In reply to chummer: Seems nobody has jon
I've used a Nikwax proofed down bag - seems to work well and it's easy to do.
Alpkit have instructions on their site about how to proof their bags.
We're Alpine bivvying and snow-holing this year and I have used proofed bags in a tent or bivvi in snow in Wales and Worcestershire several times, sometimes in very damp, muggy conditions with no problem (up to 2 nights at least).
You might need to go higher spec if you were in the Alps and high up for long periods as the effects of water getting into the down are cumulative I believe.
I used an old Blacks down bag until very recently but I'm now using (varied according to temperature and usually fully clothed) proofed Alpkit Pipedream 200 (with a thermal jacket over torso), fleece liner (proofed), Hunka (waterproof cover) on Thermarest Prolite plus (full length), with option of additional foam mat and all inside a hooped bivvi. We'll probably also take a tarp and a spade (to make walls/dig holes).
When you go might be key...the above kit is cosy (for me) to -5 ie approx summer Alps.
btw The British Standard for rating bags assumes you wear (at least) a hat, thermals and knee socks in colder weather.
Hope this is useful.
