Duplex With Shared Foundation: One Unit Tested, Two Units Occupied
You are hired to test the left unit of a side-by-side duplex built on a shared poured slab with a common basement divided by a partial cinderblock wall. Gaps at the top of the wall and a single shared sump connect the two sides. The left unit has a finished basement bedroom; the right unit is a rental with an unfinished basement and a tenant who works nights. You place one CRM in the left basement bedroom 6 ft from the shared wall and 20 in above the floor, and run a 48-h test under closed-house conditions. The result is 9.4 pCi/L average. The left owner asks whether the right tenant also has a radon problem and whether the left result can be used for the right unit. You did not enter the right unit and did not test the right basement. The owner wants one mitigation bid for the whole building.
1. How should the 9.4 pCi/L result be interpreted for the left unit and for the right unit?
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The 9.4 pCi/L is a valid result for the left unit's lowest livable level, assuming the placement and closed-house conditions were maintained. It cannot be transferred to the right unit. The right unit is a separate occupancy with a different basement finish, different air exchange, and potentially different soil-gas entry points, even though the slab, sump, and partial wall are shared. Radon can move through the gaps and sump, so the right unit may also be elevated, but the left result does not establish its concentration. The correct statement is that the left unit is above 4 pCi/L and the right unit remains untested; a right-unit test is needed before any conclusion is made.
2. What testing approach would you recommend for the right unit and the building?
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Test the right unit's lowest livable level, which is the unfinished basement, with a 48-h test under closed-house conditions. Because the tenant works nights, schedule the test around occupancy and explain that doors and windows must remain closed except for momentary entry. Place the device at least 3 ft from exterior walls and the shared wall, 20 in to 6 ft above the floor, and away from the sump. If the basement has multiple foundation areas or the building is large, use more than one device. Do not place a single device in the shared basement and call it representative of both units. For a building-wide mitigation bid, the contractor should evaluate both sides and the shared pathways rather than assume one system will serve both.
3. What should you tell the owner about a single mitigation bid for the whole building?
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A single bid may be possible, but it must be based on testing and a site evaluation of both units, not on the left unit's 9.4 pCi/L alone. The shared slab, sump, and wall gaps mean the building has connected pathways, but the two units may need separate system design, separate fans, or pressure-field extensions. The right tenant must consent to access and testing. Tell the owner that mitigation is justified for the left unit now, but the right unit's need and the scope of a building-wide system depend on its own test and a contractor's diagnostic inspection. Do not promise that one system will fix both sides without that evaluation.