Fabric with Water Barrier

That sounds like the function Gore-Tex serves, although I have always seen it as a layer embedded between other fabrics in jackets.

Yes, that is the first choice I would use.

Gore-Tex

You can buy Gore-Tex fabric from McMaster-Carr. The PTFE-coated stuff (P/N: 8321T33) is 0.015" thick and has a nice "feel" to it. Slightly spendy - a 12" x 29-1/2" piece just cost me $43. There are other "breathable" fabrics out there, but Gore seems to be the market leader. Good luck!

Thanks for the source. I am working with Gore-Tex and they indicated the cost is about $14/running yard, 56" wide or in packaging terms, $7.00/1,000 square inches. It might be acceptable but I have yet to get a sample for lab testing. I have also had a fabric with 0.001" of polyurethane adhesive laminated to it constructed for our testing. That looks very interesting because it does not let water through but water vapor can penetrate although at a reduced rate. Is there a material something like this that is used commercially somewhere out there? My application is a "safety net". We do moisture management of wooden musical instruments and so I am constructing a holder for the pouch containing the salt solution. In case of a rare package defect, I want to keep the product from touching the beautiful wood finishes. Thanks for any responses.

The patent to make expanded PTFE has expired, so you may want to try Googling the general term "expanded PTFE" and see if competitors are cheaper and more cooperative.

You may also want to check out Tyvek, which should be pretty cheap.

Tad

Or, if you want low-tech, waxed cotton had been used for a long time. I've had a Filson hunting jacket made from waxed cotton for over twenty years that I've only re-treated once about 3 years ago. It's never let me get wet despite some pretty frequent and rough use. I only re-treated it because I was feeling guilty about not ever having done it. Waxed cotton is heavy and stiff, but definitely has an appeal to us outdoorsy types.

Try Goretex.

Have you considered leather? Or check globalspec

http://materials.globalspec.com/SpecSearch/Suppliers?Comp=4314

It sounds as though the breathable membrane material that has replaced underfelt in buildings' tiled and slate roofs is the sort of thing that might work. Check out the Screwfix catalogue, item 98252-88, currently starting at about £1.50GBP/m², including local taxes. Page 645.