I bought three of the large, 250mm Backdraft Dampers, fitted them to the subfloor fans at my house and no more insects can get in. The dampers use very sensitive spring-loaded flaps which seal very well against a circular foam rest.

It is very noisy in its operation. It makes a noise every time I open the door to the room. It still allows a drought in when the wind is blowing and causes an uplift. I’m not entirely happy with its operation at all.

Product provides a good seal when the fan is not in use. Previously had some flying pests come down the ducting when cooking indoors. Has resolved that issue and less wind noise.

Value for money product, as advertised but do make sure you have the correct dimensions!!