Radon in basements: testing, finished basements, bedrooms and fixing it

Basements sit in direct contact with the soil, which is where radon comes from. They usually have the highest radon levels in a house, and they're increasingly used as bedrooms, offices, playrooms, and gyms. If you have a basement, especially one you use or plan to finish, radon testing should be part of your plans. This guide covers testing, finishing, and the mitigation options EPA describes for basement homes.

Why basements run higher

EPA explains that the air pressure inside a home is usually lower than the pressure in the soil around the foundation, so the house draws soil gas in through cracks and openings. In a basement, that contact area is large: the entire slab plus the below-grade walls. Common entry points include:

• cracks in the slab and walls;
• the joint where the floor meets the walls;
• gaps around pipes, drains, and wiring;
• open sump pits;
• the cores of hollow concrete block walls;
• floor drains connected to soil.

Testing a basement correctly

EPA recommends testing the lowest level of the home that's used, or could be used, as living space. For many homes, that's the basement.

• If you use the basement regularly, or might in the future, test there.
• Place the device in the main area, not in a laundry room, workshop, bathroom, closet, or near the sump.
• Put it about 20 inches or more above the floor, away from exterior walls and drafts.
• Use closed-house conditions for short-term tests.

EPA's retest guidance says that if your living patterns change and you begin occupying a lower level of your home, such as a basement, you should retest on that level. If you tested only the first floor years ago and have since added a basement bedroom, test again.

Finishing a basement: test first

The best time to deal with radon is before you finish a basement:

• Test before you start. If the level is high, install mitigation first.
• Plan the system route. A sub-slab suction pipe needs a path from the slab to the roof or exterior. It's much easier to plan this before walls and ceilings go up.
• Seal openings in the slab and walls while they're exposed.
• Leave access to the sump, the suction point, and the system's warning device. A manometer hidden behind drywall is useless.
• Retest after finishing. Changes to HVAC, new bathrooms, and new penetrations can affect radon levels.

A finished basement can be just as safe as any other room, as long as radon has been addressed.

Mitigation methods for basements

EPA's Consumer's Guide to Radon Reduction lists several soil suction techniques for basement and slab homes. Its comparison table gives each a typical reduction range of 50 to 99 percent:

• Sub-slab suction (sub-slab depressurization). A pipe is inserted through the slab into the soil or gravel beneath. A fan draws radon from under the slab and vents it outdoors. EPA notes that often only a single suction point is needed, depending on how easily air moves under the slab and how strong the radon source is. This is the most common method.
• Drain tile suction. If the house has perforated drain tile around or under the foundation, suction can be applied to it. EPA notes this can work with partial or complete drain tile loops.
• Sump-hole suction. If the basement has a sump pit, the sump can be capped and used as a suction point. EPA says this works best if air moves easily to the sump from under the slab.
• Block-wall suction. In basements with hollow block foundation walls, EPA describes depressurizing the voids in the wall. It requires sealing major openings, and EPA notes it's often used together with sub-slab suction.

Fans should be located outside the living space, in unconditioned areas such as attics, garages, or the home's exterior, as EPA describes. The discharge goes above the roof.

Pressurization: a niche option

EPA also describes basement pressurization, which uses a fan to blow air into the basement to keep soil gas out. Its table lists 50 to 99 percent reduction, but notes it works best with a tight basement isolated from outdoors and upper floors. It can bring in outdoor air with moisture and energy penalties, so EPA suggests it be considered only after more common techniques haven't worked.

Moisture and radon

Basements with moisture problems often have the same entry routes that let radon in. A sub-slab suction system can also reduce moisture and soil odors in many basements. That's a nice side benefit, but don't count on it as a waterproofing system. Fix drainage and grading problems separately.

After mitigation

• EPA advises a post-mitigation test within 30 days of installation, but no sooner than 24 hours after the system starts.
• Check the manometer regularly. A change in the reading can signal a problem.
• EPA suggests retesting at least every two years.
• The fan must run continuously. EPA notes fans may last five years or more, and manufacturer warranties tend not to exceed five years.

Basement bedrooms and the risk numbers

Bedrooms matter because people spend many hours there. EPA's risk tables estimate that at 4 pCi/L, about 7 of 1,000 never-smokers, and about 62 of 1,000 smokers, could get lung cancer from a lifetime of exposure. If a basement bedroom tests at or above 4 pCi/L, fix it. If it's between 2 and 4 pCi/L, consider fixing it, especially if the person sleeping there smokes or spends a lot of time at home.

Quick checklist

• Test the basement if it's used, or could be.
• Mitigate before finishing if the level is high.
• Keep the sump, suction point, and manometer accessible.
• Retest after finishing and every two years.
• Never turn the fan off.

A basement that has been tested and, if needed, mitigated is a basement you can use with confidence.