low permeability

State what else the thin flexible foil needs to do.

Aluminum foil out of the grocery store is impermeable to water. But, probably because of other considerations, you won't find this useful.

Most any metal foil will be impermeable and flexible to a degree. Well, not soduim or lithium under normal conditions.

You know there are numerous waterproof and "breathable" polymer membranes used in outdoor clothing and a variety of uniforms?

Advice - give more and more useful information. Otherwise, there are too many candidates to bother discussing blind with anyone.

Hi

You are right, there are missing alot of important informations in this message.

The foil should be used as a skin on a small open structured device (40mm x

10mm x 10mm) which undergo change of external presure between 0 to about 2 bar and a temperature shift from 0 to 50 degree C It is very important the ther will be no permeability or very limited though the foils.

Sounds like your problem will be more the hermetic sealing of such a foil around your object than the materials selection. Hope you have a helium leak tester.

Alufoil is crustalline and full of tiny cracks and pinholes. Use the metglass tape from theft tags.