Sub-Slab Depressurization — Wisconsin's #1 Radon Mitigation Method

Sub-Slab Depressurization (SSD), also called Active Sub-Slab Depressurization (ASD), is the most common radon mitigation method in the United States. The system creates negative air pressure beneath a building's foundation slab by installing a continuous-duty fan that pulls soil gas from beneath the slab through PVC piping and vents it above the roofline. ASD systems use a single (or multiple) suction points drilled through the slab into the gravel or substrate layer beneath. ASD is used in approximately 75-80% of Wisconsin residential radon mitigation installs, with the higher end seen in glacial-till basement homes (Milwaukee/Madison metros) and somewhat lower in northern Wisconsin where crawl-space construction is more common.

ASD works on a pressure-gradient physics principle. A continuous-duty radon fan creates lower air pressure beneath the foundation slab than above it. Because gas flows from higher pressure to lower pressure, soil gas (including radon) is drawn outward through the suction point rather than upward into the home through cracks and openings. The system requires: (1) a 3-6 inch suction point cut through the slab into the gravel or bedrock substrate beneath, (2) PVC piping from the suction point through the home to the roof, (3) a continuous-duty radon fan (RadonAway, Festa, or Fantech are the standard choices), (4) sealing of slab cracks and penetrations to maintain the negative pressure field, and (5) exhaust outlet above the roofline per EPA placement requirements (10 ft above grade, 10 ft from any opening). Wisconsin's cold winters add a snow-drift consideration to exhaust placement that warmer-state mitigators don't face.

Four reasons ASD dominates Wisconsin installs: (1) Effectiveness — ASD reduces indoor radon by 70-99% in properly designed systems, the highest reduction of any mitigation method. (2) Cost-effectiveness — typical Wisconsin install $1,000-$2,200, lower than block-wall or sub-membrane alternatives. (3) Foundation compatibility — ASD works with Wisconsin's most common foundation type (poured-concrete basement slab on glacial-till or carbonate substrate). (4) AARST-ANSI standardization — the method is well-documented in industry standards, training, and certification programs, ensuring consistent quality from NRPP- or NRSB-certified mitigators. In Wisconsin's no-state-license environment, AARST-ANSI compliance becomes the practical quality benchmark.

Five common mitigation methods, with ASD being the default for Wisconsin homes with poured-concrete basement slabs: (1) Sub-Slab Depressurization (ASD) — basement slab homes, $1,000-$2,200 in Wisconsin. (2) Sub-Membrane Depressurization — crawl spaces, more common in northern Wisconsin and Door County, $1,500-$3,500. (3) Block-Wall Depressurization — homes with hollow concrete block foundation walls (pre-1980 stock common in older Milwaukee neighborhoods), $2,000-$4,000. (4) Drain-Tile Depressurization — homes with perimeter drain tile systems, $1,500-$3,500. (5) Passive System Retrofit — new construction homes with builder-installed passive radon stack, $500-$1,500. Foundation type and regional substrate drive method choice; ASD covers the majority of Wisconsin cases including Driftless karst, glacial-till, and most Precambrian-granite homes.

Six core components: (1) Suction point — 3-6 inch diameter hole through the slab into the gravel layer beneath, fitted with a PVC riser. (2) PVC piping — schedule 40 or 80, typically 3-4 inch diameter, routed from suction point through the home to the roof. (3) Radon fan — continuous-duty centrifugal fan (RadonAway GP301, Festa AMG, or Fantech HP190 are common Wisconsin choices). Wausau-area Precambrian-granite homes with high pre-install readings (20+ pCi/L) often need higher-capacity fans like the RadonAway GP501 or HP-2190. (4) Manometer — pressure gauge installed inline to indicate the system is operating. (5) Sealing materials — polyurethane sealant for slab penetrations and visible cracks. (6) Electrical service — dedicated circuit (usually 110V) for the fan. Total installed component cost is typically $300-$700; the balance is labor.

Four manufacturers dominate the Wisconsin ASD fan market: (1) RadonAway GP301 — 79 watts, 195 CFM at 0 in. w.c. The industry standard for most Wisconsin residential applications in glacial-till basement homes. 5-year warranty. Component cost $250-$350. (2) RadonAway GP501 / HP-2190 — 88-130 watts, higher CFM for larger Wisconsin homes or Wausau-area Precambrian-granite homes with high pre-install readings. (3) Festa AMG-Series — 195-365 CFM options for larger homes or commercial buildings. Energy-efficient operation. (4) Fantech HP-190 / HP-220 — inline configuration option, lower noise profile, good for finished-attic installations common in Wisconsin renovation work. The right fan is determined by the mitigation system designer based on basement square footage, soil gas permeability, regional substrate (Driftless karst vs glacial till vs granite), and post-mitigation target radon levels.

AARST-ANSI (American Association of Radon Scientists and Technologists — American National Standards Institute) standards for residential radon mitigation include: (1) Suction point placement per diagnostic testing (not arbitrary location). (2) PVC piping must be sealed at all joints with primer + cement. (3) Exhaust outlet ≥10 ft above grade and ≥10 ft from any openable window, door, or air intake. (4) Continuous-duty fan rated for radon service. (5) Manometer installed inline for system monitoring. (6) Visible slab cracks sealed during install. (7) Post-mitigation verification testing within 30 days per WI DHS guidance. (8) Written documentation of design and install provided to homeowner. In Wisconsin's no-state-license environment, AARST-ANSI compliance is the practical quality standard — NRPP- and NRSB-certified mitigators are trained to it.

A typical Wisconsin residential ASD installation takes 4-8 hours of on-site work, split across three phases: (1) Site preparation and suction point creation (1-2 hours) — diagnostic measurement, drilling the suction point through the slab into gravel. Wausau-area Precambrian-granite substrates may add 30-60 minutes due to denser material. (2) Piping routing and fan installation (2-4 hours) — running PVC from suction point through the home to the roof, installing the fan, electrical connection. (3) Sealing and finishing (1-2 hours) — sealing slab penetrations and visible cracks, installing the manometer, system activation testing. Post-installation, allow 48-96 hours for verification testing before considering the project complete.

Yes — Wisconsin does not require state-level licensing for radon mitigation work. The Wisconsin Department of Health Services only RECOMMENDS national NRPP or NRSB certification; it does not require it. Homeowners may technically install on their own home. However, the practical consequences make DIY a poor choice: lower effectiveness (typical 30-60% reduction vs 70-99% for professional installs), no warranty coverage, weakened Wis. Stat. § 709.03 Form WB-3 disclosure for future home sale, and rejection by FHA/USDA/VA lenders. See our DIY vs Professional Mitigation cornerstone for the full comparison.

Quality Wisconsin ASD installations typically include: (1) Lifetime warranty on PVC piping and structural components — these components do not wear out. (2) 5-year manufacturer warranty on the radon fan (RadonAway, Festa, and Fantech standard). (3) 1-year workmanship warranty on the installation. (4) Post-mitigation effectiveness guarantee — system must reduce radon below 4 pCi/L on first verification test or contractor will adjust at no charge. (5) Some Wisconsin contractors offer 2-year service warranties that include free verification testing during the warranty period. Wisconsin Radon Experts partner contractors include these terms as standard practice, even though Wisconsin's no-state-license environment doesn't require them.