Radon Testing in Multi-Level Homes: Where to Test First

A Common Question in Split-Level and Two-Story Homes

Homeowners in single-story ranch homes rarely wonder where to test — there is one livable floor, and that is where the test goes. Multi-level homes are different. A split-level with a lower level partly below grade, a two-story colonial with a full basement, or a home with a finished walkout level all raise the same question: does radon concentrate in one part of the house, or does it spread evenly once it gets inside? The honest answer is that it does both, in a predictable pattern — and understanding that pattern is what makes a test result trustworthy rather than a guess.

This matters because a test placed in the wrong location can understate the radon level a household is actually exposed to, or it can measure a spot nobody spends meaningful time in. Neither outcome is useful. The goal of a certified test is a number you can act on with confidence, and that starts with placing the device correctly for your home's specific layout.

How Radon Moves Once It's Inside a Multi-Level Home

Radon enters a home from the soil beneath and around the foundation, drawn in by the slightly lower air pressure indoors relative to the surrounding ground. It is heaviest and most concentrated at its point of entry — typically the lowest level of the structure, wherever that level touches the soil directly through a slab, basement floor, or crawl space. From there, it doesn't stay put. Interior air moves upward through stairwells, HVAC ductwork, and simple convection, carrying radon with it into the levels above.

As that air travels upward, it mixes into a larger volume, and the concentration drops with each floor gained. This is why radon readings are almost always highest at the lowest livable level and progressively lower on each floor above it. In a multi-level home, this isn't a reason to ignore the upper floors — it's the reason the lowest level is where a first test belongs. It's the level where the reading is most representative of the home's actual radon source, and the level where, if there is a problem, it will show up most clearly.

Split-Level Homes: Why the Layout Complicates the Question

Split-level homes are a distinct challenge because "the lowest level" isn't always obvious. A typical split-level has a lower level partially below grade — often a family room, den, or bedroom — situated a half-flight below the main living floor, with bedrooms a half-flight above that. The lower level is frequently finished and regularly occupied, which means it functions as the home's true ground-contact living space even though it doesn't look like a traditional basement.

For a split-level, the correct approach is to identify whichever level has the most direct contact with the soil and is used for four or more hours a day — usually the lower level, not the main floor above it. Testing the main floor of a split-level because it "feels" like the ground floor can miss the level where radon is actually concentrating, since the true point of soil contact is a half-flight down.

Two-Story Homes with a Basement: Start at the Bottom

A two-story home with a full basement has a clearer hierarchy, but the same principle applies with extra weight. The basement is in direct contact with the soil on every side, and it is where a home's highest radon concentration will almost always be found. If that basement is finished and used as a bedroom, playroom, or home office, testing there isn't optional — it's the level where the household is actually spending time closest to the radon source.

Even an unfinished basement used only for storage or laundry deserves a first test, because the air it holds circulates upward into the first and second floors through stairwells and return-air ducts. A high basement reading is a signal about the whole home's air, not just the basement itself. Testing the basement first establishes the baseline every other floor should be compared against.

What Testing Only the Upper Floors Gets Wrong

It's tempting to test wherever a family actually gathers — often a second-floor bedroom or a main-floor living room — and treat that as representative of the home. The problem is that upper floors, by the physics described above, will typically read lower than the lowest level, sometimes substantially so. A test taken only upstairs can return a reassuringly low number while the level closest to the soil, and the level that best represents the home's radon entry, goes unmeasured.

This is the core reason testing strategy matters in multi-level homes: the question isn't "is there radon somewhere in this house," it's "what is the highest concentration this household is realistically exposed to, and where is it." A single test at the lowest regularly used level answers that question directly. Testing only the top floor answers a different, less useful question.

When It Makes Sense to Test More Than One Level

A first test should always start at the lowest livable level. There are situations where a homeowner reasonably wants a second data point elsewhere in the home — for example, after a mitigation system has been installed and the goal is confirming that radon levels have dropped throughout the house, not just at the point where the original test was taken. A second test on an upper floor in that situation tells you whether the improvement reaches every level a family actually uses.

Outside of that kind of follow-up question, one correctly placed test at the lowest livable level gives existing homeowners in Kannapolis, Concord, and Landis the information they need: an accurate, defensible reading of the level in their home where radon is most concentrated, taken with certified, professionally calibrated equipment rather than a guess about which room "feels" right.

Getting an Accurate Answer for Your Home

Wightman Inspection Group performs independent radon testing only — no mitigation sales, no pressure to buy a system, just a certified test and a result you can trust. A standard residential test runs $175–$225 and is placed according to your home's actual layout, whether that's a split-level, a two-story with a full basement, or anything in between, so the reading reflects the level where it matters most.