UKC

I am looking to buy a sleeping bag suitable for camping and bivying in the Alps from 2000-3500m during the summer (late June - early September). I am planning on using the bag for summit pushes (with all layers on) to avoid the expensive hut prices but also for down in the campsite.

I am considering either the Gruezi Bag Biopod DownWool Summer (8,3,-11 @ 1.1kg) or the Biopod DownWool Subzero (2,-4,-20 @ 1.5kg).

I haven't bivied at these elevations before and looking for some advice!

I used to use a rab 500, no zip, 500g down, 500g of cover, total weight 1000g. That was 1987, lighter materials and loftier downs are now available. Bivied up to 4000m always over warm except only just warm camping below the Cap with a thin mat.

So your wooldown options look on the heavy side. 

I have now got mountain hardwear lamina bags, fully synthetic, with a short centre zip and double puller. Good for drinking a brew , staying tied on and having a slash without getting out too much. Weighs less than a kilo, probably lighter synthetics available .

It is decades since I did this, but back when I did, I found a gortex bivi bag  the most crucial thing (sorry if you're already taking that for granted).  Adding a sleeping bag was something we did only when saving weight didn't matter too much.

An unforeseen snow shower etc will be grim without a bivi bag. 

I did several bivis above 3500m (& above 4500m) using the bivibag_but_no_sleepingbag set up and managed to sleep. 

My inclination would be to just have a nice warm sleeping bag and use that if you're not climbing etc carrying it. Whenever you are needing to climb with your bivi kit, ditch the sleeping bag.

If your planning to carry your bivvy up routes the I would suggest a blizzard bag. Very cheap, very light and fairly weather proof. Bulky when unpacked but manageable with care and an unfashionably large rucksack.