The previous owner of my house ran both vents through the attic in wire flex pipe into a Y and out one roof exhaust. I've heard that it can and cannot/shouldnt be done.
I have to redo them as nothing was insulated, had pooling water in the pipe and it rotted out and now some water damage to repair as well from the condensation.
Someone before had a similar question and was answered by many contractors. I recommend to use the search function over there type in: " bathroom vent " - 13 results including this post will show up.
3D-Tile-Design - Bertram Tasch
Maple Ridge, BC (Greater Vancouver)
Running both exhaust fans into a Y and out the roof shouldn't be a problem.
I would think, the issue is that the vents themselves were not insulated.
Every situation is unique, but the vents running from the fan to the attic must be insulated.
In regards to the water damage if the area is larger than 5 square feet I can I've you advice on how to properly handle the issue in case of microbial (mold) contamination. It is imperative that you make sure to control cross contamination of mold spores while performing the work.
James C Watson
Running two vents into a Y is not allowed. What happens is the one fan will be running and push the air into the second fan instead of out the vent, therefore defeating the purpose of the bath vents. They can only be run out the roof through a vent designed for it, not the roof vents used to ventilate the attic space.
It's best to insulate the complete pipe from the fan to the outside. Use ridgid piping, not the flexable stuff, which just creates restriction. The pipe should also be sloped towards the bath fans. I know it sounds weird, but what happens is the condensation might pool in a low spot and leak into the attic space, or freeze to the inside of the pipe and melt on a warm day.
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