bump!

We have to know how much free air area remains.

Check the pressure drop across those components.
I have a number of pages you should be interested in, look for the relevant links. udarrell

Here is a link that might be useful: udarrell.com

I'm afraid that all that information is above my head.

What I am looking for is an approximation on how much the airflow would be limited if I put either an automatic damper (like the kind on the outside of houses that don't allow backdraft or critters into the duct) or a screen on the end of a 4" flex duct. The flex duct will be about 10 foot long and have about 50 degrees of bend. The airflow will be from a 110CFM Panasonic bathroom exhaust fan.

Any approximate guesses?

I would use a flapper valve made for venting a clothes dryer through the wall - It will surely have much less blockage than a window screen. The screen will eventually plug up from the inside with dust and debris in the air coming out of the house, so you would have to clean it way more often than you would a flapper valve. Think about all the lint that gets blown out of a dryer and those valves seldom plug up

Don

That's very true Don, I never thought of the dust clogging aspect of it.

As for the flapper, how much resistance will that give? I notice my kitchen exhaust has a flapper at the bottom that gets pushed open by the airflow, it seems as if it is adding resistance since it only opens a little bit.

Try hardware cloth...more open but keeps out the critters.

"Try hardware cloth...more open but keeps out the critters."

Unfortunately the critters I am most worried about (House Centipedes from the garden center next door) will fit right thru that.

Regular window screen is about 50%.

"Regular window screen is about 50%."

That's definitely out.

So out of curiosity, how much air flow restriction do the little dampers on outside vents cause?

bump