How a radon mitigation system works
The standard system is beautifully boring: one pipe, one fan, and a pressure trick that beats the house at its own physics. Ten minutes here and you’ll read contractor quotes like someone who can’t be baffled.
For homeowners comparing quotes or deciding whether to go ahead. After this: what it costs and why · compare written offers side by side · record what gets installed · confirm the finished system actually works
The physics in one paragraph
Your house breathes. Warm air rising and exiting up high pulls replacement air in down low — including soil gas drawn through slab cracks, joints, and penetrations. That gentle suction on the soil is how radon gets in. An active sub-slab depressurization (ASD) system reverses the contest: a fan applies stronger, continuous suction to the soil under the slab than the house does, so soil gas flows into the pipe and out above the roof instead of into your basement. The radon isn’t destroyed — it’s rerouted to open air, where it dilutes to background levels.
How well that contest goes depends on what sits under your particular house. A slab over open gravel gives a fan far more to work with than stone walls or a dirt crawl space, and a finished basement changes where a pipe can legally and reasonably run. Our library of foundation and building conditions covers each of these — what it means for suction, and what to expect a contractor to propose because of it.
Three parts of that contest are worth understanding on their own terms before you read a proposal. The method itself has conditions it suits and conditions it does not. The fan is where quotes diverge most and where a homeowner has least basis for comparison. And where the pipe finally lets out is the detail neighbours and buyers ask about, and the one most often left unstated until installation day.
Every component, and what it’s for
| Component | What it does | Worth asking about |
|---|---|---|
| Suction point(s) | A cored opening through the slab with a small pit beneath it, where the system grips the sub-slab soil. | How many, where, and what diagnostics justify the count. |
| PVC piping | Sealed run from the suction point to a discharge above the roofline, away from windows. | The exact routing path — this drives cost and looks. |
| In-line fan | The always-on heart, mounted in an attic, garage, or outside — never in living space below. | Make/model, capacity, warranty length. |
| Manometer | U-tube gauge on the pipe; unequal fluid levels = the fan is pulling. Your daily-glance verification. | Where it’s mounted and how to read it (takes 10 seconds to learn). |
| Sealing work | Caulked slab cracks, sealed sump lid, crawl-space membrane — supplements that help the fan win. | Exactly what’s included; sealing alone is never the fix. |
Installation day, hour by hour
- Placement decisions confirmed — suction location and routing walked with you before any drilling.
- Core and pit — an opening through the slab, a small cavity excavated beneath for airflow.
- Pipe run — assembled and sealed through the planned route to above the roof.
- Fan mounted and wired — in its attic/garage/exterior position, on its own switched circuit where required.
- Sealing pass — accessible entry routes closed so suction concentrates where it counts.
- Manometer installed, system labeled — you’re shown the reading that means “working.”
Most single-point installations complete in one working day.
Verification: the step you never skip
A running fan is not the goal — a lower number is. Post-mitigation testing (typically conducted no sooner than 24 hours after startup and within about 30 days) proves the indoor concentration actually fell. EPA’s consumer guidance is that most homes can be brought below 4 pCi/L, and well-designed systems routinely land much lower — but the retest, not the promise, is what you accept the job on — how that retest is run is what makes it worth accepting. Then retest every couple of years like any home, which is where a continuous monitor does something a one-off kit cannot.
Common questions
Where does the radon go once it’s vented above the roof?
Into open air, where it dilutes almost instantly to the trace background levels present everywhere outdoors. Discharge standards place the outlet above the roofline and away from windows and intakes precisely so re-entry doesn’t occur. The system doesn’t create radon — it redirects what the soil was already producing away from your living space.
How loud is the fan?
Modern in-line fans produce a low hum comparable to a quiet bathroom fan, and placement does most of the acoustic work: attic and garage mounts are effectively inaudible indoors. If a pipe run passes a bedroom wall, say so at quote time — routing and isolation hardware exist exactly for that. Most owners report forgetting the system exists within weeks.
Does the system affect heating and cooling bills?
Modestly, two ways: the fan’s own electricity (roughly light-bulb to small-appliance draw, a few dollars monthly for most models) and a small amount of conditioned air drawn through slab leakage — one reason installers seal accessible openings. For typical homes the combined effect is real but minor; the sealing pass exists to keep it that way.
What maintenance does it need over the years?
Glance at the manometer when you pass it — unequal fluid means all is well. Retest the home every couple of years. Expect the fan, like any motor, to eventually need replacement after many years of continuous duty; it’s a routine service item, not a system rebuild. That’s the entire ownership manual.
Sub-slab depressurization system
What it shows. How an active system moves soil gas out of the house rather than filtering the air inside it.
Where a radon fan may and may not be mounted
What it shows. Why fan location is a safety decision rather than a convenience one, and how to check yours by looking at it.
Authored as SVG in this repository, with real text nodes and a long description for screen readers. Reusable under CC BY 4.0 — see the diagram library.
See what this system costs for your house
Free written quotes from independent, IEMA-licensed contractors — with the design spelled out.