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Radon Mitigation in a Leaky House

A reader weighs the pros and cons of various ventilation options as a way of lowering the health risks from radon

Posted on Nov 23 2015 by Scott Gibson

RadonColorless, odorless, short-lived radioactive gas that can seep into homes and result in lung cancer risk. Radon and its decay products emit cancer-causing alpha, beta, and gamma particles. mitigation in new construction is now routine when testing finds that concentrations of this odorless, cancer-causing gas exceed government-recommended levels. Writing from southeastern Wisconsin, Andrew S. has a slightly different problem: How to control radon levels when you live in a leaky log home built in the 19th century.

"Our radon issue is being worked on via traditional methods with mixed success," he writes in a Q&A post at GreenBuildingAdvisor. "The radon issue improves dramatically when windows are open — I assume this is dilution and perhaps also pressure equalization?"

That may be fine in warmer weather, but leaving windows open during the winter in Climate Zone 6 to keep the concentration of radon under control doesn't seem like much of a solution.

Andrew wonders whether other forms of ventilation might help. He weighs two possible options:

  • Positive-pressure ventilation, possibly by means of a dehumidifier that draws outside air into the house. "Might the positive pressure help reduce radon soil draw and simultaneously dilute the radon issue?" he asks.
  • An energy-recovery ventilator. "Does a balanced system ever make sense in a leaky home?" Andrew wonders. "The house also has some negative pressure devices (bath fans, boiler, wood stove insert). The goal here might be simply to dilute bad air whereas a positive pressure system may dilute but also prevent tendency to draw bad air in."

Andrew knew about the radon problem when he bought the house, but he was assured by a well-recommended radon specialist in the area that controlling it would not be a problem. When the sale went through, that expert came to look at the house — and never came back. Other contractors have looked but never followed through with a plan. Although Andrew has found a contractor who's willing to work on the problem, he still has his doubts that a traditional approach will work.

So what will?

That's the topic for this Q&A Spotlight.

What's been tried so far

There are essentially five "basement systems" in the house, Andrew explains, including two basement foundations, two dirt crawl spaces, and a slab-on-grade room. Here's what they've done:

  • Sealed the dirt floors in the crawl spaces ("one quite well, one fairly well").
  • Sealed a large, open sump pit.
  • Sealed various cracks in the basement floor.
  • Installed an active mitigation system connected to the sump pit, drain tile, and one of the basement foundations, plus the two crawl spaces.
  • Installed a separate active radon system for the room built on a slab.

The U.S. Environmental Protection Agency sets the "action level" for radon at 4.0 picoCuries per liter (pCi/LAbbreviated pCi/L, this term refers to the relative radioactivity contributed by radon gas to one liter (1,000 cc's) of air. A picocurie is one-millionth of a curie and represents about 2 radioactive particle disintegrations per minute. EPA has established an action limit (the level at which some form of radon mitigation should take place) of 4 pCi/L.), and when the door to the room built on a slab was closed radon levels there would approach 20-30 pCi/L, Andrew writes, although the mitigation system seems to be working "quite well." For the basement foundations, readings fall in the 5-12 pCi/L range, although the onset of cold-weather dynamics makes Andrew wonder how far they might climb.

"With regard to ventilation," he adds, "I just noticed that when I open some windows, the readings seem to dramatically improve. I was trying to determine how much ventilation equates to ‘some windows open’ and how viable that would be to replicate via mechanical ventilation. I don't love the energy penalty, but I also don't mind it if it brings the problem down to manageable levels. I am more concerned about introducing a new problem — mold, etc.

"I feel that with traditional methods and ventilation we would likely be OK," Andrew continues. "I just can't determine the best approach regarding ventilation so as to solve this problem without introducing others. I'm also wondering how much benefit we might get from reducing the negative pressure systems in the house — boiler, bath fans, dryer, wood burning stove, etc."

Effects of ventilation

Mechanical ventilation will help, GBAGreenBuildingAdvisor.com senior editor Martin Holladay replies, adding, "This is an expensive way to lower radon levels, however, because of the energy penalty associated with mechanical ventilation."

