Bushwalk Australia

I am after a bag that you'd use in cold weather or rainy conditions to protect a duvet sleeping bag.

It has to be breathable and a gore-tex like alternative but just as good in order to reduce the cost.

The condensation factor of bivy sacks is going to be the main issue so if gore-tex is the only real alternative, I'd be
willing to spend the extra money.

What would you recommend me?

Cheers!

I'd recommend a tent.

A bivy is really only for emergency use, or for short term use in cold but otherwise dry conditions were weight saving is absolutely critical (e.g. high alpine ascents).

In a sort-of-cold or rainy condition for any length of time any WPB fabric bivy is still going to restrict evaporation to the degree you are going to end up pretty wet inside.

If your sleeping bag insulates you well, the outside of it will be at air temperature, which means that there's not enough temperature differential on the outside of the bag to "drive" any moisture from you through the bivy bag, so the outside of your your sleeping bag will get damp. This is why goretex sleeping bag outers don't work, and why the inside of goretex bivy bags get damp.

If the outside of your sleeping bag is warm ( meaning that your sleeping bag is not insulating you properly) then there might be enough temperature differential to push the vapor through the outer layer.

The balance point is where you are at perfect temperature, and don't produce any sweat/vapor, which means that there is no vapor to go through any outer bag , in which case the outer bag could be waterproof nylon, as could the inner lining - the latter being a vapour barrier.

I suppose the ideal bivy sack might just be a lightweight nylon with a decent DWR finish, which is really the ideal sleeping bag outer fabric.

A

Tyvek works reasonably well
Evan at TerraRosa

Mountain Laurel Designs SuperLight Bivy made from 10D 3x nylon - very breathable but only splash proof - you'll need a tarp with it.