Passive vs. Active Radon Mitigation: What's the Difference?

The Same Pipe, Two Different Drivers

Both passive and active radon systems use the same basic architecture: a PVC pipe runs from a suction point beneath the foundation slab, through the home, and exits above the roofline. The difference is what moves the air.

Passive: Natural air pressure and convection. No mechanical components.

Active: A continuously running fan creates forced negative pressure.

Passive Radon Systems

Passive systems are primarily built into new construction as part of the radon-resistant new construction (RRNC) standard. The EPA and many state codes require or recommend RRNC in high-risk zones, which includes:

• A gas-permeable aggregate layer beneath the slab
• A polyethylene vapor barrier over that aggregate
• A sealed sump pit
• A PVC pipe through the slab stubbed to the attic or exterior (ready for fan addition)
• Sealed wall openings around the pipe

When natural air pressure is favorable — warm interior air rising (stack effect), outdoor wind creating positive pressure on the exterior — passive systems can meaningfully reduce radon. Some new homes with good sub-slab gravel and favorable conditions maintain levels below 2.0 pCi/L passively.

The problem: Passive systems are inconsistent. In winter when buildings are tightly sealed and the stack effect is strongest (and windows are closed, and heating is running), radon entry is also at its peak. The natural convection that powers passive systems may not be sufficient to offset increased radon infiltration under the same conditions.

Studies show passive systems reduce radon by 30–50% on average — useful, but not sufficient for homes with elevated levels.

Active Radon Systems

Active systems add a fan to the passive pipe architecture. The fan runs continuously and creates reliable, consistent negative pressure beneath the slab regardless of weather, season, or building pressure dynamics.

Performance comparison:

For a home testing at 8 pCi/L:

• Passive system might bring levels to 4–6 pCi/L (still at or above action level)
• Active system typically brings levels to 0.5–2.0 pCi/L

Active systems are the EPA standard recommendation for any home testing above 4.0 pCi/L.

When Passive Systems Are Enough

Passive mitigation can be sufficient when:

1. New construction with good sub-slab aggregate — A well-built RRNC system in a favorable geology area (good gravel, low soil radon flux) may maintain acceptable levels without a fan
2. Very low post-construction levels — If the home tests at 1.5–2.5 pCi/L passively, adding a fan delivers diminishing returns
3. Temperate climates with regular ventilation — Mild climates where windows are open frequently partially compensate for passive system limitations

The right approach: test first. If a new home with a passive system tests above 4.0 pCi/L, add the fan. The pipe is already in place — it's a straightforward, affordable upgrade.

Converting Passive to Active

If you have a passive system and your home tests above 4.0 pCi/L, conversion is simple:

1. Locate the pipe stub — typically ends in the attic or exterior wall
2. Size the fan — a radon professional assesses sub-slab aggregate to select the right fan model
3. Install inline — the fan mounts on the pipe with standard fittings
4. Wire to power — standard 120V connection
5. Retest in 30 days — confirm new post-mitigation levels

Cost for fan addition when pipe is already installed: $500–$900 at most contractors, significantly less than a full new system install.

The Bottom Line

If your home has a passive system: test. If levels are below 2.0 pCi/L, the passive system is doing its job. If levels are above 4.0 pCi/L, add the fan — the marginal cost is low and the performance jump is large.

If you're building new: insist on RRNC with the pipe stubbed out and wired for a future fan. The passive-only version is inadequate insurance for a high-radon area.

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