You can’t seal radon out
Many folks falsely believe that if their foundation has no cracks in it, then radon cannot get in. Others, falsely assume that “radon paints” or other floor seals, carpeting, etc. will keep radon out. This is not true. Radon is actively pulled into homes on a daily basis. It’s not simply leaking in through gravity or other means. There is almost always a pressure differential between outside air and inside home air. Typically, hot air leaks out the roofline and upper levels of a home while replacement air comes in from lower levels, most often the foundation and subfloors. This airflow pattern is so pronounced that the phenomenon has a name in the building world: the “Stack Effect”. Read more on the stack effect here. This is why traditional radon systems, also called depressurization systems, need to be installed. Radon needs it’s own path so it never gets in the home in the first place.
How to measure permeability
In vapor barriers and other crawlspace encapsulation materials, there are several ways that permeability is measured. Tensile strength and puncture resistance are often important because rarely does a barrier get installed without some splits or holes. However, the most common way permeability is measured is with perms. Perms measure how easily water vapor can pass through a material.
Concrete has a typical perm rating of about 3.2 perms per inch; however it can vary wildly based on the mix, cure, and other factors. We can assume a 4″ slab will have a perm rating around 0.8 perms.
Class A vapor barriers need to meet ASTM E1745 requirements which means the permeability needs to be below 0.1 perms. As was stated above, even in homes with class A vapor barriers under the concrete, radon finds a way in; often through plumbing penetrations and cracks. A simple radon test in the winter months will alert you to high radon levels. Smoke testing or diagnostics with an EcoTracker can help you find the source of the radon. Again, a sub-slab or sub-soil depressurization system is the most effective (both practically and in cost) way to reduce your radon.
Water versus Radon
A water molecule is about 275 pm (picometers) across while a radon atom is 202 pm. Water commonly soaks through concrete via gravity. It is safe to assume radon can diffuse through it as well, as it has a smaller diameter. It can absolutely make it through with a high enough pressure differential.
Side effects of a radon system
Barriers try to block water. They try to encapsulate it. Radon systems seek to neutralize a pressure differential and actively exhaust radon gas. In doing so, a properly designed radon system also removes water vapor. Engineers estimate radon fans can move up to 15 gallons of water vapor a day! That’s water that your barriers do not have to try to resist. Water that cannot rot out building materials or help mold grow. Ask any builder or engineer, then less water under your home, the better.
Don’t try to block radon out, it’s a fools errand. Redirect it with a conventional radon mitigation system and enjoy all the benefits of clean air.
Sources:
https://www.uaf.edu/ces/publications/database/energy/permeability-water-vapor.php
https://buildingscience.com/documents/information-sheets/info-312-vapor-permeance-some-materials
https://www.stegoindustries.com/blog/stego-iq-below-slab-vapor-barriers-what-does-a-perm-rating-mean
https://store.astm.org/e1745-17.html
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