Active vs. Passive Radon Mitigation: Which System Is Right for Your Pennsylvania Home

The Question Every Homeowner Asks Wrong

When most homeowners start researching radon mitigation, they ask the wrong question. They ask: how do I get rid of radon? The better question is: what is preventing radon from leaving? Because radon doesn't need to be destroyed — it has a 3.8-day half-life and will decay on its own. The only problem is that your house keeps collecting new radon faster than the old radon decays. Mitigation is a collection management problem, not a chemistry problem.

That reframe changes how you evaluate systems. A passive system tries to create a natural collection pathway. An active system forces collection mechanically. Both use the same basic architecture. The difference is one fan — and that fan matters enormously in Pennsylvania's climate and geology.

How Passive Systems Work — and Where They Fail

A passive radon mitigation system is exactly what it sounds like: a pipe running from beneath the slab to above the roofline, with no mechanical fan, relying on natural convection and pressure differentials to move soil gas upward and out. In new construction, Pennsylvania's Uniform Construction Code requires passive-ready rough-ins in all new homes in EPA Zone 1 counties — which covers most of the state.

The problem with passive systems in existing construction is consistency. Natural convection depends on temperature differential between the sub-slab space and the outside air. In summer, when outdoor air is warm and the sub-slab is cooler, the pressure differential may actually reverse — pushing outside air down the pipe and reducing or eliminating the suction effect. In winter, the stack effect works in your favor, but winter is also when the house is sealed and radon accumulates most rapidly.

PA DEP mitigation data shows that passive-only systems in existing high-risk homes — particularly on Reading Prong geology or above Pittsburgh coalfield soils — achieve reliable below-4.0 pCi/L results in only a fraction of installations. The variability is the problem. A system that works well in January may underperform in July, and a single long-term test won't capture that seasonal swing.

Active Sub-Slab Depressurization: The Engineering Standard

Active sub-slab depressurization — ASD — is the system that DEP-certified contractors install in the overwhelming majority of Pennsylvania residential mitigation projects. The architecture is simple: one or more suction pits beneath the slab, a vertical PVC pipe running through the structure to the exterior, and a continuously operating fan that maintains a zone of negative pressure beneath the floor that is always lower than indoor air pressure.

"Always" is the operative word. Unlike passive convection, an electric fan doesn't respond to outdoor temperature. It runs the same way in a July heat wave that it does in a January deep freeze. That consistency is why the EPA and AARST-NRPP list ASD as the preferred technique for existing construction, and why DEP-certified contractors default to it for Pennsylvania homes with confirmed elevated readings.

System performance is characterized by two measurements: suction at the tap point and pressure field extension across the sub-slab. A contractor performing a proper diagnostic will drill small test holes at multiple locations, measure how far the pressure field from the suction pit extends, and place additional pits if coverage is inadequate. In a standard poured concrete slab over crushed stone, a single pit commonly extends across the entire footprint. In a slab poured directly over clay or in sections, multiple pits are required.

Contractors must hold active certification under PA Code Title 25, Chapter 240 — the Radon Certification Act — to perform paid mitigation work in Pennsylvania. Verification of that certification is the non-negotiable first step in hiring anyone for this work.

Fan Selection: Why It Matters

Not all ASD fans are equivalent, and the difference matters in Pennsylvania. The state's geology creates two distinct installation scenarios. The first is high-permeability sub-slab material — crushed stone, coarse gravel, or porous fill — where radon moves freely and a modest fan generates adequate pressure extension. The second is low-permeability sub-slab material — clay, compacted fill, or slab-on-grade without a gravel bed — where resistance is higher and a more powerful fan is required.

The RadonAway RP145 is the industry workhorse for standard Pennsylvania installations. It operates at 33 CFM at 0.5 inches of water column static pressure, draws approximately 23 watts, and is specifically rated for outdoor exposure in freeze-thaw climates. For most split-levels and ranches in Lehigh Valley or Pittsburgh suburbs with standard gravel sub-base, the RP145 is the correct fan.

For homes with clay sub-slab, large footprints, or multiple isolated sub-slab zones, the RadonAway GP501 — operating at 66 CFM at higher static pressure — is the appropriate upgrade. A contractor who installs an RP145 on a clay-sub-slab slab-on-grade home without doing a pressure field diagnostic is guessing. Post-mitigation testing will reveal the error, but by then you've paid for an inadequate installation.

Crawl Spaces, Block Walls, and Non-Standard Foundations

The ASD playbook changes when there's no slab. Pennsylvania's older housing stock — particularly in the Lehigh Valley, Delaware Valley, and Pittsburgh's older neighborhoods — includes a significant percentage of homes with crawl spaces, rubble stone foundations, or hollow concrete block walls. Each of these requires a different technical approach.

Crawl spaces are addressed with sub-membrane depressurization: a heavy-gauge polyethylene barrier covers the crawl space floor, and a suction point beneath that membrane creates negative pressure under the entire sheeting. The barrier also serves as a vapor retarder, which addresses secondary moisture issues common in Pennsylvania's humid summers.

Hollow block foundations present a different problem: the hollow cores of the blocks are interconnected, creating a radon pathway from the footing up through the wall and into the basement. Block wall depressurization — drilling into the hollow cores of the bottom course and applying suction — is the correct technique. Attempting sub-slab ASD without also addressing a hollow block wall will produce incomplete results, because the two entry pathways are independent and must be treated independently.

What Verification Looks Like After Installation

A mitigation system that has been installed but not verified is a mitigation system that may or may not be working. This is the part of the process that homeowners most often skip, and it's the part that determines whether you've actually solved the problem or just paid for hardware.

Post-mitigation testing protocol, per ANSI/AARST MALB standards, requires a short-term test performed no sooner than 24 hours after system activation, under closed-house conditions, in the lowest livable level of the home. The result from that test is the number you use to evaluate system performance.

The visual indicator — the U-tube manometer or digital gauge mounted on the suction pipe — tells you the system is running and generating pressure differential. It does not tell you what the indoor radon level is. A system can show positive suction on the gauge and still fail to bring levels below 4.0 pCi/L if the pressure field doesn't cover the full sub-slab area. The test result is the only valid performance metric.

PA Radon Hub connects you with DEP-certified contractors across Pennsylvania who perform both installation and post-mitigation testing. We are an independent resource — not a service provider. Request quotes from multiple certified contractors, compare their diagnostic methodology, and verify their DEP certification number before signing any contract.

PA Radon Hub is an independent informational resource. We do not perform radon testing or mitigation. Every contractor referenced through paradonhub.com holds active DEP certification under PA Code Title 25, Chapter 240. Verify any contractor at dep.pa.gov before authorizing work.