Radon in New Construction Pennsylvania: Passive Systems, Code Requirements, and When to Activate

What Pennsylvania's Uniform Construction Code Requires

Pennsylvania's Uniform Construction Code (UCC) mandates passive radon-resistant rough-ins in new residential construction in all EPA Zone 1 counties. Zone 1 designation covers the majority of Pennsylvania — including the entire Reading Prong corridor from Bucks County northward through Northampton and into the Pocono foothills, and extending westward across the Appalachian Plateau through the central and western counties. Builders working in these counties are required to integrate radon-resistant features into the foundation and framing during construction, before the slab is poured and before the walls are enclosed.

The requirement is for a passive system specifically — one that creates the mechanical infrastructure for radon control without adding an active fan. The five required elements are:

A. Gas-permeable layer. A 4-inch layer of clean coarse aggregate — typically ¾-inch washed crushed stone or pea gravel — placed beneath the entire slab footprint. The aggregate layer allows soil gas to migrate laterally and collect beneath the slab rather than building up pressure at isolated entry points. Fine aggregate or compacted fill is not acceptable for this purpose; the material must be open-graded with sufficient void space to support free gas movement.

B. Polyethylene soil gas retarder. A minimum 6-mil polyethylene sheeting membrane placed on top of the gas-permeable layer and beneath the concrete slab. The sheeting acts as a barrier between the sub-slab aggregate and the slab itself, preventing direct soil gas diffusion through the concrete. Seams must be overlapped at least 12 inches and sealed with compatible tape or mastic to maintain continuity. Tears, penetrations around pipes, and unsealed edges are common installation failures that reduce system effectiveness.

C. Sealing and caulking. All below-grade openings in the foundation walls and floor must be sealed with permanent elastomeric materials — not standard construction caulk, which shrinks and cracks over time. This includes gaps around plumbing penetrations, floor drains, construction joints at the slab perimeter, and any access panels or utility openings. Sump pits, if present, require sealed covers with gasketed access points. Unsealed sump pits are one of the most common radon bypass pathways in Pennsylvania new construction and must be addressed during installation.

D. Vent pipe. A 3- or 4-inch Schedule 40 PVC pipe runs from the gas-permeable aggregate layer, through the interior conditioned space, and terminates at least 12 inches above the roofline in a location that prevents reentry of exhausted soil gas through windows or HVAC intakes. The pipe must follow a continuous, unobstructed path — typically routed through interior closets or utility chases to avoid condensation problems associated with exterior routing. The sub-slab end terminates in a tee fitting buried in the aggregate layer to maximize pressure distribution. This pipe is the conduit through which soil gas passively rises and escapes — and through which a fan will be inserted if activation is required.

E. Electrical junction boxes. Two junction boxes are required: one in the attic, sized and positioned to accept a vent fan wired to the existing rough-in; and one in the conditioned living space at a location accessible to occupants, intended for a future radon warning alarm or system indicator light. The attic junction box is what makes activation straightforward — rather than running new wiring through finished walls, the contractor simply installs the fan at the pre-wired location.

The cost advantage of building radon-resistant from the start is substantial. A passive rough-in installed during construction typically costs $350–$500 — largely because the foundation is open and accessible, no finished surfaces need to be cut or repaired, and the electrical rough-in is done alongside all other wiring. A complete active sub-slab depressurization system installed in an existing home with no rough-in requires coring through a finished slab, routing pipe through enclosed walls or along exterior surfaces, and running new electrical from a panel — typically costing $800–$2,000. The incremental cost to add a fan to an existing passive rough-in is only $300–$500, making the passive investment a highly cost-effective risk reduction strategy even if activation turns out to be unnecessary.

One common misconception worth addressing directly: the presence of radon-resistant features does not mean a new home is radon-safe. As documented in the radon myths guide, the myth that new homes have lower radon risk than old homes is one of the most persistently misleading beliefs in Pennsylvania. A tightly constructed new home with a passive system but no fan can accumulate higher radon concentrations than a drafty older home with the same sub-slab flux, precisely because the building envelope restricts dilution. Every new home must be tested.

Why Passive Systems Are Not Sufficient in Pennsylvania's High-Radon Geology

A passive sub-slab depressurization system operates on a simple principle: when indoor air is warmer than outdoor air, warm air rises inside the building (the stack effect), creating a slight negative pressure at low levels that draws soil gas upward through the vent pipe. The pipe becomes a low-resistance pathway, and soil gas preferentially exits through the pipe rather than through slab cracks and penetrations into the living space.

