How a radon mitigation system works

The four parts of a system, why it works, the five DEP-recognised methods, and how to tell a good installation from a bad one.

"Radon mitigation system" is what almost everyone searches for, and the thing itself is simpler than the name. In nearly every York County house it is a pipe, a fan and a gauge, arranged so that the air pressure under the slab is lower than the air pressure in the basement. Radon then flows into the pipe instead of into the house.

The components

• Suction point. A hole cored through the basement slab with a small pit dug beneath it, or the existing sump pit with a sealed lid. This is where the pipe starts.
• Vent pipe. Three- or four-inch PVC running from the suction point up through the house (or up the outside wall) to a point above the roof eave, at least ten feet from any window or door that opens.
• Fan. An in-line fan mounted on the pipe in the attic, in the garage or outside. Never in the basement or living space, because everything downstream of the fan is under positive pressure and a leak there would push radon into the house.
• Manometer. A U-shaped tube of coloured liquid on the pipe below the fan. Uneven levels mean the fan is pulling suction; level ones mean it has stopped. The DEP calls it the warning device, and every system must have one.
• Sealing. Caulk on slab cracks and the floor-wall joint, a sealed sump cover, and sometimes sealed block-wall tops. Sealing on its own does not fix radon, but it makes the fan's job possible.

Why it works

Radon comes up through the soil because the house is a chimney: warm air rises and leaves through the top, and soil gas is pulled in at the bottom to replace it. The fan reverses the pressure difference at the slab. Under the slab becomes the lowest-pressure place in the building, so gas goes there, into the pipe, and out above the roof where it disperses to nothing. A well-installed system cuts indoor radon by 80 to 99 percent.

The DEP-recognised methods

The Pennsylvania DEP's consumer guide lists five active methods. Which one you get depends on the foundation.

1. Sub-slab suction (active soil depressurization). One or more suction points through a poured slab. Most York County basements.
2. Sump hole suction. The sump pit is the suction point. Common because so many York County basements have sumps.
3. Drain tile suction. The perimeter drain pipe under the slab is used as the collection network. Works when the drain tile forms a loop.
4. Block wall suction. Suction applied to the hollow cores of a concrete-block foundation, usually alongside sub-slab suction. Older York and Hanover houses.
5. Sub-membrane suction. A plastic sheet laid over a dirt crawl space and sealed at the edges, with the fan pulling from under it.

Passive systems, meaning the same pipe with no fan, rely on the stack effect alone and typically reduce radon by only a third to a half. They are what builders install as a rough-in; see activating a passive system.

Choosing a system

You do not really choose the system; the house does. What you choose is the contractor and the details. A good mitigator will do a diagnostic test before drilling: a small hole in the slab and a vacuum to see how far suction travels under it, which decides whether one point is enough. Ask where the fan will go, where the pipe will exit, how loud it will be (fans in attics are quieter than fans on the wall outside a bedroom), and what the post-test target is.

Signs of a bad installation

• A fan inside the basement or in a closet on a living floor.
• A pipe that terminates below the eave or beside a window.
• No manometer, or one that reads level.
• An open or loosely covered sump with the pipe running past it.
• No post-mitigation test.

Any of those is a reason to call a different DEP-certified company from the directory for a second opinion.

Last updated 2026-09-28. General information for the York area, not professional advice; confirm specifics with a qualified contractor or your insurer.