Every radon mitigation system is either active or passive. The distinction controls whether your system runs on a fan or relies on natural physics — and it determines whether your radon levels will reliably stay below the EPA’s 4.0 pCi/L action level or merely reduce somewhat. Understanding the difference helps you evaluate what you have, what you need, and what a contractor is actually installing.
What Is a Passive Radon Mitigation System?
A passive radon mitigation system uses no mechanical fan. It relies entirely on natural pressure differentials — specifically, the stack effect — to draw radon-laden soil gas out from under your home and vent it above the roofline.
The stack effect is the same phenomenon that makes a fireplace draw: warm air rises, creating upward airflow through any vertical channel. In a passive radon system, a 3–4 inch PVC pipe runs from a suction point beneath the slab, through the home’s interior, and terminates above the roof. When the home’s interior is warmer than the outside air — which is most of the year in most U.S. climates — warm air rising through the pipe creates mild negative pressure at the bottom, drawing soil gas upward and out.
When Passive Systems Are Installed
- Radon-Resistant New Construction (RRNC): The standard passive system installed during home construction — pipe, vapor barrier, gas-permeable layer — before a radon problem has been confirmed. The passive infrastructure is in place; a fan can be added if post-construction testing shows elevated levels.
- Low-radon environments: A home testing at 1.5–2.5 pCi/L might achieve adequate reduction with passive-only in favorable conditions.
- Supplemental to other measures: In some crawl space installations, passive ventilation combined with encapsulation can achieve adequate reduction without a fan.
Passive System Limitations
Passive systems are inherently unreliable as standalone mitigation for confirmed elevated radon levels. The stack effect weakens or reverses under specific conditions:
- Summer months: When outdoor temperatures match or exceed indoor temperatures, the stack effect diminishes — exactly when windows are open and radon testing results vary most
- Windy conditions: Wind pressure can reverse airflow direction in the pipe
- High-efficiency sealed homes: Tight building envelopes can create neutral or positive pressure at the slab level, counteracting passive stack draw
- Poor sub-slab communication: Homes with clay or sand sub-slab fill have limited natural airflow regardless of stack effect
EPA testing has found that passive RRNC systems achieve below 4.0 pCi/L in roughly 50–70% of new construction cases. For the remaining 30–50%, activation with a fan is required. As a standalone fix for a home that has already tested elevated, passive-only is not recommended.
What Is an Active Radon Mitigation System?
An active radon mitigation system adds a continuously operating electric fan to the pipe network. The fan creates reliable, consistent negative pressure in the sub-slab zone — regardless of outdoor temperature, wind, or building pressure conditions. The fan runs 24 hours a day, 7 days a week, 365 days a year, typically consuming 20–90 watts (similar to a light bulb).
Active Sub-Slab Depressurization (ASD) is the most common form — used for slab-on-grade and basement foundations. Active Sub-Membrane Depressurization (ASMD) uses the same fan-powered approach for crawl space homes, with suction applied beneath the vapor barrier rather than directly below a concrete slab.
Why Active Systems Perform Consistently
The fan’s mechanical suction creates 0.02–0.15 inches of water column negative pressure at the slab — a controlled, measurable value. This negative pressure is:
- Independent of outdoor temperature (stack effect is irrelevant when the fan is running)
- Consistent across seasons and weather conditions
- Verifiable via the U-tube manometer installed on the pipe — the displaced liquid column confirms the fan is generating suction
- Adjustable by swapping to a higher or lower capacity fan if conditions change
AARST-ANSI standard SGM-SF governs active ASD system installation. The standard requires a performance indicator (manometer) on every active system precisely because consistent, verifiable performance is the system’s primary advantage over passive.