The problem is that this mechanism is seasonal and reversible. In Pennsylvania summers — when outdoor temperatures regularly exceed indoor temperatures — the stack effect in the vent pipe either diminishes or reverses entirely. Outdoor air flows down the pipe rather than soil gas flowing up. The pressure differential that made the passive system effective in January may be absent or inverted in July. During transitional seasons, when homes are sealed for heating or cooling but outdoor temperatures fluctuate, the passive system's effectiveness is unpredictable day to day.

Pennsylvania DEP's monitoring data from homes with passive-only systems in Zone 1 counties confirms this pattern. Passive systems achieve reliable below-4 pCi/L results in only a fraction of high-risk installations — particularly in the highest-flux geology. The installations that fail are not defective; they are operating exactly as designed. Natural convection simply cannot overcome the soil gas flux beneath foundations in Pennsylvania's Reading Prong and Appalachian Plateau geology during the portions of the year when the thermal gradient is unfavorable.

The geology context is critical. Reading Prong counties — Northampton, Lehigh, Berks, Bucks, Montgomery, Chester, and their neighbors — have some of the highest residential radon flux rates in the eastern United States. The granitic gneiss and associated uranium-bearing minerals that form the Reading Prong produce continuous radon generation that does not pause for summer. New homes built on this geology with passive-only systems frequently test above 4 pCi/L in post-occupancy testing, particularly in full or partial basements where sub-slab contact area is greatest. The Bethlehem and Lehigh Valley corridor is a particularly clear example: new subdivisions built directly on Reading Prong bedrock carry the same geological radon source term as the oldest homes in the area, regardless of construction vintage.

New growth areas in York County present a different geological challenge. South-central Pennsylvania's karst limestone terrain produces elevated radon through different mechanisms — fractured bedrock and dissolution voids that create irregular, high-permeability pathways from deep soil to foundation. Karst radon entry is characteristically difficult to intercept with a single sub-slab suction point because the entry pathways are concentrated, not distributed. A passive system designed around distributed sub-slab pressure may fail to address a dominant karst entry point entirely, leaving the primary radon source unmitigated while the pipe provides only cosmetic depressurization of the surrounding fill.

For a comprehensive comparison of what passive and active systems can and cannot achieve, see the guide to active vs. passive radon mitigation in Pennsylvania.

Activating a Passive System: Fan Installation and Testing

When post-occupancy testing returns a result at or above 4 pCi/L, the passive rough-in must be converted to an active system by adding a fan. This is the scenario the passive rough-in was designed to enable — it is not a failure of the original construction, but the intended upgrade pathway when the geology demands it.

The activation process requires a DEP-certified radon mitigation contractor. The contractor locates the attic junction box that was installed as part of the original rough-in, installs a radon vent fan in the vertical section of the existing PVC vent pipe (typically immediately below the junction box in the attic), and wires the fan to the pre-installed circuit. A suction diagnostic is performed to verify that the fan is drawing negative pressure through the sub-slab aggregate layer and that the pressure field extends across the foundation footprint. If sub-slab communication is inadequate — a finding that is more likely in karst geology or in homes with dense clay or mixed fill beneath the slab — additional suction pits may be required.

The contractor also installs a system performance indicator in the living space: either a liquid-filled U-tube manometer or an electronic alarm wired to the second junction box that was roughed in during construction. This indicator confirms the fan is running and generating suction, though as covered in the mitigation standards context, it does not substitute for a certified radon test. The system must be labeled per PA Code § 240.308(e) with the installation date, contractor certification number, and retest recommendation.

Fan selection for activated passive systems follows the same principles as any active sub-slab depressurization installation. The fan must be matched to the sub-slab conditions — a high-communication gravel sub-base in a standard residential footprint is well-served by a moderate-capacity fan such as the RadonAway RP145 or equivalent. Dense or mixed sub-slab conditions may require a higher-static fan to achieve adequate pressure field extension. The full engineering requirements are covered in the guide to ASD engineering standards in Pennsylvania. The contractor should document fan selection rationale in the system installation record.

Cost for fan activation when the passive rough-in already exists is approximately $300–$500 in most Pennsylvania markets. This reflects labor for fan installation and wiring connection, the fan unit itself ($80–$150 depending on model), the system indicator, labels, and post-activation test. The absence of slab coring, pipe routing through finished walls, and new electrical runs from the panel is what keeps the cost substantially lower than a full new installation.

