Hi bmholloway,
Just in case you haven’t seen it … some of the better options I know about in the Atlanta area are in post #2 here.
To my knowledge … the food grade polyethylene protectors such as those used by Naturepedic are not the “membrane” type which are both waterproof but also allow water vapor and air to go through them.
Having said that … I know that naturepedic also sells a waterproof/beathable protector but the membrane they use is proprietary and they don’t say what the membrane is made of. Even their CSR’s don’t know. It is a top protector with elastic straps rather than a complete surround type and it is GOTS certified. It’s also not “stretchy” though so it may affect the feel of soft latex or memory foam by creating a bit of a “drum” effect and/or reducing the amount of heat that reaches any memory foam (this came from a conversation on the phone with them). It’s only made up to Queen size and is produced a little larger than standard and will shrink to size with washing. They also have a cotton flannel protector which is more stretchy but not waterproof and is only made up to full size.
I also came across this interesting article which is more about allergy protection but has some very good information about the breathable membrane types of protectors.
Another alternative would be a thin wool protector such as the Dormeir which is thin, breathable, and fairly stretchy but only water resistant rather than waterproof.
A title search on “cover” or “protect” will bring up more information and links.
Phoenix