Product review (5 stars): I must have been wasting money on heating for years.
I was painting and had to move the dryer.  When I disconnected the vent hose a huge blast of arctic air came through the pipe from outside.  I checked outside and found the plastic vent flapper was warped.  I investigated energy saving devices and came across this closure.  I installed it onto my vinyl siding.  I used my oscillating tool to cut a square in the vinyl to allow the closure pipe to lay flat against the house, and then caulked around it.  I checked the opening again..  Nothing.  Not one bit of air came inside from the outside.  I ran the dryer and it functioned as advertised.  I immediately ordered another one for the bathroom vent.  Well engineered tool, made in the USA.  Can't wait to see how long it will be until my next heating fuel fill-up.