[HayZee518]

I had a similar installation to do last year here in NY. I wasn't able to find a damper that would close by gravity. What I ended up doing is exactly what you stated. The fan was installed on a gable end with the louver pointed outside. Fan - ON louver - open. Fan - OFF, louver - closed. Ceiling registers were wide open for air flow. A suitable "dam" was made to keep insulation from flying into the room when there was no upward air flow.