Causes of radon in the basement of a newer home?

Question

We are in the process of (maybe) buying a home. It is a newer (2007) home in Northern Wisconsin with an insulated concrete form (ICF) basement in red clay. We just got the radon test back and the 2 day average is at 5.1 pci/l. I am not concerned about the levels - mitigation devices will happen. My concern is why?

Is the presence of radon in a newer home an indicator of a not tight / poorly constructed basement or is it an indicator of a very tightly built basement/home?

I have seen some people arguing that it indicates the basement is not 'tight' / poor construction - if a basement is properly built radon should not be able to enter. However, there was no sign of water in it - and it was as dry a basement as I have seen. But, with ICF you can't see voids or cracks in the wall - but then again, no sign of moisture.

Others argue that because a home is tight it tends to trap more stuff and what does get in can't get out. This also seems plausible - there was no air exchanger installed that I saw and most newer homes seem to have them up here to control moisture. Also the sump/drain wasn't sealed so that could be a point of entry.

Answer 1

Concrete is Porous this is why we put vapor barriers down and foundation drains because if there is moisture it will permeate the concrete. Doesn’t that mean the foundation is not done correctly? No concrete is porous and if moisture can make it through it is much easier for a much smaller substance to make it through. Not only can it come just through the ground but if the well is close To the house this is a source that the radon has an easy exit compared to going through many feet of earth. If your home has an internal sump pump there is another entrance point. So radon in the home is not an indication of the quality of the foundation. Epoxy coating the floor can help as this helps to seal the floor but now the walls we can’t forget about the walls.

Answer 2

As I understand it, you cannot possibly "tighten" your basement enough to prevent radon from getting in, there is no way to attain a radon level of zero.

Answer 3

I don't think you can build a slab that can prevent radon from entering. The vapor barrier under the slab is what keeps the radon and other soil gasses out. It is really difficult to have perfect seams in the vapor barrier (there are wall seams, seams around post footings, etc). Typical modern requirements for areas known to have high radon levels are for a passive pipe the sits under the slab, extends through the slab out to the exterior of the building. In the case of high readings a fan can then be added to have active mitigation. Radon can also be in water and can come out of water.

Even a house build to passive house standard is going to have ACH50 leakage rates of .6 ACH50 / hr. A standard decent built house is closer to 3.5 ACH50. An old house is > 15 ACH50. I don't think a radon reading reveals much about the quality of a house. Even a house that meets the tightest standard is going to fully change the air weekly. Soil gas that builds up pressure (higher than exterior air) is going to get into the house. Even if the gas is not coming into the house from the basement, it can come in from the ambient exterior air and since it is heavier than air will settle into the lowest point of the house. Passive house requires .33 air changes per hour via ventilation to maintain a good AQI. Does your house have a whole house fan or HVAC? If those devices are in the lowest section of the house they should pull out the radon as combustion air when active.

What are your readings for the other floors in the house?