The radon could also go up to the first floor of your house, especially with closed windows. We had a finished basement, and bought a radon detector (radoneye), we found out we had over 1800 Becquerel/m3 downstairs and about 1500 upstairs. It quickly goes away when we opened the windows but also comes back completely within days.

It’s a huge amount so we got the fan installed and we are now at 15 and 0. It costs us about $1500 to install.

For those who get the system installed, please keep in mind that depending on how your house is built, you may need to extract the gas from the two halves of the basement if you have a slab right in the middle used as some sort of foot. You’d have to check the blueprint to know. The installer might have an idea too depending on the construction year of the house.