reasons for mounting the fan and exhaust piping outside building envelope in US Doug, well stated. I think it is important that we attempt to keep the bar a little higher rather than lower. To this day I continue to find fans in the crawl space, mounted sideways to save 2 90's with black flexible drain pipe as the exhaust that discharges at he foundation vent, below a window. And people have been living in the home for years thinking they were safe from radon. If some people in our industry fail to follow the standards that have been in place for years I would hate to see the results of a relaxed standard.