In reply to gethin_allen:
> You can't argue that a bivy bag, no matter how good the material is, will allow water vapour to pass through like it doesn't exist.
Especially if the sleeping bag is doing its job of insulating, thus reducing the temperature gradient between inside and outside of the breathable membrane.
Equivalent experience comes with wearing a breathable membrane waterproof jacket over a down jacket, or a waterproof-shelled down jacket. Both cause more condensation within the insulation than a simple microfibre-shelled down jacket. It's why we don't like wearing waterproof clothing unless we really need to, preferring to wear more breathable, non-waterproof shells that allow water vapour to escape more easily.
Ye cannae break the laws o' physics, captain...
What may happen is that you get the condensation on the inside of the bivvy bag, rather than on the surface of the down bag, since the bivvy bag traps some warmer air around the sleeping bag. If your bivvy bag has a fluffy inner scrim, the condensation may be trapped in this, and not find its way back to the cover of the sleeping bag.