I found a new Wander Ridge brand bag on Amazon1 which is opaque and seems to be made out of foil or Mylar… in fact they claim both on the listing… LOL. I sent them a question asking for the WVTR. They’re ~2x the price of their regular bags (which had a 50% failure rate for me).
Could be aluminized Mylar, which should have a low WVTR (good barrier). Mylar itself is not necessarily a good moisture barrier, the transmission rate still depends on what it is combined with (and thickness).
There are a few more (newer?) filament bag offerings in what appear to be at least partially aluminized/opaque plastic, but they all seem to focus on light blocking and as usual none state a WVTR, thickness, or material.
Made me wonder if antistatic aluminum bags would help – no, not the “typical” ones apparently. But there’s a whole classification known as “Moisture Barrier Bags” (MBB) for electronics, with their own international standards and very low WVTRs. Not sure yet if any of those are practical for our application.
-Max
1 https://www.amazon.com/Filament-Storage-Aluminum-Light-Blocking-Moisture-Proof/dp/B0GY2L4HZV
(no tracking, no affiliate, not an endorsement)