What is an inline attic ventilation system, and when is it used?
Short answer. An inline ventilation system uses a fan mounted partway along a duct run, usually up in the attic, to pull air from one or more bathroom grilles or a whole house exhaust point and push it outdoors through a roof or wall cap. It is an exhaust system for the living space, not a substitute for attic ventilation, and it must never discharge into the attic itself.
How it works
Instead of a fan in each ceiling grille, one quieter fan sits in the attic between two duct sections. Ducts run from the grilles to the fan, and a single duct runs from the fan to an exterior termination with a backdraft damper. Because the motor is away from the room, these systems are quiet and can serve two or three bathrooms.
Getting the attic side right
- Terminate outdoors. Code requires exhaust to go outside, and specifically not into an attic, soffit, or ridge vent.
- Use a real roof or wall cap. That means a roof jack with a damper and flashing, or a sidewall cap. Pointing a duct at a soffit vent lets moist air get pulled right back into the attic.
- Insulate the ducts in the attic so warm, humid air does not condense inside them and drip back into the fan or ceiling.
- Keep runs short and smooth, with rigid or semi rigid duct where possible, sloped to drain toward the exterior.
- Leave access to the fan for service.
Why roofers care
Bath exhaust that ends in the attic is one of the most common reasons roofers find mildewed decking and rusty nails. If you add an inline system, plan its roof cap with your roofer so the flashing is integrated into the shingles rather than caulked on top. See roof ventilation basics and roof flashing explained.