Passive vs Active Radon Systems

If you bought a newer Colorado home — say, anything built after 2009 — there's a good chance your home has a passive radon system in it. Maybe nobody mentioned it. Maybe you saw a white PVC pipe in the basement or an exhaust pipe sticking up through the roof and wondered what it was for.

That's the rough-in. Whether it's actually doing anything for you depends on whether it's been tested and activated. This page explains the difference between passive and active systems, when each is right, and what to check if you bought a home with a passive system already in place.

Reduction figures: ASTM E1465 and AARST standards.^[1]

A passive radon system is a complete mitigation system minus the fan. There's a suction point through the slab (or beneath a crawlspace membrane), a sealed PVC pipe running up through the home or up an exterior wall, and an exhaust point above the roofline. What's missing is the inline fan that would create active suction.

Instead, the system relies on the stack effect — the natural tendency of warm air to rise and create slight upward draft in vertical pipes. The temperature difference between the soil and the outside air, combined with the height of the exhaust stack, creates a small pressure differential that pulls soil gas up through the pipe.

It works modestly. In a tight, well-sealed home in a cold climate, a passive system can reduce indoor radon by up to 50%. In warmer weather, with the stack effect weaker, the reduction is less.

Starting in 2009, the International Residential Code (IRC) introduced an optional appendix for radon-resistant new construction. The appendix was originally called Appendix F, became Appendix AF in 2021, and is now Appendix BE in the 2024 IRC.^[2] The appendix requires a passive radon system to be installed during new construction in EPA Zone 1 — which includes all of Colorado.

What this means for buyers of newer Colorado homes:

• If your home was built after Colorado jurisdictions adopted IRC Appendix F or its successors, there's likely a passive system already roughed in.
• The white PVC pipe you may see in an unfinished basement, garage, or coming through the attic and roof is part of the passive system.
• The system is doing some work — but unless the builder ran a post-construction radon test, you don't know if it's enough.

The 2021 and later IRC versions say new-home radon control is incomplete unless a post-construction test confirms radon levels below 4.0 pCi/L.^[2] If the test fails, the system is upgraded to active — the contractor adds a fan to the existing pipe and verifies the result.

Activation cost is typically $300–$800 because most of the system is already there. The work is:

• Install a name-brand fan (RadonAway RP145 or equivalent) in the existing pipe, usually in the attic or in an exterior enclosure.
• Add a manometer at the existing suction point.
• Run electrical to the fan (sometimes a separate $150–$400 line item).
• Run a confirming post-mitigation test.

If you bought a newer home with a passive system and haven't tested, that test should be your next move. If the result is at or above 4.0 pCi/L, activating the existing rough-in is the cheapest mitigation path you'll find.

• Find the pipe. It usually starts in an unfinished basement, exits through the rim joist or up through the attic, and emerges above the roofline. The pipe is white 3-inch or 4-inch PVC.
• Look for a manometer. A passive system typically has no manometer; an active one does. If you see a U-tube gauge mounted on the pipe with both columns at the same level, the fan may have been installed but isn't running. If the columns are at different levels, the fan is running.
• Check builder documentation. If a post-construction radon test was run, the result should be in your home's construction paperwork.
• Test the home. A short-term DIY kit ($15–$40) is the fastest way to know whether the passive system is keeping levels below 4.0 pCi/L.
• If the test is high, get an activation quote. Activation is far cheaper than a full new system.