This photo attached is of an old bathroom fan ducting section we removed today in Burnaby BC. This whole section of ducting is hidden from view. It is up in your attic and in behind the soffit screening. So to analyze the ducting – the top left is where it joins to the fan housing and where the fan motor is… so you can see that it is expected that when the ducting does a 90 degree downturn it is required that the moist and warm bathroom air must travel about 12 inches straight down towards the ground before it even has a chance to exit the ducting. Logic says, even with a very high powered fan, this route is difficult enough to perform never mind that the old installers now expect the moist bathroom air to then travel horizontally a distance so it escapes the coverage of the soffit overhang before it is free to dissipate into the general air. How do the original installers think this is possible without allowing some, OR ALL, of that moist air back into your attic space? It simply is not possible. That is why we re-duct bathroom fans, and kitchen hood fans, to a vertical configuration. We know warm air rises so to vent it up and out through the roof is the best. There is only one reason for builders to not do it the correct way. That is cost. It is simply put, easier and cheaper to do a horizontal ducting then to do it the proper and obviously necessary way. Call Greg @ Crucial Roof Services for more information on this and other design failures that builders have done to your home.