Finding the pipe your builder left you
Somewhere in a lot of newer homes around Sioux Falls there’s a PVC pipe doing absolutely nothing — on purpose. It’s a passive radon rough-in, and if your house was built in the last couple of decades, it’s worth five minutes to find out whether you have one.
Start in the mechanical room or an unfinished basement corner. You’re looking for a vertical pipe, usually 3 or 4 inches, coming straight up out of the slab and continuing into the framing above. Builders often label it — “RADON” or “RADON VENT” stenciled or stickered on the pipe. Follow it upstairs if you can: a rough-in runs through the house’s guts, often beside the furnace flue chase, and exits through the roof like a plumbing vent. The giveaway is in the attic, where there’s typically a straight section of pipe with room left around it. That gap is a parking space for a fan that was never installed.
Why would a builder run a pipe and skip the fan? Money and timing, sensibly. Running pipe through open walls during construction is cheap and tidy. Retrofitting that same route through a finished house costs multiples. So some builders rough in the path, cap the work, and leave the fan decision, called activation, to the eventual owner and their test result.
Passive doesn’t mean protected
Here’s the part the sales brochure tends to blur: a rough-in without a fan is mostly a pipe.
A passive stack does move some soil gas. Warm air in the pipe rises, drawing a mild draft from under the slab, the same physics as a chimney. In a mild climate on forgiving soil, that draft alone sometimes keeps a house low. But on eastern South Dakota’s gas-rich glacial ground, plenty of homes with rough-ins still test at or above the EPA action level of 4 picocuries per liter. New construction earns no pass: fresh concrete, tight modern envelopes, and long heated winters can add up to high readings in a house that still smells like paint.
The sober framing is this. A rough-in changes the cost of fixing a radon problem, not the odds of having one. The odds are set by the soil under your slab, and around here the soil deals a tough hand about half the time.
So the move for any newer home, pipe or no pipe: test. A winter test with closed-house conditions gives the truest look. The testing guide covers kits versus monitors and how placement works. Low result: enjoy the house, keep the report, retest in a few years. High result: read on, because you’re holding the easy case.
Found the capped pipe and a high number? That's the easy case
Turning the pipe into a system
Activation is the trade’s word for finishing what the builder started, and it’s typically the shortest distance between a high test and a verified fix.
The usual scope: an inline fan goes into that waiting attic section, wired to run continuously. A u-tube gauge gets mounted on the pipe downstairs, so you can see the system pulling. Seals get checked where the pipe meets the slab. Then comes the step that makes it real: a follow-up test confirms the house came in below the action level. “Typically” is doing honest work in that sentence: a rough-in’s hidden parts occasionally disappoint, and the assessment checks what’s actually under the cap before anyone promises anything.
Compare that to a from-scratch retrofit (coring the slab, routing pipe through finished rooms, a roof penetration) and the appeal is obvious. The disruptive majority of the job is already in your walls.
If your test came back high and you’ve found the capped pipe, passive system activation is the page that covers the work itself. Your builder left the sentence half-written. Finishing it is the cheap part.