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We have two bathrooms side by side on our second floor. They both have the same exact layout (about 6'x7'). Both have their own separate exhaust fans (Nutone, 80CFM, 1.6amps), but share the same vent to the roof through a Y-shaped connector and flex 4" pipe. When we take steamy showers in bathroom A, paint from the ceiling of bathroom B is showing signs of moisture and paint is peeling in one area of the ceiling. We even noticed some condensation on the tile wall of bathroom B.

What could be causing the problem in bathroom B and how can we fix it?

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If both fans are venting out the same exhaust port, i would check you have back-draft Dampers (one on each section before the presumed Y connector).

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Its possible your first extractor is just blowing the damp air into the second bathroom - a damper should add enough resistance to the flow of air to make the damp air "prefer" exiting via the exhaust port rather than going through the second damper (and in to the second bathroom, causing the damp), without restricting airflow noticeably.

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Your fans apparently don't have built-in dampers on their outlets, so you'd need to either add some to each inlet branch of the Y such as the one below or what James pictured, or remove the Y and run a second line to the outside.

Metal backdraft damper

I personally find the white flex duct inferior as well. Traps dust and moisture which could mold over time if it is in an attic. I'd use metal pipes for a short run or an insulated flex duct for a longer run.

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One solution is to have two input grilles (one per bathroom), then a Y-connector, then an in-line fan, then the exhaust vent.

This ensures that when the fan is running, air is pulled out of both bathrooms through the grilles, and cannot go in to either bathroom through the grille.

(This solution may not be the most cost effective in your situation given you already have two fans.)

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