How quickly do radon levels change?

Radon levels change fast. They can swing hour to hour and day to day, and they shift a lot more between seasons. Winter usually runs the highest. So a single short test is just a snapshot, and your real number is the average over time.

That’s the part most homeowners miss. You test once, you get a number, and you treat it like it’s fixed. It isn’t. Radon comes up out of the soil under your house, and how much gets in depends on pressure, weather, and how your home is running that day.

Why the number moves

Radon is a gas that seeps up from the ground through your foundation. Cracks, the floor wall joint, the sump pit, gaps around pipes. All of it is a path.

Your house pulls that gas in. Heating, exhaust fans, a clothes dryer, even wind across the roof can lower the pressure inside and suck soil gas up through the slab. That’s why the same basement can read low one week and high the next.

Weather drives it too. Cold weather, snow cover, and frozen ground all cap the soil, so the gas has fewer ways out except up into your home. That’s why winter readings tend to be the worst, and why the EPA points people toward longer-term testing to get a true average. Here in New Hampshire that seasonal swing is real, and our ground freezes hard.

A 2-day test in July and a 90-day test through the winter can tell you two very different stories about the same house.

What this means for testing

A short-term test of a few days gives you a quick read. Useful, fast, fine for a real estate deadline. But it can land high or low just because of the weather that week.

A long-term test of 90 days or more averages all that swinging out. It’s the number you can trust. The EPA action level is 4.0 pCi/L, and they say to consider fixing between 2 and 4 (EPA radon, epa.gov/radon). NH DHHS has guidance for New England homes specifically, where granite bedrock makes radon common.

If you’ve got a fieldstone foundation or a dirt crawl, expect more variability and usually higher numbers, because there’s more open soil contact.

We can run a test for you at $50, credited toward the job if you move ahead. You don’t always need us for this part though. A hardware-store kit works, and if your number’s borderline, a longer test is the smart next move before you spend on a system.

How a mitigation system handles the swing

This is where it stops mattering so much. An active sub-slab system runs all day, every day. It doesn’t wait for good weather.

A fan pulls the soil gas from under your slab and vents it up and out above the roofline before it ever reaches your living space. Because it’s always on, it flattens out the hour-to-hour and season-to-season swings that a passive house lives with. For a dirt or fieldstone crawl we use sub-membrane depressurization, which seals the floor first and then pulls from underneath. Branden, our radon guy, the crew calls him B-Radon, installs these all over the region.

A radon system runs around $900 to $6,000, and most homes land around $1,950 to $2,250, and it carries a 10-year warranty. One note. We don’t guarantee a specific radon number unless it’s in writing, because the soil and the house both vary. What we guarantee is the system, installed right.

You can read more on our radon mitigation page, and our promise on the work is spelled out on the 603 guarantee.

If your long-term average comes back under 2 pCi/L, you don’t need a system yet. Retest down the road and move on.

If your basement is also damp, radon and water often travel the same paths up through the slab, so it can be worth looking at basement waterproofing or crawl space services at the same time. Sealing those entry points is part of a good radon fix anyway.

We serve homeowners across New Hampshire, Maine, and Massachusetts, and 5,000+ New England homeowners have trusted us so far. Free inspection, free estimate, quote back to you within 24 hours.

Call us at 603-610-1770 or book your free inspection. We’ll give you a plain read on your numbers and what, if anything, you actually need.

Related questions

How long should a radon test run? A short-term test runs a few days. A long-term test runs 90 days or more and gives you a true average, which is the number to act on. The EPA recommends longer testing where you can (epa.gov/radon).

Is radon worse in winter? Usually, yes. Frozen, snow-covered ground caps the soil so radon has fewer escape paths and pushes up into the home instead. Heating systems also pull more soil gas inside. Winter readings often run the highest of the year.

Do I need to retest after a radon system goes in? A retest is the homeowner’s call, and we’d encourage one to confirm your average dropped. Our system runs continuously to keep levels down, and it’s backed by a 10-year warranty on the install.

What’s the EPA radon action level? 4.0 pCi/L is the action level. The EPA also says to consider fixing between 2 and 4 pCi/L (epa.gov/radon). Long-term testing gives you the most reliable average to compare against it.