Radon and a Passive House

Let me start out by stating that I am neither a radon expert, Passive House expert or a builder.  That said, I do have experience with radon gas in a Passive House that I'd like to share.

According to The Passive House Institute US FAQ, Passive Houses are by design, well-protected from radon.  They attribute the radon-preventing attributes of a Passive House to its rigorous air tightness metrics and balanced ventilation.  Our home is a certified Passive House that had a radon problem until a conventional radon mitigation system was installed.  I don't believe that achieving the Passive House standard did much to reduce the likelihood of our having a radon problem.

According to the EPA's A Citizens Guide to Radon, radon occurs naturally in soil and rocks in which home foundations are placed.  Radon can be found all over the country.  In our area, northwest CT, there is approximately a 40% chance of radon existing on the site of your home according to a local expert.

Radon gas enters a home through gaps in the structure (typically a foundation) that separates the below grade interior of a home from the earth that surrounds it.  The local expert we saw explained that even a tiny gap the size of a dime can lead to elevated levels of radon in a home.  The atmospheric pressure in the ground tends to be higher than the atmospheric pressure in a house's interior.  As a result, a basement can act as a sort of vacuum cleaner motor that sucks in radon in soil air through any of these small gaps.  Radon gas can then accumulate to levels that according to the EPA can lead to increased risks of lung cancer.

In houses without balanced ventilation this vacuum cleaner effect can be exacerbated by exhaust fans, dryer vents and mechanical heating and cooling systems.  As air is forced out of a house unconditioned air enters through gaps in the lower portions of the house.  Although a Passive House has a balanced ventilation system, the pressure in the soil around the foundation still tends to have higher pressure than the house's interior. I conclude this based on the fact that my house has a balanced ventilation system as well as a radon problem isolated in the basement.

A Passive House's balanced ventilation can also reduce the level of radon in a house by exhausting the air with elevated levels of radon from a basement and supplying fresh air with lower levels of radon found in outdoor air.  Doubling the rate of ventilation cuts the radon level in half.  In our house, I saw short term (7 day) radon levels range over the course of a year from 3 pCi/liter up to 12 pCi/liter using a Safety Siren Radon Gas Detector.  We would have had to quadruple the rate of ventilation in order to reduce the highest detected levels below 4 pCi/liter as recommended by the EPA.  This was not possible they way our ventilation system was designed.  So, while our continuous exhaust of basement air reduces the amount of radon in our basement, it doesn't reduce it nearly enough to mitigate our moderate radon problem.

A Passive House's balanced ventilation has two seemingly positive characteristics with respect to radon infiltration and exhaust which, at least in my case, fell short of preventing elevated indoor radon levels.  So, the question now is why don't the air-tightness requirements of a Passive House prevent enough soil air containing high levels of radon from entering the building?  Our house beat the Passive House standard by more than 35%.  It had an air-tightness measure of 0.38 ACH50 (air change per hour at 50 pascals) with a corresponding reading on the blower door test of about 180 cfm.  This means that under normal unpressurized conditions air leaks into our house at a rate of about 300 liters/minute.  I'll assume that the air entering our house through the balanced ventilation system has radon levels of 0.5 pCi.  Soil air in the US typically has radon levels of 200 to 2000 pCi.  I'll assume that our soil has a radon level of 750 pCi which is considered in the middle of  an average risk area (270-1350 pCi, according to A Living Radon Reference Manual) for radon.  If the source of air in our basement were a mixture of 99.5% 0.5 pCi air from the ventilation system and 0.5% 750 pCi soil air, then we'd see radon levels of 4 pCi in the basement.  Our basement contains about 300,000 liters of air.  0.5% of this is approximately 1500 liters.  If all the air leakage in our house occurred below grade with soil air, it would take about 5 minutes to leak 1500 liters of 750 pCi soil air.  If only1% of the leakage in our house occurred below grade it would take 8 hours.  The entire volume of air in my basement is exhausted out by the ventilation system every 16 hours, so 1500 liters leakage in 8 hours should be enough maintain radon levels at 4 pCi in the basement.  I don't know the actual soil air radon levels (although our well water tested at 2500 pCi and 5000 pCi, so 750 for the soil seems reasonable to me) or the percentage of our house's leakage occurring below grade, but nonetheless, I'm not at all comforted that the Passive House air-tightness requirements have any meaningful effect on preventing radon infiltration.  In fact, our house exceeded the Passive House standard on the initial blower door test under conditions that were ideal for radon infiltration, with a gravel basement floor prior to the concrete slab being installed. 

To prevent radon from infiltrating a house through air-tightness requires close to 100% effectiveness below grade.  Our house was designed and built with an elaborate air and vapor barrier surrounding the foundation (although based on our initial blower door test this barrier was not at all necessary to achieve the Passive House air-tightness standard).  I suspect that others who build Passive Houses have similar below grade air and vapor barriers.  The barrier was designed to be continuous with both penetrations (well and septic) sealed with spray foam.  My expectation was that this air and vapor barrier would keep all soil air with its high radon concentrations out of the house.  Unfortunately, it seems to have at least one dime sized gap as evidenced by the elevated radon levels in the basement.  Here is a photo album with pictures of the foundation and barrier for anyone interested.

Fortunately, we were able to prevent radon from entering through the small gaps in our air and vapor barrier through  a conventional radon mitigation system.  The conventional system was built by drilling an 8" diameter whole through are basement slab and removing about 5 gallons of the gravel found there.  A 4" PVC pipe was inserted into the hole and the hole was sealed back up.  The pipe travels out of the basement and into the adjoining garage and then out the garage wall.  A 20 watt fan was placed in the pipe and run continuously to create negative pressure underneath the slab.  Because the pressure under the slab is now less than the pressure indoors in the basement, there is no longer a vacuum effect and soil air is not sucked into the basement through existing gaps.  Fortunately, we were able to achieve the desired result with a small fan drawing only 20 watts to keep electricity usage to a minimum.

4" PVC pipe penetration through basement slab for conventional radon mitigation system

So, while a Passive House's balanced ventilation and air-tightness sound like they'd be helpful preventing elevated levels of indoor radon, they don't do enough to make a difference.  In particular, air leakage permitted by the Passive House standard can allow substantial amounts of radon to infiltrate a home.  I don't believe, unfortunately, that achieving the Passive House standard significantly reduces the risk of having a radon issue in your house.  On the positive side, if you have a Passive House with a radon issue, it can be addressed with a conventional radon mitigation system.