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% |
What a Radon System Costs to Run & Maintain
Once installed, a radon system is nearly hands-off. The fan runs continuously for about $6–$11 a month, the piping is maintenance-free for 20+ years, and the only periodic tasks are a glance at the manometer and a re-test every two years.
| Item | Typical figure | Notes |
|---|---|---|
| Fan electricity | ~$6–$11 / month | Fan draws ~60–90W continuously (the GP301 is 79W) — like a light bulb, at Wisconsin's ~16¢/kWh |
| Fan lifespan / replacement | 5–10 yrs / $150–$300 | A failing fan shows on the manometer (columns level out) or gets louder; often covered by a 5-year fan warranty |
| Piping lifespan | 20+ years | No maintenance; lifetime piping warranty typical |
| Fan noise | ~30 dB (quiet models) | Soft hum; mounted in the attic or outside, away from living space |
| Re-test interval | Every 2 years | Radon levels drift with the seasons and home changes |
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.
How is a radon mitigation system actually installed, step by step?
A standard active sub-slab depressurization system is installed in 4-8 hours: (1) a 3-6 inch suction hole is cored through the basement slab; (2) a PVC vent pipe is run from that suction point up through the house or along an exterior wall; (3) an inline radon fan is mounted in the attic or outside — never in livable space; (4) the pipe is vented to discharge at least 10 feet above grade and at least 10 feet from any window, door, or opening; (5) slab cracks and penetrations are sealed to improve suction; (6) a manometer (U-shaped pressure gauge) is mounted so you can confirm the system is running; (7) a post-install verification test (48-96 hours) confirms the radon level dropped. About 90% of Wisconsin installs use this method.
What is sub-slab depressurization (ASD)?
Sub-slab depressurization (also called active soil depressurization, or ASD) is the standard radon mitigation method: a continuously-running fan creates suction underneath your concrete slab, intercepting radon gas before it enters the home and venting it safely above the roofline. Because the pressure under the slab is now lower than the air pressure inside the house, soil gas flows out the pipe instead of up into your living space. It's the most common approach — used in 90%+ of installs — and reduces radon by 50-99%.
What does a radon mitigation system physically look like? Will it be ugly?
A finished system is mostly a single 3-4 inch white PVC pipe running from the basement floor up and out of the house, plus an inline fan. How visible it is depends on routing: an exterior run puts the pipe and fan on an outside wall (most affordable, more visible), while an interior run hides the pipe through a closet, garage, or chase and places the fan in the attic (cleaner curb appeal, sometimes higher cost). The only indoor evidence is usually a small manometer gauge on the pipe. Most homeowners find it unobtrusive once routed thoughtfully.
Where do the radon fan and vent pipe go?
By code and standard practice, the radon fan is never installed inside a livable space or basement — it goes in the attic or outside, so that any leak in the pressurized section discharges outside the home rather than into it. The vent discharges at least 10 feet above ground level and at least 10 feet away from any window, door, or other opening so the exhausted radon disperses and cannot re-enter. The pipe itself can run inside (through a closet or garage) or along an exterior wall depending on your preference for appearance versus cost.
What do I need to do to prepare for the install, and what happens on install day?
Clear access to the basement area where the suction point will go and to the planned pipe route (attic, closet, or exterior wall). On install day a crew is on site 4-8 hours — a single day for most basements; crawl spaces can take 1-2 days. They core the slab, run and seal the pipe, mount the fan and manometer, and activate the system. You don't need to leave the home. About 48-96 hours later, a verification test confirms the radon level dropped below the EPA action level of 4 pCi/L.
How much electricity does a radon fan use, and what will it add to my power bill?
A radon fan draws about 60-90 watts running continuously — comparable to a standard light bulb. At Wisconsin's residential electricity rate of roughly 16¢ per kWh, that adds about $6–$11 per month, or $80–$130 per year. It's the single largest ongoing cost of owning the system, and it's small. Energy-efficient fan models and correctly-sized systems keep it at the low end of that range.
How loud is a radon fan? Will I hear it?
A properly installed radon fan is quiet — most homeowners describe a soft hum similar to a refrigerator, and the fan is mounted in the attic or outside, away from living space. Manufacturers rate quiet fan models at around 30 decibels. You may hear a faint airflow 'whoosh' near the pipe, but correct fan sizing and placement keep it unobtrusive. If a fan gets noticeably louder over time, it's usually a sign the fan is nearing the end of its life.
How long does a radon fan last, and what does replacement cost?
A radon fan typically lasts 5-10 years; the PVC piping lasts 20+ years. When a fan eventually wears out, replacement typically runs about $150-$300 including parts and labor. A failing fan is usually obvious: the manometer gauge levels out (no pressure difference) or the fan gets louder. Many systems carry a 5-year fan warranty, so an early failure is often covered — ask your contractor for warranty terms in writing.
What maintenance does a radon system need, and how do I read the manometer?
Very little maintenance is needed. Check the manometer — the small U-shaped gauge on the pipe — periodically: when the two liquid columns sit at different heights, the system is pulling suction and working; if they sit at the same level, the fan has likely failed and needs service. Beyond that, test your home's radon every 2 years (levels drift over time and with the seasons) and replace the fan when it reaches end of life. There are no filters to change and no routine servicing.
What happens if the system doesn't get my radon below 4.0 pCi/L?
A properly designed system almost always succeeds — active systems cut radon 50-99%, and most Wisconsin installs reach below 2 pCi/L, well under the 4.0 action level. AARST-ANSI-standard installs succeed 99%+ of the time when verified. If a verification test still shows an elevated level, the contractor adjusts the system — adding suction points, upsizing the fan, or sealing additional entry routes — until it passes. Ask any contractor before hiring what they do if the first test doesn't pass; reputable installers stand behind the result.
Can radon come back after mitigation?
An active system runs continuously, so as long as the fan is operating, radon stays suppressed — it does not 'come back' while the system works. Levels can rise again only if the fan fails, the pipe is damaged, or new foundation cracks open over time, which is why you check the manometer and re-test every 2 years. A failed fan is the most common cause and is a straightforward repair.
What warranty comes with a radon mitigation system?
Partner contractors typically provide a 5-year warranty on the radon fan and a lifetime warranty on the PVC piping, though terms vary by installer. Warranty specifics are set by the installing contractor, so ask for them in writing on your quote. A written warranty plus a post-install verification test are two of the most important things to confirm before hiring.
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.