How to Vent a Bath Fan Through the Roof | This Old House
Why Home Improvement Stack Exchange recommends it
DIY Stack Exchange answerers point people to it when explaining why bath fans should not exhaust into or under a soffit and how to vent through a shingle roof instead.
Recommended by 1 people across 1 different months, 2018 to 2023.
What it is
This Old House general contractor Tom Silva walks through moving a bathroom fan vent from a roof soffit to a flashed vent hood on the roof. The episode shows a real case where soffit venting pulled moist air into the attic and caused mold, with a materials and tools list for the fix.
What people said
Recommends Bryce · 2019-10 · Home Improvement Stack Exchange · on “How to cut round hole in aluminum soffit after installation”
This old house has an excellent video on why the soffit location is bad, and how to vent through a common shingle roof. [this video] How to Vent a Bath Fan Through the Roof, August 2014. The homeowner installed the same way your contractor did, and in just one season there was mold under the roof. Your contractor should pay for this: the soffit installation was incorrect and hazardous to your health. You certainly don't want the vent inside the soffit, nor do you want it under the soffit.
Recommends Bryce · 2019-10 · Home Improvement Stack Exchange · on “Will I be able to run a duct over the wall to a soffit vent for a bathroom fan?”
This old house has an excellent video on why the soffit location can be bad, and how to put it through a common shingle roof instead: [this video] How to Vent a Bath Fan Through the Roof, August 2014. The problem with the soffit is depending on climate, warm air heading back up and causing mold under the roof sheathing. This is worst if your soffit is vented in the same area as the bath exhaust.
Cites Machavity · 2018-12 · Home Improvement Stack Exchange · on “What Type of Ducting Should Be Used for a Bathroom Exhaust Fan?”
Most places don't require rigid ductwork. it also appears that 2014 NY code doesn't either [LINK] 603.6.1 Air Ducts Air ducts shall be permitted to be rigid or flexible and shall be constructed of materials that are reinforced and sealed to satisfy the requirements for the use of the air duct system, such as the supply air system, the return or exhaust air system, and the variable volume/pressure air system. And Class 0 or Class 1 rigid or flexible air ducts tested in accordance with UL 181 and installed in conformance with the conditions of the listing. As DaveM noted, insulated lines are your friend, especially in NY in the wintertime. But be sure to exhaust your line through the roof, if
Cites Armand · 2023-01 · Home Improvement Stack Exchange · on “Bathroom Exhaust Vent - Out Shingle Roof? Or Out Siding?”
@Huesmann It certainly happens with dryer vents (personal experience) but of course those exhaust warmer, wetter air (warm air rises) than bathroom vents. Ask This Old House has a video at [this video] showing a case where a bathroom fan was vented out through a soffit, and the warm moist air went back up and along the soffit to the next roof soffit vent, leading to mold on the underside of the roof sheathing inside. They fixed the issue by installing a through-the-roof exhaust vent for the fan.