Radon mitigation systems: how they work and what a good install looks like

How does a radon mitigation system work?

The goal is to make the air pressure under the floor lower than the air pressure in the house. Once that happens, soil gas gets pulled into the pipe instead of leaking in through cracks and gaps.

A typical system has a suction point drilled through the slab, a small pit dug out under it, plastic pipe running up to the outside, and a fan that runs all the time. The EPA notes that a single suction point is often enough. A small gauge on the pipe shows the system is pulling. If you haven’t tested yet, start with how to test for radon.

1. Suction point and pit under the slab
2. Cracks and the floor-wall joint sealed
3. Sealed pipe, sloped so condensation drains back
4. U-tube manometer
5. Fan outside the living space
6. Discharge above the roof, away from windows and doors

SoilConcrete / masonryWoodGravelSoil gas with radon

What is a radon mitigation system made of?

• Suction point: a hole through the slab with a small pit dug under it.
• Pipe: usually PVC, sealed at every joint, sloped so condensation drains back down.
• Fan: in the attic, the garage or outside, never in or below the living space (EPA).
• Discharge: above the roof and away from windows and doors.
• Warning device: a U-tube manometer, a gauge or an alarm that shows whether the system is working. The EPA says active systems must have one.
• System label: who installed it and when.

How does radon mitigation work in different homes?

If you have a basement slab, the usual first choice is active sub-slab depressurization. That’s what I find in most homes I visit in Virginia, Pennsylvania, Maryland, Ohio, Kentucky and New Jersey. A house built on a slab with no basement works on the same principle.

In North Carolina and Tennessee we get asked about crawl spaces a lot. There, sub-membrane depressurization is usually the solution. If a house has both kinds of foundation, you have to check whether one system can control both.

I won’t give you percentages of how many systems are one type or another without going through my job files. It varies by state and by house. What I decide at the house isn’t the method that “sells” best, but where I can get suction that actually works.

Sump pits

A sump pit is an open path to the soil. It needs an airtight, rigid lid that can still be removed: whatever gets sealed, you have to be able to reach the pump. The float can’t touch the lid or a radon pipe, and the pump still has to discharge.

I’ve seen lids that just sit on top, lids with open gaps around the cords, and lids glued shut. None of those are good. If the sump pit is part of the suction system, I make sure it doesn’t interfere with drainage and that the suction reaches where it needs to. See our guide to sump pumps.

Crawl spaces

The EPA calls sub-membrane suction the most effective method for crawl spaces: a sheet of plastic covers the soil, and a pipe and fan pull the gas from under it.

For a crawl space that will get maintenance visits, I use a heavy-duty membrane, 12 to 20 mil. Thinner sheeting can meet the minimum, but with rocks in the soil, a lot of foot traffic or piers, I’d go thicker. The soil has to be prepped, the seams overlapped and sealed, the piers and walls sealed, and a suction point connected under the membrane. No soil can be left open to the air.

If you already have an encapsulation liner, I check its seams and openings to see whether it can work for a radon system. Some can be used; others need sections pulled up and redone. You can keep running your dehumidifier for moisture, but it won’t take the place of suction under the membrane. See also crawl space encapsulation and crawl space vapor barriers.

1. Membrane over the soil, seams overlapped and sealed
2. Membrane sealed to the foundation walls
3. Membrane sealed around piers
4. Suction point under the membrane
5. Sealed pipe
6. Fan outside the living space

SoilConcrete / masonryWoodSoil gas with radon

How many suction points does a house need?

I look at any plans, the type of slab and the foundation sections to see where a pipe could go. If I’m unsure, I do a communication test under the slab: I make a small test hole, use a device that pulls suction, and check other test holes to see how far it travels.

With a good layer of gravel under the slab, one suction point may be enough. With packed clay, interior foundation walls or slabs that don’t connect, you may need more than one. The communication test and the airflow resistance decide how many suction points you need and which fan to use, not a formula like “one per X square feet.” A bigger fan won’t help if the suction doesn’t reach.

