In general, no ventilation fan should be terminated in the attic.
Doing so can result in adding moisture to the attic space which can do damage to the roof structure and may lead to mold growth in the attic. All exhaust fans should properly vent to the exterior of the home at a proper vent cap with back-draft damper. While bathroom fan vent pipes do not “have” to be smooth wall metal pipe, they will vent better if they are. All exhaust fan vent pipes can benefit from being insulated—and in new construction are required to be insulated.
This post will focus on kitchen exhaust fans. These duct pipes should always be smooth-wall type metal pipe. Grease in the air that is being exhausted from the home can build up on the inside of the pipe—-especially un-insulated pipe that does not have a back-draft damper at the exterior vent cap. This is exacerbated when the home is in a colder climate.
In this picture we see where the corrugated vent pipe has never been connected to the roof vent cap and the pipe is not insulated. Also the roof vent cap is not a type with a back-draft damper—just an ordinary attic roof vent cap with a screen. Although some jurisdictions accept this type of pipe, I don’t like it. I do not see this as “smooth wall” type pipe. All the little grooves and ridges will collect grease.
Many times these exhaust pipes are simply “aimed” at screened attic vents without a back-draft damper. The screens of these vents will clog with grease over time which allows for even more grease build-up in the pipe itself.
Grease build-up in the pipes and screens can become a fire-hazard and should be regularly cleaned by a qualified duct cleaning company. Most homeowners have experienced how nasty the grease filter of their kitchen range hood can get. The insides of the vent pipe can get equally nasty and any screens in the exterior cap can get even worse.
Since the building codes require screens in these caps, it is very important that they be maintained clean.
I think the worst and/or strangest case of a kitchen exhaust vent terminating in an attic that I have ever come across is the one pictured next.
Yes that is cardboard.
What condition is your kitchen range exhaust vent in?
Charles Buell, Real Estate Inspections ins Seattle
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