Outside cap for exhaust fan
I have two bath fans that are the inline style, 1 for each bathroom. one fan is 4&quot; solid duct end to end, the more powerful uses a 6&quot; solid duct end to end.
From the vent side in the bathroom ceiling, there is a butterfly damper that remains closed due to gravity and lifts up when the bath vent is turned on. On the exhaust side, the 4&quot; bath fan has a plastic hooded exhaust cap, while the 6&quot; duct has a plastic louvered cap.
I would say on very windy days, my 4&quot; duct probably opens up, as I can hear my damper lift up and down sometimes.
I also recently installed a new dryer duct, 4&quot; diameter, where I used a heavier gauge exhaust cap which is damper style, with magnets. This cap could probably work for a bath fan, as long as the fan is powerful enough to lift up the damper flap. I used this one.
I would try adding a damper inline and a beefier exhaust cap.