Product review (4 stars): Useful device
I have no way to calculate the energy saving from installing this device although there certainly will be some because cold air can't simply fall down the dryer vent pipe. I look at it as a useful device to keep outside junk from getting into my dryer vent and fouling up the exhaust. It will keep out leaves, insects, birds, and maybe snakes.
My installation was harder than most because it was into concrete and the upper half of concrete overlapped the lower half by about 3/4 inches. So, I had to make a spacer from scrap wood and drill into concrete.
I have a hammer drill so drilling small holes in concrete is no problem for me. Check to see if you are going to have to fasten this gadget to concrete. If I didn't have the tools and accessories to fasten this to concrete I wouldn't have been able to do it.