I get asked this a lot, as all of a sudden it seems radon readings are higher than we are used to. I admit I could only refer everyone to the EPA website for a "non-answer".
But then I inspected a house in Brewster that was admittedly a fairly unique setup but none the less completely explained - visually - the answer.
Now this gets into 'my brain hurts' territory, but here goes
Its due to a principle caused the 'Stack Effect'. Everyone knows that hot air rises and that cool air sinks, thats why we insulate our attics to stop such a thing.
So pretend you're underground, where the temperature is a fairly constant 55 degrees. In Summer the air temperature is much higher - 75 or so. There is no movement due to the stack effect upwards as there is no 'pressure' to make it happen. But bring on Winter, and yes, we have pressure. This pressure creates convection currents that move radon and other gases naturally tending to rise at a much more concentrated pace.
So that's the science, and now for the visual. The residence in Brewster had a modern addition off an existing basement that was only a crawl space. But they had done it right and covered the ground witha nice thick poly but had also spray foamed the crawl space foundation walls so the plastic was sealed. So watch the video and see the plastic was pressurised by the stack effect. My sources tell me that in Summer, the reverse should happen and the plastic should 'stick' to the ground as the stack effect will be operating in reverse as the warm air migrates to the cool ground