Radon mitigation systems
A mitigation system keeps radon from entering at the foundation rather than trying to filter it out of the air. Here's what one looks like in a Helena basement or slab house.
Most basement and slab houses get a sub-slab depressurization system, also called active soil depressurization. A fan creates a gentle, constant vacuum under the floor, so soil gas flows into a pipe and out above the roofline instead of into the house. Air cleaners, dehumidifiers and opening a window now and then don't do the same job.
What does the installer put in?
- Suction point. A hole is cored through the slab and some fill is removed to make a small pit. Many installers test how easily air moves under the slab before picking the spot.
- Pipe. PVC runs from the pit up through a closet or garage into the attic, or out through the rim joist and up the outside of the house.
- Fan. An inline radon fan goes in the attic, the garage or outdoors, never in the living space, and runs all the time.
- Exhaust. The pipe discharges above the roof edge, away from windows, doors and other openings.
- Sealing. Reachable cracks, the floor-wall joint and the sump get sealed so the fan pulls from the soil, not from the basement.
- Gauge and label. A U-tube manometer on the pipe shows the fan is pulling, and the system gets a label.
Inside or outside pipe, given Helena's cold?
Helena's January nights regularly drop below zero. An interior run through the garage or a closet into the attic keeps the pipe hidden and protected from the worst of it. An exterior run can be simpler to install, but soil gas carries moisture, and on the coldest nights that moisture can condense and freeze at the outlet or in an outdoor fan. A good installer plans pipe size, slope and fan location for winter. Ask how they'll handle it on your house.
What else could the house need?
- An airtight, sealed lid on an open sump basin.
- A second suction point if the fill under the slab is tight or the slab is divided by footings.
- A sealed membrane over a dirt-floor crawl space; see crawl space radon.
- An outlet or circuit near where the fan will go.
Is this the same as radon-resistant new construction?
Close. Montana DEQ describes radon-resistant construction as a gas-permeable layer under the slab, plastic sheeting, sealed openings, a vent pipe and an electrical box for a future fan. If you're building in the Helena Valley, ask your builder about it. DEQ still recommends testing a new home in its first year, because a passive pipe may not bring levels down enough without a fan.
How do I know it's working?
Get a follow-up test within the first month, no sooner than 24 hours after the fan starts, ideally from an independent tester as DEQ recommends. Glance at the manometer every so often; uneven liquid levels mean the fan is pulling. Then retest at least every two years.
We don't publish or quote prices. The radon company prices the job after it understands your foundation, and usually after someone has walked through the house, so treat any figure given over the phone as a starting point, not a quote.
Where we connect you with help
Common questions
How much will a system lower my radon?
It depends on the house, but a well-designed sub-slab system usually brings levels well below 4 pCi/L. The follow-up test tells you where you actually landed.
Will the fan be loud?
A properly mounted fan in the attic or outdoors is usually a low hum. Ask the installer where they plan to put it and how it will be isolated from the framing.
Related services
- Radon testing
- Crawl space and mixed foundations
- Radon for buyers and sellers
- Radon fan replacement and repair
More background: Montana radon guide.
Radon result came back over 4 pCi/L?
Call and get connected with a Helena-area radon company that can look at your foundation and design a system.