Heat Loss through bathroom ventilator - passive

When ever its windy I can hear the bathroom vent flap opening and closing. This is an obvious heat loss. I suppose all fans are prone to this. But I have a roof flap, an extra in line damper used for dryers and a Panasonic fan with a nearly useless flap. The pipe from the ventilator to the roof is only about 2.5 feet in length. I can't figure what to do to stop it other than removing the fan or replacing the ventilator. I have a window, so I don't need a fan. What can I do other than finding a "better" ventilator?

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
Add image file

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.