Made a pertex bag with material from Pennine Outdoor weight about 150gms .Ideal for use with a tent.Sleep outside if its good weather,keeps the sleeping bag clean.

The key to the breathability is the coating rather than the outer fabric so whether it's Pertex or another synthetic probably makes little difference. The original Survival Zone (mine is 11 years old) had a PU coating that was moderately breathable, Like all bivi bags I've used, including Gore-Tex ones, I've had much condensation in the Survival Zone on humid nights. I'd never rely on any bivi bag to keep me dry for several wet nights.