New construction in Lancaster County's growing suburbs is a particularly common activation scenario. Lancaster County has experienced significant residential development over the past two decades, with large subdivisions built across both the Reading Prong fringe and Lancaster Plain limestone terrain. Post-occupancy testing in these developments frequently identifies homes above 4 pCi/L that were built with compliant passive rough-ins — the geology demands activation, and the rough-in makes it straightforward. Homeowners who discover this situation after moving in should contact a DEP-certified contractor promptly rather than waiting for a second test cycle.

For full technical requirements governing post-activation testing and system certification, see the Pennsylvania Radon Mitigation Standards guide.

New Construction in Pennsylvania's High-Risk Growth Areas

Pennsylvania's residential growth corridors intersect directly with its highest-radon geology, creating a structural overlap between new housing supply and radon risk that builders, buyers, and code officials all need to understand.

The Lehigh Valley corridor. The Lehigh Valley is one of Pennsylvania's fastest-growing residential markets, with significant new construction activity in Bethlehem Township, Forks Township, and adjacent communities in Northampton County. This entire growth corridor sits on or immediately adjacent to the Reading Prong. New subdivisions built on this geology carry the same radon source term as the oldest homes in the area — the geology does not care that the slab was poured in 2024 rather than 1962. Easton and its surrounding Northampton County townships are experiencing active residential development on Reading Prong terrain. Buyers of new construction in this corridor should treat post-occupancy radon testing as mandatory rather than optional, regardless of what the builder represented about radon-resistant features.

York and Lancaster County suburban fringe. South-central Pennsylvania's suburban fringe — where Harrisburg, York, and Lancaster metropolitan areas are expanding outward — encompasses both limestone karst terrain and the southern reaches of the Reading Prong. New construction in York County communities like Shrewsbury, Spring Grove, and Red Lion sits on geology that produces above-average radon through karst dissolution pathways. New construction in Lancaster County communities like Manheim Township, Elizabethtown, and East Hempfield Township spans both limestone plain and Reading Prong fringe, with radon risk varying significantly by specific site location. In both counties, passive rough-ins are code-required, but activation rates in new construction suggest a meaningful fraction of homes are being occupied without post-occupancy testing.

Western Pennsylvania growth areas. New construction in the Pittsburgh metropolitan area — particularly in Washington County, Westmoreland County, and Butler County — sits on Appalachian Plateau geology influenced by Marcellus and other black shale formations. Zone 1 designation applies across most of this area, and passive rough-ins are required accordingly. The radon dynamics here differ from the Reading Prong: rather than the high-flux uniform granite mineralization of the southeast, western Pennsylvania radon tends to reflect localized black shale outcropping and glacial till variability. This means inter-home variability can be higher than in Reading Prong areas — adjacent homes may test quite differently — reinforcing the importance of testing each new home individually rather than relying on nearby test results.

The builder's responsibility and buyer's due diligence. Pennsylvania buyers of new construction in Zone 1 counties should take several specific steps that are not commonly emphasized in the builder's sales process. First, ask the builder for written documentation of which radon-resistant rough-in components were installed — a list of materials and a photo of the vent pipe stub-out before slab pour is ideal. Second, commission an independent post-occupancy radon test rather than relying on any builder-provided test result. The conflict of interest is the same reason that post-mitigation tests should be performed by a contractor independent from the one who installed the system: when the entity being evaluated is also the entity performing the evaluation, the result is inherently less credible. Third, if the builder claims the home was tested and passed, request the actual test report — the device type, laboratory analysis certificate, placement location, and test dates — rather than a verbal assurance.

SB 760 and New School Construction Requirements

Pennsylvania Senate Bill 760, enacted in 2025 and effective September 19, 2025, includes a provision specifically addressing new school building construction that goes beyond the existing testing and remediation requirements for existing schools. Section (g) of the bill requires that each new school building be constructed using radon-resistant new construction techniques in accordance with the version of ANSI-AARST CC-1000 — the standard titled "Soil Gas Control Systems in New Construction of Buildings" — in effect on the effective date of the subsection.

ANSI-AARST CC-1000 is a more comprehensive standard than the UCC's residential passive rough-in requirement. CC-1000 specifies soil gas control systems designed as permanent, integrated building systems — installed during construction as part of the structural envelope rather than as afterthought retrofits. The standard covers gas-permeable aggregate layers, soil gas retarder membranes, sealed penetrations at all utility and structural crossings, and vent piping systems with provisions for active fan installation. CC-1000 also addresses multi-zone buildings where a single vent pipe cannot serve the entire building footprint — a common condition in school construction where wings, additions, and varying slab elevations create multiple sub-slab compartments.

