Should I worry about radon before finishing my basement in Minnesota?

What is radon, and why does Minnesota have so much of it?

Radon is a radioactive gas you cannot see, smell or taste. It comes from the natural decay of uranium found in nearly all soils, according to the EPA's A Citizen's Guide to Radon. It moves up through the ground and into a house through cracks in the slab, construction joints, gaps around pipes, wall cavities and openings like an unsealed sump basket.

Minnesota is a high-radon state. The Minnesota Department of Health (MDH) says more than two in five Minnesota homes have radon levels that pose a significant health risk, and that the average radon level in Minnesota is more than three times the U.S. average. MDH points to two causes: the state's geology and its cold climate. In winter, a heated house acts like a chimney and pulls soil gas in through the foundation, and Minnesota homes stay closed up and heated for a large part of the year.

The statewide number comes from real test results. When MDH released its interactive testing map, it reported that 44 percent of tested Minnesota properties came in at or above 4.0 pCi/L, while only about 1 percent of properties are tested each year. That map shows testing rates down to the census tract, roughly neighborhood-sized areas inside each county, through the MDH radon data portal.

Why your county map is not your answer

People look up their county and decide they are safe. MDH warns against that: any home can have high radon, old or new, tight or drafty, with or without a basement, and neighboring houses can differ widely. The county map tells you how many people near you have tested and what they found. It cannot tell you what is under your slab. Only a test in your house does that.

How dangerous is radon?

Radon is the reason this topic belongs in a basement design guide and not a footnote. The EPA estimates about 21,000 lung cancer deaths a year in the U.S. are radon-related, and the Surgeon General has identified radon as the second leading cause of lung cancer after smoking. MDH adds that radon is the leading cause of lung cancer in people who have never smoked, and that smoking plus radon is far worse than either alone.

MDH puts the risk at the action level into numbers. Its estimates for a lifetime of exposure at 4 pCi/L:

There is no safe level. MDH says any radon level carries some risk, so the goal is to get it as low as you reasonably can.

What is the 4 pCi/L action level, and what does MDH recommend?

Radon is measured in picocuries per liter of air (pCi/L). The EPA action level is 4 pCi/L. MDH recommends a mitigation system at 4 pCi/L or higher and says one can be considered between 2 and 4 pCi/L. MDH makes that decision on either the average of two short-term tests or one long-term test, not a single short-term reading. A well-built system, MDH notes, can often bring year-round levels below 2 pCi/L.

MDH also tells you what to do after a first short-term test: under 2 pCi/L, retest every 2 to 5 years; 2 to 7.9 pCi/L, follow up with a long-term test; 8 pCi/L or higher, follow up quickly with a second short-term test (a long-term test is also an option).

How do you test a basement for radon?

Where to put the test

MDH's placement rules matter more than most people think, because a test in the wrong spot gives you a wrong number. Test the lowest level you regularly use; if you spend more than 10 hours a week in the basement, test there. Then:

• Set the kit at least 20 inches off the floor, ideally at breathing height.
• Keep it at least 1 foot from exterior walls and 3 feet from exterior windows.
• Keep it away from drafts, humid rooms (bathroom, laundry, kitchen) and heat sources like the furnace or a fireplace.
• For any test under 3 months, keep windows and exterior doors closed except for normal coming and going, and run heating and cooling as you normally would.
• Skip short-term tests during severe storms or unusually windy weather. The heating season is the best time to run one.

In the reference basement, I put the test in the middle of what will become the family room, on a shelf or stool at seated breathing height, well away from the sump corner and the furnace room. Not on the floor next to the sump, and not in the mechanical room, which nobody lives in.

Test before and after finishing

MDH specifically says to test before and after finishing a basement, and also after energy upgrades, additions, a new kitchen hood, or changes to heating or central air. The EPA guide says the same: test the area before a basement renovation begins, and test again when the work is complete. I treat these as two separate jobs with two different purposes:

• Before: this tells you whether to mitigate before the walls go up. It is far easier to cut a suction point and route a pipe through open framing than through a finished room.
• After: finishing changes how air moves through the house. New ductwork, a new bathroom fan and a tighter envelope can all shift the number. This test confirms where you landed.

