Radon and your new custom home: What homeowners must know

Radon is an invisible, naturally occurring gas that seeps from the soil into your home. You can’t see, taste, or smell it yet at high levels, it’s the second-leading cause of lung cancer after smoking! The EPA’s action level is 4 pCi/L. Anything higher demands immediate attention.

What is a radon mitigation system?

Imagine a gentle vacuum beneath your slab or crawl space. A standard system includes:

1. Gravel layer allows gas to flow freely

2. Perforated PVC pipe embedded in gravel

3. Sealed plastic membrane traps gas and directs it to the pipe

4. Vent stack routes gas through the attic or exterior wall

This creates a “passive” radon system. If tests exceed 4 pCi/L, a quiet fan attaches to the stack, converting it to an “active” system that vents gas before it enters living spaces. This is detailed in IRC Appendix AF.

Bellevue’s rules for basements vs. crawl spaces

Effective March 15, 2024, Bellevue follows the 2021 IRC. Since King County isn’t among Washington’s seven "high-radon" counties, new basements don’t require pre-installed radon systems (WAC 51-50-1202.7.3). Still, many builders add a 3" PVC stub-out as a low-cost safeguard that avoids drywall demolition later.

Crawl spaces? It’s more nuanced:

• Fully vented: No system needed for ≥1 sq. ft. fixed ventilation per 300 sqft

• Reduced or closed vents: Triggers full system requirement (roof-vented)

• Unvented: Require a continuous exhaust fan + sealed plastic liner

See WAC 51-51-0408 for more details.

Pro Tip: IRC Appendix AF103.5 offers an alternative. Skip the roof vent by installing a mechanical ventilation system moving 1 cfm per 50 sqft of crawl space.

Your radon action plan

Decide early which crawl-space approach works for you as that choice sets your code requirements. While the walls are open, have your builder install the gravel base, plastic membrane, and PVC stub-outs. It’s inexpensive peace of mind. After you move in, pick up a $20 test kit in winter when radon levels peak. If you measure over 4 pCi/L, simply hook a fan to the pipe you roughed in or call a radon pro. Tackling radon during construction lets you avoid drywall repairs, design headaches, and most importantly, protects your family’s health. Then you can get back to agonizing over cabinet pulls.