reasons for mounting the fan and exhaust piping outside building envelope in US Let me add a relevant experience.
I have several thousands of multiple branch active radon mitigation systems installed.
There were two documented failures by me of activated systems of passive systems that had originally been installed in the USA by others, where discharge of radon vent pipes were under soffits that also have (soffit) vents. The ultimate proof that this was the cause for high indoor radon levels was that after several other modifications that did not work and a pressure field that was complete from the beginning by moving the discharge to above the upper roof in one case and in the other case more than 10 feet away from the soffit vents solved the problem. I gave a talk about this at the San Diego AARST Symposium with an article that can be found on the AARST website.
Soffit vents at regular distance along all soffits are very common in the area of the USA where I live and work due to potential Summer attic overheating issues. It is my understanding that soffit vents are not common in Canada because due to its northern climate this overheating issue does not appear to be much of a problem, (although they have their function also in moisture removal, if entered). Conversely, I have been told the danger of these vents in the climate North from where I live allows snow blowing into attics via these vents. This latter argument was used to me by a builder in Wyoming for not having installed roof vents on his house (where that Summer the attic overheated and the 150 W fan died. An IR-temp meter registered 155 degree F in that attic (Yes fans stop running at 140 F having a themal protection, but this fan really was fried) when I replaced the fan, which is why I urged the owner to talk to his builder).
Unless a rim-joist discharge study includes Homes with soffit vents that are directly above the rim joist exhaust it does not reflect the stock of houses built in our area, and I have documented the potential of a negative effect of radon reentrainment on discharges under soffit vents.
Leo Moorman
Radon Home Measurement and Mitigation, Inc.
Fort Collins CO, USA