Half of Madison seems to be finishing a basement at any given moment — rec room, office, the teenager's bedroom. Radon belongs at the front of that project, not the end: once people sleep down there, exposure hours multiply, and once drywall hangs, every mitigation step gets harder and uglier.
Test before the design is final
A 48-hour closed-house test in the basement, ideally in heating season, tells you whether you are building around an active mitigation system or just providing for a future one. Both are cheap now; neither is cheap after carpet.
Finishing also changes airflow — tighter, warmer, more occupied — so plan to retest after the project regardless of today's number.
Rough in the pipe while walls are open
With joists and stud bays exposed, a suction pipe runs from slab to attic invisibly. The same run after finishing means opening drywall or accepting an exterior pipe up the siding.
Even at a low test number, a capped rough-in from a sealed pit to the attic is cheap insurance during framing. If a future winter test comes back high, the fix is a fan and an electrician — not a remodel of your remodel.
Seal while everything is exposed
Caulk the floor-wall joint and slab cracks, put an airtight lid on the sump, seal around plumbing penetrations. Trivial line items with the basement open; miserable retrofits with a finished room in the way.
The bonus is comfort: the same sealing that blocks radon blocks soil moisture, and a Madison basement that stays dry stays finished.
Two code notes worth your time
A basement bedroom needs an egress window — non-negotiable, and easiest planned early. And if the project adds a bathroom fan or big exhaust, know that more exhaust means more soil-gas pull; it is one more reason the retest after finishing matters.
The order, one more time
Test with the basement open. Mitigate and verify if high. Rough in the capped pipe even if low. Seal sump, joints, and penetrations. Then finish, then retest. Six steps, and the expensive rework scenarios all disappear.
Your future self — the one whose kid sleeps down there — signs off on this order every time.
