What is a material that allows air to pass but not water vapor?

Question

What are some materials that allow air to pass but not, or to a lesser extent, water vapor?

Here are two materials I am aware of:

• Teflon is a material that allows water vapor to pass, but not air. Water itself can't pass.

What are some other sourceable substances which have this feature? What are some defining terms and measurements used to identify this property in a material, and how can the rate of water vapor / air transfer be manipulated in such a substance?

Answer 1

I just check on Google and I find the following results

• TEMISH (TM) of Nitto
• Silicone (yes, you read that right) (fixed for spelling mistake) - Apparently, the American company GE tested it out in 1950s
• Tyvek (R) of DuPont
• Gore-Tex (R)

The stuff above can happen due to something called "Molecular Sieve", find quick reading here. Basically, the material concerned is porous ("with holes"), but these holes are very, very, very small, at the atomic level. The holes can block bigger molecules/atoms (like water - H2O), but allow air (say oxygen - O2) to pass through. Any material with sufficient small holes can "block" the water, and "unblock" air.

Of course, said is easier than done, because the water molecules have the size of nearly 3 angstroms (2.75 if you want more precise value), so... yeah. Building material like that is possible, but very hard.

Answer 2

Pinhole-free silicone rubber membranes , ~.001 inches thick, can pass oxygen but will prevent water vapor from passing through. General Electric developed these types of membranes back in the 1960's but I don't know if anyone makes these or similar membranes anymore.