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

If you’re researching radon mitigation, you’ll quickly encounter two broad categories: passive systems and active systems. The difference comes down to one component — a fan. Understanding how each approach works, and what that fan actually does, helps you evaluate quotes, understand your existing system, and set realistic expectations for results.

How Passive Systems Work (No Fan)

A passive radon mitigation system uses natural pressure differentials and stack effect to draw radon-laden soil gas away from the foundation — no mechanical help required.

The typical passive setup installs a vertical PVC pipe through the foundation slab into a gravel sub-base beneath it. The pipe routes upward through the home’s interior and exits above the roofline. Because the soil beneath the slab is at higher pressure than the upper atmosphere (particularly in cold weather when warm indoor air rises), soil gases including radon are drawn up through the pipe and vented outside.

Passive systems are most effective when:

• The sub-slab material is highly permeable gravel or crushed stone
• The home is well-sealed at the foundation
• Local climate and building conditions create consistent stack effect

Many newly constructed homes in high-radon areas are built with passive radon-ready systems — a sleeved pipe roughed in and capped, ready for a fan if post-occupancy testing shows it’s needed.

The key limitation of passive systems is reliability. Stack effect varies with season and weather. A system that reduces radon adequately in winter may underperform in summer when temperature differences between inside and outside are smaller.

How Active Sub-Slab Depressurization Works

An active radon mitigation system adds an inline fan to the same basic pipe configuration. The fan creates continuous, consistent negative pressure beneath the slab regardless of weather or season, drawing soil gas out and venting it above the roofline or through a side-wall exit.

This approach — called sub-slab depressurization (SSD) — is the most common and most effective radon mitigation method for slab and basement foundations. The fan runs 24 hours a day and is engineered to move the precise volume of air needed to maintain negative pressure under the slab.

Active systems also enable the contractor to perform a diagnostic before installation. By depressurizing test points in the slab, they can map how air moves through the sub-slab material and determine how many suction points are needed for complete coverage.

Cost Comparison

Passive systems installed during new construction typically add $400 to $800 to building costs. Retrofitting a passive system to an existing home — if the sub-slab conditions are favorable — runs $600 to $1,000.

Active systems installed in existing homes range from $800 to $2,500 depending on foundation type, number of suction points required, pipe routing complexity, and local labor rates. The fan adds to the upfront cost and draws roughly $5 to $10 per month in electricity.

Converting an existing passive system to active typically costs $300 to $600 — the pipe is already in place, so the work involves installing a fan and connecting it to the existing vent.

Effectiveness: Which Reduces Radon More?

Active systems reliably outperform passive systems in real-world conditions. Studies comparing paired systems find that active SSD reduces radon levels by 80% to 99% in most installations. Passive systems in optimal conditions may achieve 50% or more reduction, but results are inconsistent and season-dependent.

If your goal is to reliably stay below the EPA’s action level of 4 pCi/L — or ideally below 2 pCi/L — an active system is the more dependable choice for most homes.

When to Upgrade from Passive to Active

A passive system is worth upgrading when post-installation testing shows radon levels remain above 4 pCi/L, or when seasonal retesting reveals levels climbing in warmer months. Adding a fan to an existing passive system is typically the most cost-effective mitigation path — you’re not starting from scratch.

Signs it’s time to add a fan:

• Post-mitigation test results are above 4 pCi/L
• Levels vary significantly between seasons
• You’re selling the home and want to provide a guaranteed result for buyers
• The foundation type (crawl space, multiple sub-slab zones) doesn’t favor passive performance

A qualified radon contractor can test your passive system’s sub-slab pressure extension and recommend whether adding a fan will achieve the necessary reduction. Find local radon mitigation contractors in your area to get an evaluation.