Hi Nathan,
Thank you for your review on the AERN80LK fan and light model. TrueSeal™ Damper Technology, featured on InVent® Ventilation Fans, are specifically designed to remain open slightly when the fan is off. This protects the fan components from any light bulb-generated heat on lighted models, by allowing the heat to move into the ductwork and away from the fan. Any backdraft in the ducting will cause the damper flap to fully close, or “seal”, effectively preventing outside air from entering the room. It is the job of the roof or wall cap damper to stop the outside air from returning back through the duct work. If your wall cap or roof cap doesn't include a built-in back draft damper, it may be good to consider this option.