Quick update:
I noticed that nowhere on the North Face literature did it actually say ‘waterproof’, merely ‘keeps you dry and comfortable’ and ‘DRYWALL technology for single wall breathability’. It also said the bag was designed for ‘high altitude’ expeditions.
None of this concerned me though, after all, what would be the point of a bivvy bag that wasn’t waterproof? Tiso’s certainly didn’t hesitate to sell me it even when I mentioned bivvying on Scottish summits…
Anyway, long story short: It’s not waterproof. In fact, I suspect this is one of those ‘windproof’ bivvies that ianfitz suggested. That being said, it was my friends in their NF bags who actually discovered this as I had a lovely dry night under my tarp 😀 (and was impressed at how little condensation had formed in the bivvy bag).
So in conclusion, does what it says on the tin, will probably keep you dry at high altitude where rain would fall as snow and condensation is your biggest concern. Utterly useless for highlands bivvying unless you’re committed to using a tarp as well. Lets hope Tiso agree that they should perhaps have told me this before I bought it, at least I have the receipt… 🙄