Bathroom fan vent 5/12 downward path OK?
We are in the process of designing a house for West Yellowstone, Montana. This is both deep snow country and zone 7 cold. I’m trying to minimize roof penetrations, so I want the second floor bathroom fan to vent out the side wall. I would rather not vent to the gable ends, as this is a longer run and an aesthetic issue. The best path looks like going from the bathroom ceiling, hugging the top of the ceiling and following the 5/12 lower chord of the trusses down to the outside wall. The truss heals are 2′-6″ deep, so this will exit about 3 feet from the soffit vents. Total straight run will be about 8 ft. My question is, this will place the outlet about 3 1/2 feet below the fan itself. Will this cause any problems, such as with airflow? Thanks!
Replies
Airflow will be fine with proper CFM fan. Venting down is actually preferable for moisture control, but often we have to vent up due to obstructions. You may encounter a problem of moisture entering back into the roof with soffit venting depending upon your design etc. If you can get the vent to go down a little further and exhaust out the wall vs the soffit it would be better. When possible, I usually try to get the vent into the dead space area at tub/shower or build a 2x6 wall at tub/shower for duct space. I go down that wall or dead space and turn into floor out the wall. If you are building extra thick exterior walls you may be able to run pipe inside the exterior wall. I also use solid pipe, tape all seams and make sure pipe seam is facing up not down. Pitch all the pipe down like plumbing waste line, so condensation can roll out the vent. Another big thing in my opinion is using a timer switch to control the fan. It’s good to run the fan 30 min after the shower as there is still a lot of moisture present.
jlyda has it right. Most fan manufacturers recommend that the vent be sloped down if possible to drain the inevitable condensation. It also helps to insulate the duct. Personally, I recommend either a humidity sensor fan control or a 60 minute timer.
Thanks for the thoughts and advice. I won't be venting out through the soffit--these trusses are 2'-6" deep at the heals, so the vent will be at least 2 feet below the soffit. Sounds like it can work, so thanks for the help.