If you vent the bathroom exhaust fan to close to the soffit vents which are vented plates under your homes outer edge and roof your home the air can be sucked right back into the attic from the soffit.
Bath fan vents to attic.
The best exhaust fan venting is through smooth rigid ducts with taped joints and screwed to a special vent hood.
When venting a bathroom exhaust fan make sure to vent the air to the outside rather than into your attic where it can cause mold and mildew to form.
This article describes routing bath exhaust fan duct upwards through an attic or roof space or downwards through a floor or crawl space.
Options for venting a bathroom exhaust fan include best to worst.
The lowdown on venting your bath fan into an attic is that it can be done but it should terminate on the outside.
In fact i have seen numerous houses that haven t yet had signs of mold with the bath fan venting incorrectly.
In all cases the ducting needs to conduct the exchaust to the building exterior and needs to terminate in an animal proof vent cover.
I would by lying if i told you that it will cause problems it may be totally fine.
Venting through a roof vent or exhausting them in the attic could cause moisture problems and rot.
No you cannot vent the bathroom exhaust fan into the soffit vents.
This setup requires an in line centrifugal fan mounted in the attic drawing air simultaneously from both bathrooms see photo.
A grille in each bathroom attaches to ducts which then fasten to a y connector at the fan.
The truth about attic bathroom venting.
But while you can t have two fans with one vent you can make one fan and one vent serve two bathrooms.
Bathroom ventilation fan duct routing routing a bath vent duct down out or up through an attic or roof out.