The following discussion is long and somewhat technical. It is probably only interesting to those who have or think they might have radon problems. If you would like to get a measure of how likely you are to have problems, visit the EPA map of radon zones. From there, you can go to a map of your state that shows radon risk by county.

Interesting Radon Facts

  • The record high reading for radon levels in a home was in Pennsylvania. The person living in it set off the radiation detectors upon starting a new job at a nuclear power plant.
  • Elements emit radiation as the atoms "decay" to become lighter elements. Radon gas atoms began as uranium, and ultimately become lead (yes, radon atoms are heavier than lead atoms).
  • Along the way to becoming lead, the atoms are polonium-210 for a while.
  • The national average for indoor radon is between 1 and 1.5pCi/L; in Gallatin County, the average is 7.5pCi/L.
  • A home with radon problems will continue to have problems for billions of years unless countermeasures are taken.
  • Alpha particles emitted by decaying radon cannot penetrate tissue paper, but the damage they do is greater than that of beta or gamma particles if the source is ingested or inhaled.

Phase II

Phase I was in the Carriage House, where we had low levels of radon and a yearly average well below 4.0pCi/L after a little work. We used our radon monitor to start our analysis of the Main House soon after our early September move. Upstairs had a reading over 6.0pCi/L and downstairs over 10.0pCi/L. Both of these are higher than recommended for continuous exposure in a living area. In the crawl space we got an alarming reading of over 46.0pCi/L.

The Carriage House and Main House are only 600' apart, so why is there such a large difference? One possible explanation is that the Carriage House required very little excavation and sits on several feet of gravel fill, while the Main House sits on a platform created by substantial cutting and filling of a rocky ridge, which may have exposed radon-emitting strata.

As with the Carriage House, we expected higher readings when Winter is in full swing, due to the stack effect extracting gases from the exposed soil in the crawl space and the frozen ground outside limiting other avenues of escape. Therefore, we took two immediate actions:
  1. We had a fan installed to pull air from under the west end slab and exhaust it outside using the pipes we had installed during construction. Total cost: about $1,000.
  2. We changed one of the air-to-air exchange units to take air from the crawl space and bring fresh air into the upstairs, thus creating negative pressure in the crawl space relative to the pressure in the living areas.

After these changes, our late October readings fell to about 2.0pCi/L upstairs and 4.1pCi/L in the downstairs "Family Room". However, the reading was over 8.0pCi/L in the downstairs "Exercise Room" located next to the crawl space. And, we were not yet into the time of year that should produce the worst readings.

Phase III

Convinced that further measures would be needed, we proceeded with the installation of plastic sheets to cover the bare areas in the crawl space. These sheets are sealed to the foundation and air is drawn from under them by the same system that exhausts air from under the slab. Cost: an additional $1,200. The picture below shows a portion of the crawl space; the white pipes are part of the exhaust system and yellow foam seals the sheets to the foundation and other penetrations.

Our countermeasures were installed at the end of November, when snow and cold weather had combined to freeze the ground. We waited 7 days (about two half-lives for radon) to give the system time to clear a bit before resuming measurements. The radon level in the Exercise Room was 10.1pCi/L – slightly higher than October. This implied one or more of several possibilities:
  1. The countermeasures were not effective,
  2. the cold weather had raised levels over 40.0pCi/L since our last measurement and they had only just decayed to a lower level [½×½×40 = 10] or
  3. the Exercise Room has problems unrelated to the countermeasures taken (for example, we know it has less fresh air coming in).

In order to get a baseline, we attempted to measure the radon levels of air being brought into the house by placing the monitor into the intake chamber of the the air exchange system. This is questionable practice: the monitor is not to be used in a direct air flow and there may be some leakage between incoming and outgoing air chambers. We got a reading of 2.8pCi/L, which seemed a bit high but still below the recommended levels. Then, we put the monitor into the open air of the crawl space and got a reading of 2.3pCi/L. By this time, 4 half-lives had elapsed so the reading was consistent with a decline related to decay of pre-existing radon coupled with the relatively higher air exchange in the crawl space. This suggests that the countermeasures were effective. It also suggests the reading of incoming air does not correctly reflect ambient levels outside (otherwise we would not expect a reading much lower than 2.8).

The next step was to verify that levels had also declined in the Exercise Room. About 5 half lives (19 days) after the installation of the plastic sheeting, the Exercise Room reading levelled out to 1.9pCi/L, consistent with possibility #2 above assuming that we are starting to reach a base level related to the ambient levels of incoming air.

Finally, there was one area of concern in the crawl space – the alcove where the water tank sits is filled with several feet of gravel in case the tank develops a leak. The area beside and behind the tank would be difficult to seal off with the plastic barrier, so we left it open in the hope that it would not be a problem. We placed the radon monitor over the exposed gravel and got a reading of 1.8pCi/L, which assured us that it is not a problem.


We solved our radon problem at the Main House with about $2,200 of mitigation technology. Our measurement techniques were a bit haphazard:
  1. We sometimes altered more than one thing between measurements.
  2. Outside weather conditions changed dramatically over the course of our measurements; controlling for this would require one or more years.
  3. We were monitoring multiple sites; it would have been better to run 3 or more monitors continuously, but this would be expensive (about $120 per monitor).
  4. We could have left our single monitor running continuously, but when levels exceed 4.0pCi/L it sounds a very annoying alarm heard throughout the house. For this reason, we often left it off for several days once a measurement had been taken.
Our priority was to determine the need for and effectiveness of mitigation technology rather than to conduct a truly scientific study. Now that levels should be constantly below 4.0pCi/L, we leave the monitor on to warn us if a problem develops in the mitigation system.