Our second floor bathroom’s ceiling vent fan exhausts through the attic to the roof, where there’s a vent cap with a flapper that opens when the fan pushes stale air against it. Having just read a recent article by Martin Holladay about air leaks and the chimney effect of multistory buildings, I got this vision of the flapper open just a crack all winter as warm house air, having risen to the ceiling of the second floor, oozes out day and night, allowing cold air to ooze in at the lower level. And it never showed up on the blower door test because that sucked the flapper tightly closed!

So, anybody know of a product that addresses this problem? Something like the powered damper on boiler and furnace flues? Something to keep the bathroom fan vent closed except when the fan is running?