Cape backdraft damper vs butterfly damper

A homeowner in an online building forum asked about a bathroom exhaust fan part called a backdraft damper — the flap inside a duct that's supposed to stop outside air and wind from blowing back into the house through the fan when it's not running. They'd bought a "cape" style damper (a fabric flap design) to replace one that kept flapping in the wind, expecting a tighter seal. But after installing it, they questioned whether the fabric flap actually seals well, since there's no frame or mesh holding it in place against backward airflow. They also pointed to the manufacturer's own test data, which showed a more traditional butterfly-style damper (a hinged, rigid disc) actually sealed better at most wind pressures, with the fabric version only winning at the highest pressure tested.

Other homeowners and one building expert weighed in with their own experiences. One pointed to a different, less attractive damper design that reportedly seals more tightly. Another found a video showing how the fabric-flap dampers work: the fabric collapses under gravity once the fan shuts off, and outside wind is thought to press it flatter against itself rather than force it open. That poster noted these dampers likely need to be installed with the duct running horizontally to work as intended, and that how long the fabric lasts probably depends on its material and how it's attached.

For anyone dealing with a drafty or noisy bathroom fan damper, the takeaway is that not all backdraft dampers perform the same, and the fabric-flap style may not be the automatic upgrade it's marketed as. Orientation, installation, and material all seem to affect how well one of these seals against wind.