway GP301: 79 watts, roughly 195 CFM at 0 inches water column, running 24/7 for 7–10 years before the bearings give out. It generates between 0.5 and 1.5 inches of water column of vacuum beneath the slab, which is small in absolute terms but plenty to redirect soil-gas flow across the entire footprint of a typical basement.
Why does radon leave the soil through the system instead of staying put?
Soil gas behaves like any other gas — it flows from higher pressure to lower pressure. Under normal conditions, soil-gas pressure runs slightly above basement interior pressure, especially during Wisconsin winters when warm indoor air rising creates stack-effect suction inside the home. Once the fan establishes a still-lower pressure zone under the slab, the entire gradient reverses. Soil gas that would otherwise leak upward into the basement is now drawn outward and downward through the gravel layer, into the suction point, and up through the riser to the roof.
How is the system tested after installation?
Post-mitigation verification testing runs 24–96 hours after the fan is activated. A continuous radon monitor or charcoal canister is placed in the lowest livable level of the home, under closed-house conditions (windows and doors closed except for normal entry and exit), for at least 48 hours. The result is compared to the pre-mitigation reading and to the 4 pCi/L EPA action level. AARST-ANSI standards require this verification step before a job can be considered complete, and Wisconsin DHS recommends it within 30 days of system activation.
How long does a Wisconsin install take?
A typical Wisconsin residential ASD install is on-site for 4–8 hours. Sub-membrane systems for crawl spaces — more common in older Driftless-region farmhouses and Door County lake homes — run 1–2 days because of the time required to seal a 20-mil vapor barrier. Add 48–96 hours for post-mitigation verification testing, and the full cycle from install kickoff to verified completion is roughly 3–5 days.
Why does mitigation drop indoor radon so sharply?
Pre-mitigation, soil gas leaks into the basement through dozens of small openings spread across the slab — control joints, sump pit covers, wall-floor seams, utility penetrations. Each leak contributes a small share, and they sum to elevated indoor radon. Post-mitigation, the fan establishes a negative pressure zone that covers the entire slab footprint and redirects essentially all the soil-gas flow into the riser. Residual indoor radon decays and is diluted by normal air exchange within hours. Wisconsin Driftless-region homes pre-testing at 12–25 pCi/L routinely verify at 0.5–2.0 pCi/L after a properly designed install.
What if my Wisconsin home has a crawl space instead of a basement?
Crawl spaces — common on older Driftless-region farmhouses and Door County lake homes — use sub-membrane depressurization instead of sub-slab. The installer lays a 20-mil polyethylene vapor barrier across the dirt floor, seals every seam and the perimeter with butyl tape and urethane caulk, ties in a sealed suction point and PVC riser, and runs the same continuous-duty fan as a slab install. The physics is identical: negative pressure beneath the membrane pulls soil gas outward through the riser instead of letting it permeate upward.
How does the installer pick a mitigation method for my home?
Five factors. (1) Foundation type — poured-concrete basement gets ASD, crawl space gets sub-membrane, hollow-block-wall foundation gets block-wall depressurization. (2) Radon source location — diagnostic micro-pressure-field testing during the assessment visit identifies the strongest entry point. (3) Existing infrastructure — homes with perimeter drain tile can use drain-tile depressurization. (4) Basement layout — finished basements often require an exterior chase; unfinished basements allow interior routes through closets or mechanical chases. (5) Homeowner preferences on fan location and aesthetics. An NRPP- or NRSB-certified mitigator working a Wisconsin home runs through all five before quoting.
What installation mistakes kill system performance?
Six recurring failure modes: (1) suction point placed without diagnostic testing, missing the actual strongest soil-gas entry point; (2) PVC joints slip-fit instead of sealed with primer plus cement, causing pressure leaks; (3) fan undersized for the basement footprint or sub-slab permeability; (4) exhaust outlet placed within 10 feet of an operable window or fresh-air intake, causing soil gas to re-enter the home; (5) slab cracks and sump pit covers left unsealed, undermining the negative pressure field; (6) verification testing skipped, so nobody actually knows whether the system worked. NRPP/NRSB-credentialed Wisconsin installers are trained to avoid all six.
How can I verify my installer did the job right?
Five checks at install completion. (1) The manometer is inline on the riser and shows unequal fluid columns or a digital reading of 0.5–1.5 inches water column — proof of vacuum. (2) The exhaust pipe terminates at least 10 feet above grade and at least 10 feet from any operable window, door, or air intake. (3) Visible slab cracks, sump pit covers, and the suction point itself are sealed with butyl or urethane — no open gaps. (4) The riser is fastened to the building and properly supported, not just hanging from elbows. (5) The verification radon test report shows indoor pCi/L below 4 (ideally below 2). Wisconsin Radon Experts partner contractors document all five in writing.
Ready to install a system in your Wisconsin home?
Wisconsin Radon Experts routes your quote request to an NRPP- or NRSB-certified Wisconsin partner contractor. Free quotes, no upfront cost, 50–99% radon reduction is the standard outcome.