One theoretical drawback to pressurizing a house with outside air is the risk of introducing moisture into wall and ceiling assemblies during the winter, Holladay adds. "At normal ventilation rates (60 cfm to 120 cfm), this risk is very low, because wind and the stack effectAlso referred to as the chimney effect, this is one of three primary forces that drives air leakage in buildings. When warm air is in a column (such as a building), its buoyancy pulls colder air in low in buildings as the buoyant air exerts pressure to escape out the top. The pressure of stack effect is proportional to the height of the column of air and the temperature difference between the air in the column and ambient air. Stack effect is much stronger in cold climates during the heating season than in hot climates during the cooling season. usually overwhelm ventilation at these low rates," he says. "If you anticipate higher rates of ventilation, though, you might need to consider this risk."

Holladay is not aware of any studies that compare the impact of different types of ventilation (supply, exhaust, or balanced) on radon levels, but he guesses all of them would have comparable effects because they all work by dilution.

Charlie Sullivan adds, however, that he's heard anecdotal evidence that adjusting the balance of a heat-recovery ventilator "can have a dramatic effect on radon — in a tight house."

"The effect was presumably that with the exhaust exceeding supply, radon was sucked into the house through cracks in the foundation, despite this being passive-house or near-passive house construction," Sullivan writes. "With improved balancing, the radon level went down. I would expect based on logic and based on that one data point that the best to worst ventilation strategies for radon would be supply only, balanced and exhaust only. The effect would be smaller with a leaky house."

Sullivan makes reference to a house built to the Passivhaus standard where an out-of-balance ERVEnergy-recovery ventilator. The part of a balanced ventilation system that captures water vapor and heat from one airstream to condition another. In cold climates, water vapor captured from the outgoing airstream by ERVs can humidify incoming air. In hot-humid climates, ERVs can help maintain (but not reduce) the interior relative humidity as outside air is conditioned by the ERV. was indeed contributing to high radon levels when it expelled more air than it brought in.

That's interesting, Holladay replies, but it doesn't change his basic premise that in general, dilution usually lowers radon levels. "The only way to determine indoor radon levels is to measure them," he adds. "Tight houses as well as leaky houses can have radon problems."

Exhaust-only ventilation may actually help

The notion that an exhaust-only ventilation system would make radon worse seems reasonable. If the house is depressurized, wouldn't it make sense that radon would be drawn in via cracks in the foundation?

It sounds like a logical premise, Holladay says, but there is evidence that exhaust-only ventilation can lower radon levels. Holladay refers Andrew to an article he wrote on the topic last year ("Exhaust-Only Ventilation Systems and Radon").

"You should go ahead and install an HRV(HRV). Balanced ventilation system in which most of the heat from outgoing exhaust air is transferred to incoming fresh air via an air-to-air heat exchanger; a similar device, an energy-recovery ventilator, also transfers water vapor. HRVs recover 50% to 80% of the heat in exhausted air. In hot climates, the function is reversed so that the cooler inside air reduces the temperature of the incoming hot air. (not an ERV) with dedicated ventilation ducts," Holladay says. "Running ducts in an old house can be tricky, but even one supply register and one exhaust grille should provide some dilution of your radon levels. If you can manage two supply registers and two exhaust grilles, even better."

And don't, he adds, put in a whole-house dehumidifier. They are "notorious energy hogs."

Given the difficulty of installing ductwork in Andrew's house, Sullivan suggests that he look at a Lunos through-the-wall HRV. Andrew has seen the Lunos and finds it a "really neat system," although boring through log walls up to 18 inches thick to install one would be no piece of cake.

Our expert's opinion

GBA Technical Director Peter Yost adds this:

I am going to use the approximately 15 years of radon measurements and strategies from my own home to drive my recommendations on this one.

When we moved into our 100-year-old, approximately 1,800-square-foot, home in 2000, I tested the radon in our full basement at about 6 pCi/L and in the conditioned spaceInsulated, air-sealed part of a building that is actively heated and/or cooled for occupant comfort. above at about 3. These measurements were done with alpha-tracker test kits provided by the state of Vermont.

