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What is the best cap to use for a thru-wall bathroom exhaust fan install? The ducting is 4" smooth aluminum.

Winters here are brutal. I'm concerned about wind sucking the cap flaps open, letting in cold air. Thanks.

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I have two bath fans that are the inline style, 1 for each bathroom. one fan is 4" solid duct end to end, the more powerful uses a 6" 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" bath fan has a plastic hooded exhaust cap, while the 6" duct has a plastic louvered cap.

I would say on very windy days, my 4" duct probably opens up, as I can hear my damper lift up and down sometimes.

I also recently installed a new dryer duct, 4" 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.

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