Basics · 7 min read
New Homes and Radon
Why new construction is not automatically radon-safe, what radon-resistant construction includes, and how to activate a passive system in a newer home.
New homes are often assumed to be free of radon problems. They are not, and in some respects modern construction makes indoor accumulation easier rather than harder.
The relevant question is not how old a home is but what was built beneath the slab and how tightly the envelope was sealed.
Why tighter homes can measure higher
Modern building codes require significant air sealing, high-performance windows, and insulated envelopes. All of that reduces uncontrolled air exchange, which is exactly what used to dilute indoor radon in older houses.
The soil emits the same amount of gas regardless of the building above it. With less outdoor air entering, the same source produces a higher indoor concentration.
What radon-resistant construction includes
When builders include radon-resistant features, they typically consist of a standard set of elements.
- A layer of clean aggregate beneath the slab to allow gas movement
- A polyethylene vapor barrier over the aggregate
- Sealed slab joints, penetrations, and the perimeter gap
- A vertical vent pipe from beneath the slab to above the roof
- An electrical junction box near the pipe for future fan installation
Passive versus active
A passive system relies on the natural buoyancy of warm air in the vent pipe. It provides some reduction, but it is weather dependent and modest, particularly in a mild climate where the temperature difference driving the effect is small.
Adding a fan converts it to an active system and typically multiplies the reduction. Because the pipe already exists, this is one of the least expensive mitigation projects available: a properly sized fan, a manometer, electrical connection, and verification testing.
Testing a new home
Test after occupancy, once the home is being lived in and the HVAC is operating normally. Testing an empty house with construction ventilation still in play produces an unrepresentative number.
If the home has a passive stub-out, testing determines whether activation is needed. If it has no radon features at all, testing is the only way to know whether a full system is warranted.
New construction in San Diego
Radon-resistant construction is not universal in local building practice, so its presence should be verified rather than assumed. A pipe running from the slab through the roof, often labeled, is the visible sign.
Newer master-planned communities are more likely to include the feature than infill or custom construction, but there is wide variation between builders.
Key takeaways
- New homes can measure higher because tighter envelopes dilute less.
- Radon-resistant construction is inexpensive during the build and valuable later.
- Passive systems provide modest reduction; adding a fan transforms performance.
- Test after occupancy, not before.