[Pantaloon]

A friend and I have been discussing this for the past few hours and are curious.  We've looked online and found different patented versions of this (Gore-tex, Sympa-tex, etc.), but were wondering if there exists a way to do it relatively simply that isn't patented by any companies.

[Arkcon]

It's a little hard to follow your application.  Those high tech patented water vapor permeable membranes are you mention are expensive to produce industrially.  However, gas permeable, water resistant substances are common -- example, porous clay pots.  So some more info on your application and scope of your needs of is needed now.

[Enthalpy]

Cork is one other example.

Wouldn't any fabric of hydrophobic fibre do the trick? Say, if you impregnate the fabric with a water repellant like silicone or perfluorinated thing?

[Pantaloon]

Essentially what a friend and I are looking to do is make a hydrophobic window screen.  A way you can leave your window open when it's raining, still allowing air flow into the room without water getting into the building. I don't believe cork would do the trick here, and those patented membranes are too expensive.

[Enthalpy]

A possible solution is to have shutters, whose tilt and overlapping prevent rain entering even if the wind blows (there are limits). If you want sunlight to enter the room, just build them of transparent material: polycarbonate (Pc), polymethylmethacrylate (Pmma), maybe glas.

Possibly the only way to get a significant air movement. Watertight materials would let very little air pass through.

Marc Schaefer, aka Enthalpy

[Intanjir]

Spray scotchgard or the like on a fine mesh screen?

[Enthalpy]

Here's how the shutter could look like (log in to see the sketch). Open the complete panels on hinges, or adjust the louvres' angle. I expect such shutters to exist already, but they're easy to assemble from glued polymer sheet if needed.