I'm not happy with the damper in my cooktop hood vent.  It's a spring-loaded damper that opens when the interior pressure exceeds the exterior pressure by a small amount.  Since the house is ultra tight construction, virtually any positive interior pressure opens the damper to let air escape.  Also, the damper doesn't always close completely.  I can tell that significant air is escaping through the hood vent because, even when the hood is not operating, moisture condenses on the cold metal roof vent and wets the shingles as it runs down the roof.  I suspect that I'm losing a lot of heat energy via this leaky damper.  BTW, I use mechanical ventilation (HRV) to achieve the recommended 0.3 ACH.

My question is:  Does anyone know of a 6" round duct damper that provides a good postive seal?  Maybe an electrically actuated damper that opens only when the cooktop hood fan is turned on?