Insect screen mesh for vent fan outlet

[AnthonyL]

Good points thanks. How much do flaps impede the flow?

[Tim Watts]

I was referring to mine :)

As it's a downwards facing soffit vent, it's open all the time - and the fan's an axial in line, so that doesn't have one either (never had a problem with wind blowing in with that arrangement).

But point taken WRT to the OP...

[soup]

Flying? Doubt it.

Crawling? Pretty conceivable see my PP (But NOBODY lives in a completely sealed house).

[Vir Campestris]

Depends.

When two successive house painters have covered the light plastic flaps with masonry paint - quite a lot.

(I gave up. There's a flap on the fan too, so I fixed the outside ones open at 45 degrees, enough to let air through but keep rain mostly out)

Andy