The significance of SB 760's new construction provision extends beyond school buildings. It establishes a legal and regulatory precedent that radon-resistant new construction is an expected standard for publicly occupied buildings in Pennsylvania — not a voluntary best practice. As the DEP begins administering the existing school testing requirements in the 2025–2026 school year and publishing results beginning in 2026–2027, community awareness of radon risk will increase substantially. Parents who learn that their children's school tested above 4 pCi/L will ask about their own homes. Buyers who see that the elementary school two blocks from the house they are purchasing tested elevated will understand that the surrounding residential geology carries the same risk. For full coverage of the SB 760 framework and Pennsylvania radon law, see the Pennsylvania radon overview.

Builders in Zone 1 counties who proactively install and document radon-resistant features — and who commission and certify independent post-occupancy tests — will have a measurable advantage in this environment. As awareness builds, the question "does this new home have a radon-resistant rough-in?" will become as routine in Zone 1 buyer negotiations as questions about HVAC efficiency ratings and energy certifications are today. Builders who treat radon-resistant construction as a documented selling point rather than a code compliance checkbox will be positioned ahead of the curve.

Frequently Asked Questions

Does Pennsylvania require radon-resistant construction in new homes?

Pennsylvania's Uniform Construction Code requires passive radon-resistant rough-ins in new residential construction in EPA Zone 1 counties — which covers the majority of the state. The required elements include a gas-permeable layer beneath the slab, a polyethylene soil gas retarder, sealed foundation openings, a 3- or 4-inch PVC vent pipe from the sub-slab to above the roofline, and an electrical junction box in the attic for future fan installation. This is a passive system — it does not include an active fan. Every new home should be tested after occupancy regardless of whether passive features were installed.

Why is a passive radon system not enough for Pennsylvania new construction?

Passive sub-slab depressurization relies on natural convection and pressure differentials to draw soil gas upward through the vent pipe. In Pennsylvania's high-radon geology — particularly in Reading Prong counties and across the Appalachian Plateau — passive systems frequently fail to maintain levels below 4 pCi/L year-round. Natural convection reverses in summer when outdoor air is warm, reducing or eliminating the suction effect. Pennsylvania DEP data shows passive-only systems achieve reliable below-4 pCi/L results in only a fraction of high-risk installations. If post-occupancy testing shows levels at or above 4 pCi/L, a fan must be added to activate the system.

How much does it cost to add a fan to a passive radon system in a new Pennsylvania home?

Adding an active fan to an existing passive radon-resistant rough-in costs approximately $300 to $500 in most Pennsylvania installations. The passive system already includes the vent pipe and electrical junction box in the attic, so the contractor only needs to install the fan and connect it to the existing wiring. This is significantly less expensive than installing a complete active sub-slab depressurization system in a home with no existing rough-in, which typically costs $800 to $2,000.

What does ANSI-AARST CC-1000 require for new school construction under SB 760?

Pennsylvania Senate Bill 760 requires that new school buildings be constructed using radon-resistant new construction techniques according to the version of ANSI-AARST CC-1000 (Soil Gas Control Systems in New Construction of Buildings) in effect on the effective date of the subsection. CC-1000 specifies the installation of soil gas control systems during construction — including gas-permeable layers, soil gas retarder membranes, sealed penetrations, and vent piping — as permanent integrated building systems rather than afterthought retrofits.

When should I test a new Pennsylvania home for radon after occupancy?

Pennsylvania DEP recommends testing every new home after occupancy, even if it was built with radon-resistant features. The recommended timing is after the home has been occupied and under normal living conditions for at least a few weeks — allowing the HVAC system, occupant patterns, and building envelope to reach typical operating conditions. A short-term test of 48 to 96 hours under closed-house conditions provides an initial result. If the result is at or above 4 pCi/L, a DEP-certified contractor should be contacted to activate the passive system with a fan.

What are the five basic elements of a radon-resistant new construction system in Pennsylvania?

The five basic elements required for radon-resistant new construction per EPA and PA DEP guidance are: (A) a gas-permeable layer — typically a 4-inch layer of clean coarse gravel beneath the slab to allow soil gas to move freely; (B) plastic sheeting — polyethylene soil gas retarder placed on top of the gravel and under the slab; (C) sealing and caulking — all below-grade openings in the foundation and walls sealed to reduce soil gas entry; (D) a vent pipe — 3- or 4-inch PVC pipe running from the gas-permeable layer through the house to above the roofline; and (E) electrical junction boxes — one in the attic for a future fan, one in the living space for a future warning device.