Do I need a separate radon test for each unit of a duplex?

For a side-by-side duplex, generally yes — one test in one unit does not characterize the building. Radon measurement in a multi-unit building follows a different consensus standard than a single-family test. ANSI/AARST MA-MFLB-2023, the protocol for measuring radon in multifamily, school, commercial and mixed-use buildings, with a compliance effective date of December 1, 2023, sets initial testing locations at 100 percent of ground-contact dwelling units plus 10 percent of dwellings on upper floors. The reason is straightforward: radon enters from the soil, so every unit that touches the ground is its own exposure case. A side-by-side duplex has two ground-contact units and both are tested. A stacked duplex has one ground-contact unit, which is tested, and an upper unit that carries lower risk but not zero risk, because soil gas moves through stairwells, plumbing chases and the pressure-driven stack effect a cold Iowa or Minnesota winter creates. This matters regionally. The EPA designates the entire state of Iowa as Zone 1, the highest radon potential classification, and Iowa HHS states that the Iowa Radon Survey found Iowa has the largest percentage of homes above the EPA action level of 4 pCi/L in the nation, at 71.6 percent. The Minnesota Department of Health reports that in Minnesota, more than two in five homes have radon levels that pose a significant health risk. There is also an ownership wrinkle worth knowing before you test: characterizing an entire building under the multifamily protocol is an association-level or owner-level undertaking, while a buyer purchasing a single unit typically has a single-unit test performed as part of the transaction. Tell us which one you need and we will scope it accordingly. Add radon when you use the free instant quote tool below, or call us to include it.