Radon-Resistant New Construction in Washington

Radon is a naturally occurring soil gas that enters homes through foundation contact with the ground. Long-term exposure at elevated levels is linked to lung cancer. The Washington State Residential Code requires passive radon-resistant construction in new residential builds located in seven counties designated as high radon potential under WAC 51-51-60101 (Appendix AF).

Passive mitigation isn’t complicated. It’s a vapor barrier, a vent stack, and an outlet for a future fan. Installed at framing stage on a new build, the cost is minor—a few additional materials and a little plumbing labor folded into work that’s already happening. Retrofitting the same system into an existing crawl space is a different story: you’re crawling, you’re working around existing plumbing and ductwork, and every joint has to be sealed in place. Build it right the first time and it’s cheap insurance.

Which Washington counties require passive mitigation?

Per WAC 51-51-60101 Table AF101(1), the following seven Washington counties are designated as high radon potential (EPA Zone 1) and require compliance with Appendix AF on new construction:

Clark · Ferry · Okanogan · Pend Oreille · Skamania · Spokane · Stevens

In these counties, unvented crawl spaces are not permitted, and the full Appendix AF requirements apply to any new construction.

Counties outside this list are not required by state code to install the system, though some show elevated radon potential on the EPA map and the code recommends supplementing the state list with local data. Radon doesn’t check a map before entering a foundation—testing after occupancy is worth doing anywhere in Washington. Every county page on wsec.ai lists the radon zone for that jurisdiction.

What passive mitigation actually is

Five parts, all installed before the slab pour or crawl-space seal-up:

Soil-gas retarder under the slab or across the crawl-space floor. Code minimum is 6-mil polyethylene (per AF102.1, the defined “soil-gas-retarder”). For durability, a 10-mil or 15-mil reinforced barrier is worth the small upcharge—it resists tears during the rest of construction and lasts the life of the building. Overlap seams by at least 12 inches and tape every seam and penetration with a compatible seaming tape.
A soil-gas collection point under the barrier. On a crawl-space build, a sanitary tee laid on its back with the branch opening facing up works well, tied into the vent stack. On a slab build, 4 inches of clean gas-permeable aggregate under the barrier (material passing a 2-inch sieve and retained on a ¼-inch sieve) serves the same purpose per AF103.1.
A vent stack routed through conditioned space to above the roof. 3-inch or 4-inch ABS or PVC, run continuously from below the barrier up through the conditioned space of the home and out through the roof. This is a code requirement, not just best practice: AF102.1 defines a passive system as one using a “vent pipe routed through the conditioned space of a building”—the heated interior creates the stack effect that makes passive mitigation work.
A junction box in the attic near the vent stack. Wired for 120V but with no fan installed. If post-occupancy testing shows elevated radon, adding an inline fan is a one-hour job. Without the junction box in place, that upgrade means pulling new wiring—a much bigger project.
Sealed penetrations everywhere the vent stack meets the envelope. Through the crawl-space ceiling, through the roof, around any other plumbing that shares the chase. This is where air sealing and radon mitigation overlap—leaky penetrations hurt your blower-door test at the same time they reduce the system’s effectiveness.

What contractors get wrong most often

Three recurring mistakes worth flagging for anyone specifying or reviewing this work:

Vent stack routed through unconditioned space. If the stack runs outside the thermal envelope for any significant distance, you lose draft in cold weather and create a condensation pathway. It also stops meeting the code definition of a passive system. Keep it inside the insulated envelope.
No outlet box for the future fan. Skipping this saves twenty dollars at framing and costs several hundred in retrofit wiring if the system later needs activation.
Soil-gas retarder torn during concrete pour or follow-on trades. 6-mil survives in theory. In practice, rebar, boots, and dropped tools tear it. The extra cost for 10-mil is minor, and inspection after the pour or before crawl seal-up catches anything that got damaged.

When does passive become active?

A passive system relies on stack effect—warm air rising through the vent stack creates natural suction that draws soil gas out from under the building. That works well most of the time, but isn’t guaranteed to bring radon levels below the EPA action threshold of 4 pCi/L.

After occupancy, the building should be tested. Short-term tests (2–7 days) and long-term tests (90+ days) are both available; long-term is more accurate. If results come in above 4 pCi/L, an inline fan is installed in the attic junction box. The passive system becomes an active system. A well-built passive system usually keeps levels low enough that the fan isn’t needed.

Does this affect WSEC energy code compliance?

Directly, no. Radon requirements live in Appendix AF of the building code, not the energy code. But the two overlap at one place: the vent stack penetrates your thermal envelope at the roof, and if you’re planning to earn air-sealing credits under R406.3 Option 2, that penetration has to be sealed correctly or your blower-door numbers suffer.

Budget a few extra minutes during air-sealing for the radon penetration. Not a conflict, just a sequencing note.

Finding a certified installer

Radon installation is a specialized trade. A list of certified installers by county is coming to wsec.ai. In the meantime:

Your local building department can often recommend installers they’ve worked with before.
The National Radon Proficiency Program maintains a national directory of certified professionals.
For questions specific to your project, email hello@wsec.ai.

Sources

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