Windstopper is First Generation Goretex film.

IE. Goretex without the
plastic layer which was added to stop contamination of the
teflon laminate.

Windstopper is also known as Dryloft when they use it in sleeping
bags.

It is waterproof and transfers water vapor MUCH better
than Goretex used today in raingear.

However, with dirt, salt etc. getting
in the pores, it loses it waterproof function.

Great for bivysacks, certain garments for wind and water
and in other garments for windproofing.