How radon mitigation actually works — the physics and the install

A radon mitigation system uses pressure differential to redirect radon-laden soil gas away from your home. The most common system — sub-slab depressurization (SSD) — installs a fan that draws air from under the basement slab and vents it above the roofline. Done correctly, it reduces indoor radon by 50-99%, typically dropping levels below the 4 pCi/L EPA action threshold within 24-72 hours of activation.

The physics in one paragraph

Radon is a heavy radioactive gas (specifically radon-222, the longest-lived radon isotope) that's continuously produced by uranium decay in soil and bedrock. It rises through soil pores under slight upward pressure. When a home's foundation is intact, much of that radon vents harmlessly to outdoor air. But homes create stack-effect (warm air rising) that creates negative pressure at the foundation level — pulling radon-laden soil gas IN through cracks, the slab-to-wall joint, sump pits, and openings around pipes. A mitigation system reverses that pressure differential under the slab, so soil gas vents up the system stack rather than into the home.

Sub-slab depressurization (SSD) — most common system

What gets installed

1. Suction point — a 4" plastic pipe inserted through the basement floor into the gravel layer beneath the slab. Sometimes two points if the slab is large or has separate gravel zones.

2. PVC pipe run — 3-4" diameter, runs vertically through the home (often inside an interior wall or chase, sometimes via a utility room) up to and through the attic, exiting above the roofline.

3. Mitigation fan — installed in the attic or on the exterior of the home. Specialized low-power fan (RadonAway RP145, RP265 are industry standards). Runs continuously, draws 60-110 watts.

4. U-tube manometer — small clear plastic tube mounted to the visible pipe, with fluid that shows the pressure differential between system stack and ambient. Lets you visually verify the fan is working.

5. Roof vent cap — exhaust point typically 10+ ft above grade, 10+ ft from any openable window.

Standard install timeline

• Day 1, hours 0-3: site survey + suction point installation (drill core through slab)
• Day 1, hours 3-6: PVC pipe run from suction point to attic
• Day 1, hours 6-8: fan installation + electrical wiring + roof vent
• Day 1, hours 8-9: commissioning + manometer mounting

A typical install completes in 4-6 hours for a standard basement home. Crawl-space-only homes (sub-membrane systems) take 6-8 hours. Multi-zone or block-wall systems can take 1-2 full days.

Visible result

The fan creates negative pressure under the slab — usually -50 to -150 Pascals — drawing soil gas continuously up the stack and out above the roofline. The manometer shows the pressure differential as a fluid-column reading.

Within 24-72 hours of activation, indoor radon levels typically drop:

• 90-99% reduction for properly-sealed homes
• 50-90% reduction for homes with significant air leakage

Other mitigation systems (less common)

Sub-membrane depressurization (SMD)

For crawl-space homes. Same physics as SSD, but the suction point pulls from beneath a sealed plastic membrane covering the dirt floor (rather than from under a concrete slab).

Install: $1,200-$3,500 typical (more variable than SSD because membrane install is part of scope).

Block-wall depressurization

For homes with hollow concrete block foundations where radon migrates through the block cores. The mitigation system draws air from inside the block walls themselves.

Install: $1,500-$4,500 typical. Specialty work — fewer contractors do this competently.

Heat recovery ventilator (HRV)

A different physics: instead of pressure differential, an HRV creates positive ventilation throughout the home. Pushes radon out via dilution rather than re-routing it.

Install: $2,500-$6,500. Reserved for homes where SSD/SMD isn't feasible (high radon levels combined with foundation conditions that prevent active depressurization).

Increasingly used in modern tight homes (post-2010 ENERGY STAR construction) where the home is sealed enough that SSD alone can pull dangerous quantities of conditioned air out the stack.

Sump-pump cover seal + active venting

When the home already has a sump basin and the radon entry path is primarily through that basin: seal the sump cover, run a vent pipe + fan from inside the sealed basin up through the roof.

Install: $600-$1,800. Smallest scope system.

What "success" looks like

Industry standard: indoor radon below 4 pCi/L confirmed by a long-term test (90+ days) AFTER mitigation is installed.

Reputable contractors guarantee this and include a post-mitigation re-test ($150-$300) in the quote.

Some scenarios:

• Initial reading 8 pCi/L → post-mitigation typical 0.8-1.5 pCi/L (80-90% reduction)
• Initial 15 pCi/L → post-mitigation typical 1.5-2.5 pCi/L
• Initial 40 pCi/L → post-mitigation typical 3-6 pCi/L (may need 2-pipe enhanced system)
• Initial 100+ pCi/L → may need multi-stage or HRV combined with SSD

When the system underperforms

The 5-10% of installs that don't reach below 4 pCi/L on first test typically have:

1. Air leakage in suction zone — slab has unsealed cracks or gaps that pull conditioned air from the basement instead of pulling soil gas. Fix: seal cracks + slab-to-wall joints ($200-$600 added).

2. Insufficient fan capacity — fan undersized for slab area or sub-slab gravel layer depth. Fix: upsize fan ($300-$500).

3. Block-wall pathway not captured — radon entering through hollow block walls that the SSD system doesn't reach. Fix: add a second suction point to the wall ($500-$1,200).

4. Sump pit not sealed — sump cover allowing soil gas to bypass the system. Fix: gasketed sealed sump cover ($150-$300).

All of these are addressable with $200-$1,200 in additional work. Reputable contractors include re-mitigation in their warranty if first test shows above 4 pCi/L.

Maintenance

Annual:

• Check manometer fluid level (top up with mineral oil if low)
• Verify fan is running (audible hum from attic or exterior)
• Test for radon every 1-2 years to confirm system performance

Every 5-15 years:

• Replace fan (60-110W fans typically last 5-15 years)
• Cost: $300-$500 including labor

Every 5-10 years:

• Re-seal suction points if cracks develop

Operating cost

The fan runs continuously, drawing 60-110 watts. Electricity cost:

• 80W average × 24h × 365 days = 700 kWh/year
• At $0.13/kWh national average: ~$91/year
• Range across electricity markets: $50-$160/year

Minor heating loss from drawing conditioned air through the slab:

• Estimated 200-400 cubic ft/min of conditioned air pulled into the system
• Annual cost: $40-$120/year in additional HVAC load

Total annual operating cost: ~$130-$280/year. Lower in mild climates, higher in cold climates with high heating costs.

What the system does NOT solve

• Existing radon decay products in indoor air — these decay quickly (half-life 30 minutes) and clear within hours of mitigation activation. Not a long-term concern but explains why you should re-test 30-90 days post-install, not 1-2 days.
• Bath water or well water radon — separate issue. Some private wells in granite-bedrock regions deliver radon-laden water. Requires aeration tank or granular activated carbon (GAC) filter at point-of-entry. Different remediation system.

A properly-designed SSD system is the single most cost-effective health remediation in residential construction. $1,500-$2,500 reduces lung-cancer risk dramatically and operates for 15+ years with minimal maintenance.