I have an old house where the soil stack (4" cast iron hub) was installed on the exterior wall out side of the house. At the second floor level there is a sanitary "Y" into the house second story bath. Above this "Y" the cast iron reduces to 2" cast iron vent but does not extend to the roof and is within 5 feet of 2 windows. The stack services a single bathroom with tub, toilet and lavatory.

I am renovating the bath.

I think I can use the existing 4" cast iron from the point it enters the house.

I thought I would install a 3 inch "no hub" "Y" fitting above the existing "Y" and either bring the vent into the house to extend the vent to through the roof.

Can I use no hub in this exterior exposed application? Can I use a "doughnut" to join the no hub to the existing stack? Or can I have a 3" plastic fitting at this exposed location?

Any thoughts would be appreciated.

You can bring the cast iron pipe on up the outside of house, It!only a vent use a rubber piece That is made to go from cast to pvc. You can get it to go from say 4" to" or 4" or 3" to 4" use a short piece of pvc with a reduceing coulping ( from say 3" down to 1 1/2" and go up thru the soffit and thru roof extend at least 18"".

No hub is for 'like' pipes, a black rubber couple [fernco] is for 'unlike' pipes. Cast iron and pvc seem like but actually are unlike. The pvc is slightly larger. The doughnut is difficult to work with but your thought process is OK to continue.