Why Radon Readings Climb in Winter — and When Chattanooga Homeowners Should Test

The same house can test 2.8 pCi/L in July and 6.1 pCi/L in January. Nothing about the soil changed. What changed is how your house breathes — and that is why the season you test in matters more than most Chattanooga homeowners realize.

Why winter readings run higher

A heated house acts like a chimney. Warm indoor air rises and escapes through upper-level gaps, and the slight negative pressure that creates pulls replacement air in from the lowest point available — which, in a house on a basement or slab, means soil gas drawn through foundation cracks, sump pits, plumbing penetrations, and block cavities. Add closed windows, closed crawlspace vents, and a furnace cycling for months, and you get the annual peak.

The Chattanooga area amplifies this. Our karst geology — the limestone and dolomite that gives the valley its caves and sinkholes — provides fractures and voids that move radon from bedrock toward foundations efficiently. Two neighboring homes on the same street can test years apart in result, because what matters is the specific pathway under each one.

What the numbers actually mean

The EPA action level is 4.0 pCi/L, and mitigation is recommended at or above it. Between 2.0 and 4.0, the EPA says to consider mitigation — most homeowners in that band who are planning to stay long-term choose to fix it, because there is no threshold below which radon is known to be harmless. Below 2.0, retest every two years and after any major foundation, HVAC, or envelope work.

The trap is a borderline summer number. A 3.4 in August with the windows open is not the reading your family lives with in February. If a short-term test lands anywhere near the action level, either repeat it in heating season or move to a long-term test that averages conditions across months.

Testing correctly in cold weather

Closed-house conditions are the standard for a reason, and winter makes them easy to hold. For a valid short-term test:

Keep windows and exterior doors closed for 12 hours before the test and throughout it, opening only for normal entry and exit. Run heating systems as usual, but do not run whole-house fans, window fans, or fireplace flues during the test. Place the device in the lowest lived-in level — a finished basement counts, an unfinished crawlspace does not — about three feet off the floor, away from drafts, exterior walls, and direct sun. Leave it undisturbed for the full 48 hours.

Weather does add noise. Heavy rain, snow cover, and sharp barometric drops can push readings up temporarily. A single storm-week result that comes back high is worth confirming, which is why a follow-up or continuous monitor test is standard practice rather than an upsell.

What mitigation looks like if you need it

An active sub-slab depressurization system is the standard fix: a suction point cut into the slab, sealed piping routed to a fan above the living space, and a discharge above the roofline. It runs continuously, draws about as much power as a light bulb, and typically brings a home well under 4.0 pCi/L. Crawlspaces are handled with a sealed vapor membrane tied into the same system. Post-mitigation testing is not optional — a system without a verifying test is a guess.

Winter is also the easiest season to prove a system works, because you are measuring against the year's worst conditions rather than its best.

Where to start