Sealing cracks alone doesn’t work

Sealing is part of the job, but the EPA doesn’t recommend sealing on its own: it hasn’t been shown to lower radon much or consistently. A high level usually needs the pressure under the floor controlled and the gas pulled out of the soil.

What a good install looks like

Use this list to check a quote or a system that’s already there:

• The fan is outside the living space: attic, garage or outdoors (EPA).
• The discharge is above the roof, at least 10 feet above the ground, and at least 10 feet from windows, doors and other openings unless it’s at least 2 feet above them (EPA).
• The pipe has no low spots where condensation can sit.
• Joints and floor openings are sealed.
• There’s a warning device you can easily see or hear.
• A post-installation test is part of the plan.

There are five things I don’t like to see: a discharge that’s too low or near an opening, a fan inside the living space, leaky joints, pipe sections where water collects, and no warning device on the system. Number six, more common than you’d think, is not testing after installation. A system can look perfect and still not control the whole slab. Testing tells you. To keep it working, see radon fan replacement and system checks.

Radon mitigation in a finished basement

Inside or outside, the pipe route doesn’t matter to me as long as it’s done right and it works. In a finished basement I like to come out through a utility area and run the pipe up the outside of the house, because opening finished walls gets expensive.

If the pipe has to run inside, I look for a chase or utility space first; I’d rather not drill through floors or touch the structure. The fan has to go in an allowed location outside the living space. Planning to finish your basement? Test first, and plan the pipe route before the walls go up.

Can you do radon mitigation yourself?

A homeowner can run a test properly, keep the sump pit area clear, check the crawl space membrane and keep an eye on the gauge of an existing system.

I wouldn’t design the suction yourself, drill into a slab you don’t know, or do the electrical and discharge work. Even if someone assembles the pipes, they need to show the suction reaches where it should and test after installation. Many states also have rules for professional mitigation: Virginia requires national certification, and Pennsylvania and Ohio certify or license mitigators themselves.

Two things are worth knowing before anyone drills, from our structural contributor, Daniel Mercer:

• Post-tensioned slabs. Some slabs are reinforced with steel cables under high tension. Daniel wouldn’t drill a slab where there’s any sign of post-tensioning. Find the construction documents and have the cables located with a proper scan by qualified people; a pipe locator won’t do it. If the spot isn’t safe, the suction point moves. There’s no benefit to lowering radon by cutting a cable.
• Framing. Trusses, beams and engineered wood shouldn’t be cut because “the pipe is in the way.” Holes in solid joists have limits on size and distance from the edges and supports; engineered lumber follows the manufacturer’s instructions. A 4-inch pipe can force a different route. If the pipe passes through a fire-rated wall or ceiling, its protection has to be restored.

A small suction pit under the slab normally isn’t structural excavation. But near a footing, a pier or a wall, you need to know where the slab ends and the soil that supports the house begins. Daniel’s rule: don’t undermine a footing to get better airflow. If you find washed-out soil, voids or signs of settlement, stop and find another spot. Radon can be fixed without taking support away from the house.

Buying a house with a radon mitigation system

Ask the seller for the original test report, the installation paperwork and the post-installation test, plus when the fan went in and who did the work. Check the gauge and the outside discharge, and test the house again. A fan that’s running doesn’t tell you the radon level inside. For prices, see radon mitigation cost.

Radon-resistant new construction

Some new homes are built with radon-resistant features: a gravel layer under the slab, plastic sheeting over it, sealed openings, and a vent pipe from under the slab to the roof, ready for a fan (EPA). Whether a builder has to include them depends on where you live. In Virginia, for example, counties and cities in the EPA’s highest radon zone can adopt the radon appendix of the International Residential Code, and some have. Either way, the EPA says every new home should be tested after you move in. If the result is 4 pCi/L or higher, a fan can be added to the existing pipe.

If you’re having foundation work done, such as an interior drain or a new slab, plan the radon system with that work. Daniel would rather install the radon system around it than have new pipe cut two weeks later. See foundation repairs.