After that, MDH recommends retesting every 2 to 5 years. If you already have a mitigation system, retest at least 24 hours after it starts running to confirm it works, and MDH's free inspection is available for systems installed after June 2020.

Do you need a licensed tester?

Not to test your own house. Under the Minnesota Radon Licensing Act (Minn. Stat. 144.4961, signed in May 2015), anyone who places or retrieves radon test devices in a building they do not own or lease must be licensed by MDH, but you do not need a license to test a home you own. If you hire someone, check their license in MDH's search and ask what device they use; for a continuous monitor, MDH suggests asking whether it was calibrated within the past year.

What does finishing a basement change about radon?

Two things. First, exposure. An unfinished basement where you do laundry is a place you pass through. A finished family room, office or guest bedroom is where someone spends evenings or sleeps eight hours. Same radon level, many more hours breathing it. That is exactly why MDH ties the testing location to the space you use more than 10 hours a week.

Second, the false sense of a sealed room. Drywall, paint, a vapor retarder on the walls and new flooring make a basement feel tight. None of them is a radon fix. Radon is pulled in by pressure differences between the soil and the house, and finishes do not change that pressure. The fix that MDH and the EPA both describe is soil suction: a pipe and fan that pull gas from under the slab and send it outside. Both agencies treat sealing cracks and openings as a supporting step that makes a suction system work better, not as a fix on its own.

Covering a slab with flooring can also make it harder to reach cracks and joints later. If your test says you need a system, decide before the floor goes down.

How does a radon mitigation system work in a finished basement?

The standard system in a Twin Cities basement is sub-slab depressurization, which MDH calls sub-slab suction. Here is each part, and what it means for your design.

The suction point

A licensed mitigator drills through the slab and digs a small suction pit in the soil or gravel below. The radon pipe connects there. In many Minnesota homes, the system can instead tie into the drain tile through a sealed sump basket, which MDH calls drain tile suction. Either way, the suction point needs to sit somewhere you can live with a pipe coming out of the floor: the mechanical room, a storage room, the back of a closet or the sump corner.

The pipe

The pipe is PVC and carries the gas outdoors. MDH says it can run inside or outside the house. Inside, it usually rises through a closet or chase on the basement level and continues up through the house to the attic. Outside, it goes through the rim joist and runs up the exterior wall. MDH requires the exhaust to be at least 10 feet above the ground and above the roofline, and at least 10 feet horizontally from windows, doors and other openings, unless it discharges at least 2 feet above them.

The fan

This is the rule that shapes your floor plan. Per MDH, the fan must be in an unconditioned space, such as an attic, a garage or outdoors, and cannot be located in or below any livable space. The reason is leaks: the pipe after the fan is under positive pressure, so a leak there pushes radon back into the house. So the fan will never be in your finished basement, and it should not be in the mechanical room under the kitchen either. It plugs into a junction box or is hardwired.

The manometer and the tag

Every system has a way to show you it is working. MDH describes a U-tube manometer, a small gauge on the pipe with colored liquid that shows whether the fan is pulling suction. Some systems add an alarm that sounds if the fan fails. After installation, the licensed professional attaches a radon system tag listing the installer, phone number, install date and license number; Minnesota requires these tags on professionally installed systems (Minn. R. 4620.7600). MDH also says the system must be clearly labeled so no one alters it by accident.

Who can install it

In Minnesota, mitigation in a home you do not own must be done by an MDH-licensed mitigation professional working for a licensed mitigation company, and MDH says those individuals are required to be licensed. MDH's work-practice rules adopt the AARST Soil Gas Mitigation Standards for Existing Homes (SGM-SF-2023). MDH's advice when hiring: get a site visit before a bid, ask for the individual license number, ask whether electrical work is needed and who does it, and get a passing post-mitigation test before final payment.

Pricing for mitigation depends on the house and is quoted by the licensed mitigator after a site visit. For what the rest of the basement costs, see basement finishing cost in Minneapolis.

How do you design a finished basement around an existing radon system?

Many reference-age basements already have a system, either retrofitted by a previous owner or built in. Finishing around it is easy if you follow a few rules.

