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Home / Code Guides / Radon-Resistant Construction

Radon-Resistant Construction — Passive Stack, Gravel & Sealing

Radon-resistant construction is cheap during framing and expensive afterward. It is a passive system with one idea behind it: give soil gas an easy path up a stack instead of an easy path into the house.

RequirementModel Code StandardSection
Where it appliesThe radon provisions sit in an appendix, so they apply only where the jurisdiction adopts them — most commonly in counties the EPA maps as Zone 1, where predicted average indoor levels exceed 4 pCi/LIRC Appendix F; EPA Map of Radon Zones
Gas-permeable layerNot less than 4 in. of clean coarse aggregate under the entire slab, or an approved sand-and-drainage-mat alternative, so soil gas can move laterally to the suction pointIRC Appendix F
Soil-gas retarderA minimum 6 mil polyethylene sheet (or 3 mil cross-laminated) over the permeable layer, with separate sections lapped not less than 12 in.IRC Appendix F
Vent pipeA minimum 3 in. diameter pipe picking up from the sub-slab layer and routed up through conditioned space to the roof, with a tee or equivalent fitting embedded in the aggregateIRC Appendix F
TerminationThe stack terminates not less than 12 in. above the roof surface and not less than 10 ft from any window or other opening that sits below the point of dischargeIRC Appendix F
SealingSlab penetrations, control and construction joints, sump covers, crawl space penetrations, and openings in below-grade walls are sealed with caulk or polyurethane, so the stack pulls from the soil rather than from the houseIRC Appendix F
Provision for a fanAn electrical circuit terminated in an approved box in the attic near the stack, so a fan can be added later without opening finished ceilings, plus a label on the exposed vent pipe on each floor identifying it as a radon systemIRC Appendix F

Worth knowing

  • Passive means passive — a stack with no fan usually reduces indoor radon somewhat but is not a guaranteed fix. The only way to know is to test the finished house.
  • The EPA action level is 4 pCi/L. If a post-occupancy test comes back at or above it, the retrofit is adding a fan to a stack that already exists, which is exactly why the rough-in is worth doing.
  • Route the stack inside the thermal envelope wherever possible. A stack run up an exterior wall or through an unconditioned garage loses the buoyancy a passive system depends on, and it can condense.
  • The sump pit is a major entry point in a basement, so a sealed gasketed lid with sealed cord and pipe penetrations is part of the system rather than an upgrade.
  • Some states enforce radon-resistant construction statewide regardless of zone, and some counties amend the appendix with stricter details — confirm locally rather than off the national map.

Common questions

What does the code say about where it applies for Radon-Resistant Construction?

The radon provisions sit in an appendix, so they apply only where the jurisdiction adopts them — most commonly in counties the EPA maps as Zone 1, where predicted average indoor levels exceed 4 pCi/L. (IRC Appendix F; EPA Map of Radon Zones)

What does the code say about gas-permeable layer for Radon-Resistant Construction?

Not less than 4 in. of clean coarse aggregate under the entire slab, or an approved sand-and-drainage-mat alternative, so soil gas can move laterally to the suction point. (IRC Appendix F)

What does the code say about soil-gas retarder for Radon-Resistant Construction?

A minimum 6 mil polyethylene sheet (or 3 mil cross-laminated) over the permeable layer, with separate sections lapped not less than 12 in.. (IRC Appendix F)

Adopted edition by state

Base-code year from BuildSpec’s adoption records. States often amend the base code, and your city or county may differ, so confirm with your building department.

This guide is BuildSpec’s own summary, not code text. It cites the 2021 International Residential Code and the 2023 National Electrical Code as model standards. The IRC 2021 text itself is in the Code Library for signed-in members; BuildSpec publishes no NEC text, which is at nfpa.org.Neither is an adoption record: your state may be on a different edition with amendments — check your jurisdiction page or get an Address Report.
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