In reply to neuromancer:
I've had a Rab Storm for several years now, used a handful of times a year either as protection for the sleeping bag in grubby bothies or for sleeping out on the open hill if the forecast is good. In practice I experience little or no condensation as long as I avoid breathing into the bag; if I have to zip it up, some condensation is inevitable, but I suspect that would be the same with any bivi. The only time I have had noticeable condensation while not breathing into the bag was on a clear frosty autumn night when dew formed on the outside of the bivi and then froze, which would certainly inhibit breathability. I was also using my down bag and not my 2 season synthetic, so while I was toasty warm there probably wasn't enough body heat escaping from it to force water vapour through the bivi fabric. This wasn't much of a problem as only the outer shell of the down bag got a bit damp, and it soon dried out when draped over a boulder in the morning sun while I had breakfast. I'm sure an eVent bag would be much better, but it's also much more expensive. Bivvying is a hit and miss affair at the best of times. If you want to stay perfectly dry, take a tent.