Subsequently, I insulated and air-sealed the vented attic (but later learned that my air sealing was pretty poor) first, then insulated and air-sealed the basement. Radon levels in the basement rose to 12 pCi/L; I did not test the conditioned space above at this point.

I was doing enough retrofitting and was concerned enough about radon levels in different parts of our home that I purchased a continuous electronic radon tester, Safety Siren Series 3. I have subsequently compared its measurement to alpha-tracker tests with no more than 0.1 – 0.2 pCi/L variation (although I have heard that over time they can really go wonky). And I have done enough short-term (three day running average) and long-term testing (cumulative average for up to six months) to have a good sense that the device is still reasonably accurate (and I now have two of the SSS3s).

Since the basement at the time was not living space, my main goal was to make sure that the high readings in the basement did not translate into higher concentrations in the living spaces above. So, as I was insulating and air sealing the front porch, built over an unvented crawl space, into a home office, I pulled a concrete block from the common wall between the office crawl space and full basement and then installed an ECM Fantech exhaust fan in the crawlspace to depressurize the basement and crawl space (see Image #4, below). The result was the basement radon levels went up to 13 pCi/L and the living space above went to 2.1 pCi/L.

I had shifted the neutral pressure plane of building such that we were keeping the radon mostly below grade. And because the Fantech exhaust was a variable-speed fan, I could “dial in” just how much negative I wanted to pull the basement, even checking it when the atmospherically vented boiler was running (making sure I was not creating back-drafting or exhaust spillage from the boiler). Problem solved…

But some years later, our kids wanted the basement to becoming a rec room — is that a living space? Shoot. I ended up decommissioning the crawl space exhaust and had a Fantech HRV installed to service just the basement as our next radon control system. The HRV has three speeds: 100 cfm (100 watts); 150 cfm (150 watts); and 200 cfm (200 watts). For shoulder seasons I seem to be able to keep the living space at 4 pCi/L or less at low or medium speed but in the winter, I have to run the HRV at full speed to keep at or below the EPA action threshold. It’s that bloody stack effectAlso referred to as the chimney effect, this is one of three primary forces that drives air leakage in buildings. When warm air is in a column (such as a building), its buoyancy pulls colder air in low in buildings as the buoyant air exerts pressure to escape out the top. The pressure of stack effect is proportional to the height of the column of air and the temperature difference between the air in the column and ambient air. Stack effect is much stronger in cold climates during the heating season than in hot climates during the cooling season. in the winter that seems to pull more radon in to the house.

I have tried more than once to seal basement floor cracks and wall joints to little effect on radon levels throughout the house. And with a four-gable gambrelThis is a gable roof with two pitches, the bottom pitch being steeper than the top. The term gambrel is also used to describe the hing leg of a horse, with a angle at the joint that looks like a gambrel roof, or much more likely, the other way around., and room-by-room renovation, I just can’t seem to get the second-floor ceiling plane tight enough to really stymie the stack effect in the winter.

In the summer, our six basement hopper windows are pretty much open except during short stints of really high humidity outdoors, and that pulls the radon levels throughout the house well below 2 pCi/L.

So, lessons learned:

  • We really don’t know much about why there is no consistent relationship between radon levels and airtightness. You still need to measure and then know whether to mitigate.
  • You can use pressure regimes to manage radon floor-to-floor. Just be careful if you have atmospherically vented appliances in the basement.
  • Dilution can work, but as Martin points out, not without a pretty significant energy penalty.
  • You can have two homes sitting right next to each other in a high-risk zone (see the map shown in Image #3 below) and which one, if either, ends up with actionable levels of radon inside the home is anyone’s guess.
  • Evaluating radon and patterns over time and in different locations can be done pretty well with a continuous electronic monitor. But it’s good practice to check these periodically against a 3-month, closed-up-for-the-winter alpha tracker test.
  • If you are using a device such as an HRV for radon control, its regular inspection and maintenance is just that much more important for good indoor air quality.


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Image Credits:

  1. Images #1 and #2: Andrew S
  2. Image #3: Kansas Geological Survey
  3. Image #4: Peter Yost

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