Side-by-Side Comparison
| Feature | Passive System | Active System |
|---|---|---|
| Fan | No | Yes (runs continuously) |
| Operating cost | $0/year | $30–$75/year in electricity |
| Reliability | Variable (weather/season dependent) | Consistent year-round |
| Typical radon reduction | 30–50% | 85–99% |
| Installation cost | $350–$700 (RRNC during construction) | $800–$2,500 (post-construction retrofit) |
| Fan-to-active upgrade cost | $200–$500 (fan + labor) | N/A (already active) |
| Performance indicator required | Not required | Required (AARST SGM-SF) |
| Suitable for confirmed elevated radon | Rarely | Yes — standard of care |
Converting Passive to Active: The Fan Activation
If you have a home built with RRNC passive infrastructure and post-construction testing reveals levels at or above 4.0 pCi/L, activating the system is the simplest mitigation work available:
- The existing pipe runs from sub-slab to above the roofline — no new routing required
- A radon fan is installed in the pipe run (typically in the attic between the riser and discharge pipe)
- Fan connects to the pre-installed electrical outlet in the attic
- A U-tube manometer is installed on the visible portion of the pipe inside the home
- Total installation: 1–2 hours, $200–$500 in labor plus $100–$300 for the fan
Post-activation radon testing confirms results (48-hour charcoal test under closed-house conditions, placed at least 24 hours after activation).
Fan Selection for Active Systems
The fan is the heart of an active system. Fan selection is based on the sub-slab diagnostic test — specifically, the measured airflow resistance (static pressure) the fan must overcome to achieve adequate negative pressure across the full slab footprint.
- RadonAway RP145: Low-static, 20W, ~40 CFM at 0.5″ WC. Best for excellent aggregate, small slab. Quietest option.
- RadonAway RP265: Mid-range, 55W, ~75 CFM at 0.5″ WC. Most commonly installed residential fan. Covers most standard conditions.
- RadonAway GP501/GP301: High-static, 85–90W. For dense fill, multiple suction points, or large footprints requiring greater suction field.
- Festa DP3: Alternative brand in the RP265 performance range used by some contractors.
Oversizing a fan (installing a GP501 when an RP145 would suffice) wastes electricity and can create too much depressurization — pulling conditioned air into the soil and increasing heating/cooling costs. Undersizing leaves radon reduction incomplete. The diagnostic test, not guesswork, determines the right fan.
Maintenance: Active vs Passive
Passive systems require essentially no maintenance — no moving parts, no electrical connections. Annual visual inspection to confirm the pipe is unobstructed is sufficient.
Active systems require:
- Monthly manometer check: Confirm the liquid column is displaced (fan generating suction)
- Annual visual inspection: Fan housing for cracks, pipe connections for separation, discharge cap for obstruction
- Fan replacement when needed: RadonAway fans carry 5-year warranties; typical lifespan is 10–15 years for interior/attic-mounted fans, 7–12 years for exterior-mounted fans exposed to weather
- Periodic radon retesting: EPA recommends retesting every 2 years even with an active system — to confirm continued performance and catch any new entry pathways that develop from foundation settling or remodeling
Frequently Asked Questions
Is a passive radon system good enough?
For new construction in lower-risk zones, a passive RRNC system reduces radon risk and provides fan-ready infrastructure if needed. For a home that has already tested at or above 4.0 pCi/L, passive-only is rarely sufficient — active (fan-powered) mitigation is the standard of care for confirmed elevated radon.
How much electricity does an active radon fan use?
Most residential radon fans consume 20–90 watts running continuously. At average U.S. electricity rates (~$0.13/kWh), a 55-watt fan (RP265) costs approximately $63/year to operate. A 20-watt fan (RP145) costs roughly $23/year. This is comparable to leaving a small light bulb on permanently.
Can I add a fan to my existing passive radon pipe?
Yes — if you have RRNC passive infrastructure (pipe already routed from sub-slab to above roofline), adding a fan is a 1–2 hour job. The fan is installed in the pipe run in the attic and connected to an outlet. If there is no pre-installed outlet, an electrician may need to add one first.
What happens if the fan in an active radon system stops working?
If the fan fails, the system reverts to passive-only operation. Radon levels will likely rise — potentially back toward pre-mitigation levels over days to weeks depending on soil conditions and building pressure. The U-tube manometer will show a level (not displaced) liquid column — the homeowner’s alert that the fan needs replacement. Most fan failures are caught this way during routine monthly checks.
Do both active and passive systems need to vent above the roofline?
Yes. Both active and passive systems must discharge radon above the roofline per AARST standards — at least 12 inches above the highest eave and at least 10 feet horizontally from any window, door, or ventilation opening. This ensures discharged radon disperses into the atmosphere rather than being drawn back into the home through openings.
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