One bathroom has a fan that vents up a flexible duct to the roof.
The other bathroom does not... Instead it has a window... however leaving that open all night to get rid of steam in not an attractive option.
If I install another celing vent fan (probably a combo with a light) can I make a "Y" joint and vent BOTH celing fans through the one existing roof penetration?
I have read somewhere this was a bad idea, but can not remember where or if it was a reputable source.
Yes, you can combine bathroom exhaust vents, as long as you direct the exhaust fan airflow in one direction (up with a Y and not into a T), if allowed by local code. Call your Building Inspection Department.
You need to be aware that if you're using both baths/showers and exhaust fans at the same time that you're dumping a lot of moisture into that single vent.
Insulate the vent ducts in any unheated space (attic) to prevent warm, moist air condensation from building up inside the duct during cold weather and running back down into your exhaust fans.
Personally, if it were me, I would keep them separate and exhaust one near the ceiling through the exterior wall, and wouldn't combine them.
Forgot to thank you for the reply on this. Will call the building department and then go for the double. Chances of using both bathrooms at the same time are slim.
Would love to vent via a wall, but there is a patio and a window there and this is a half bath.... oh and we used to have had problems with water in this area thanks to the excellent roofing job done there *cough*