• Box the pipe in a chase, do not bury it. Frame a small chase around a vertical pipe in a corner or alongside a wall, and finish it to match. Leave an access panel at the manometer and at any joint near the floor. Do not reroute, cut or reconnect the pipe yourself. Any change to the system is mitigation work and belongs to a licensed mitigator.
• Keep the manometer visible. A gauge behind drywall is a gauge no one reads. Either leave the manometer in an unfinished room or put a clear cover or access door in the chase at eye level. If the system has an alarm, keep it where you will hear it.
• Keep the tag and labels visible. The radon system tag and the system labels exist so the next contractor or buyer knows what the pipe is. I place them on the exposed section or inside the access panel, never painted over.
• Keep the sump lid sealed. If the system pulls through the sump, the lid is part of the system. An open or cracked lid lets the fan pull house air instead of soil gas. Plan the sump corner so the lid stays accessible for pump service but sealed the rest of the time. See the sump pump guide for how I lay out that corner.
• Seal new penetrations. A new bathroom drain, an ejector basin or a floor outlet cut into the slab is a new opening. Each one should be sealed.
• Retest when you are done. New ductwork, a new bath fan and a tighter room can all change performance. The post-finishing test confirms the system still keeps up.

If you are finishing and do not have a system yet but your test says you need one, put it in before framing. The suction point, pipe route and chase are just a few more lines on the plan at that point. This fits in the early steps of the basement finishing process, right after moisture checks and before framing.

What are radon-resistant construction details?

Since 2009, MDH says, all new Minnesota homes must be built radon-resistant. The building code sections (Minn. R. 1303.2400 to 1303.2402) and MDH describe a passive system with these parts:

For the reference home, this splits by age. A house built in Lakeville or Shakopee after 2009 should already have a labeled passive vent pipe, usually near the sump or the mechanical room. A 1990s house in Plymouth or Maple Grove probably does not, unless someone added a system later. A passive pipe is not a guarantee. MDH says that if a passive system tests high, it can be converted to an active system by adding a fan, which is usually the simplest upgrade there is.

Check with your city: radon-resistant requirements apply to new construction through the building code. Finishing an existing basement is a permitted remodel, and inspectors in different cities treat existing radon pipes and new slab penetrations differently. See basement finishing permits in Minnesota.

What does Minnesota's radon disclosure law require when you sell?

Under the Minnesota Radon Awareness Act (Minn. Stat. 144.496), effective January 1, 2014, a seller must give the buyer written radon disclosure before signing a purchase agreement for residential property. The disclosure covers:

• whether any radon test has been done,
• the most recent test records and reports,
• any known radon levels, mitigation or remediation,
• a description and documentation of any installed mitigation system,
• a required radon warning statement, and
• a copy of the MDH publication Radon in Real Estate Transactions.

The law does not require anyone to test or mitigate. But a seller who knows material radon facts and does not disclose them can be sued by the buyer, generally within two years of closing. For your basement project, the practical takeaway is to keep your before and after test reports and the mitigation paperwork with your permit file. When you sell, a finished basement with a documented system and a recent low test is a much easier story than one with no records.

How this plays out in a typical Twin Cities basement

Take the reference home: a 2003 two-story in Prior Lake, poured walls, about 1,100 square feet below grade, sump in the back corner, furnace and water heater in the opposite corner. The owners want a family room, a bedroom with an egress window and a three-quarter bath.

• Before design: they run a short-term test in January in the middle of the future family room, closed-house conditions, 20 inches up. It reads 6.1 pCi/L. Because that is between 2 and 7.9, MDH says follow up with a long-term test. If the schedule cannot wait 90 days, a second short-term test gives an average to decide on.
• Decision: the average is over 4, so a licensed mitigator designs a system before framing. The house was built before the 2009 requirement, so there is no passive pipe.
• Layout: the suction point goes in the unfinished mechanical room or ties into the sealed sump. The pipe exits through the rim joist and runs up the outside wall, or through the garage to an attic fan. Either way, the fan sits outside living space, as MDH requires.
• Finishing: the sump gets a sealed lid with an access plan; the manometer and tag stay in the mechanical room where they are visible; the new bath drain and ejector basin penetrations are sealed.
• After: once the basement is finished and the HVAC is rebalanced, they retest. The report goes in the house file with the permit and the system documentation.

The order is the whole lesson: test, decide, mitigate, then frame, finish and retest.

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