What is the proper way of venting a bathroom air duct through the soffit?

It's common enough to just run the vent duct from the fan out into the soffit and let it blow. Mostly this wouldn't blow back into the attic but I suppose it depends on how wide your overhangs are. I do this with my fan and it doesn't make any problem either in the attic or on the roof. However, if you are getting melting and blowback into the attic, you should get an appropriate hood for the soffit. A quick google check shows that these are readily available.

Stan,

It's really preferable to vent it through the roof or a side wall using either a roof jack or wall jack. Even if you use a fittling to vent it through a screen in the soffit, much of the moist air can make it's way back into the attic because the normal flow of air is in through the soffit, and out through the ridge. You really don't want all that moisture in your attic. Not only does the heat cause problems like you describe, but the warm moist air will condense on the cold roof surface, which can lead to rot and mold.

You see a lot of bath vents done this way because it is easier and cheaper. But they violate all the guidelines for proper venting.

HTH,

Paul

Sad to say you are right, but they are not, it is a bad idea. Roof or sidewall venting is far better.

Around here, all the builders just use a 3' piece of flex duct and point it up into the attic, or else they just drywall the thing in with no duct.

I have a hipped roof (no gables) and a ventilated drip edge (no venting in soffit itself, no blowback) But my builder didn't even put ducting on the bath fans. (I'm second owner)

I installed _insulated_ ducting to a soffit vent like this UEV 4

Dave.