I am new to camping the in cold climates and I am wondering is condesation an issue in temps 0-10°C C (32-50°F), I am expecting the humidity to be 60-70%
I only as since I plan to use a down sleeping bag. My alternative would be to take my hiking tent.

There are a lot of variables that might make it hard to definitively say what might work for you, but I have used mine successfully in a much wider range of conditions than that. The range you specify sounds like ideal conditions for easy bivy use, but it does require some care on your part.

I have found that any moisture tends to condense near the foot end of the bivy. It is usually mostly under the the sleeping pad (which is inside the bivy) and on the top outside of the sleeping bag near my feet. It is typically pretty easy to manage. It just brushes off of my DWR sleeping bag and gets mopped up or shaken out of the bivy.

Any condensation is often in the form of ice in really cold weather which is a bit harder to deal with but still not a big problem.

It is key that you not bundle up to the point that you sweat a lot. That may mean adjusting ventilation as the temperature changes. Also key is that you don't breathe into the bag or the unscreened portion of the bivy.

The one condition where my regular bivy is kind of a fail for me is when it is hot and the bugs are biting. An mesh top bug bivy is handy for that. If a trip is going to have very hot weather and very cold weather taking both my bivy and bug bivy would still be under a pound for the two combined. I have never actually taken both, but did wish that I had on the Southern Tier. I guess I could have mailed the bivy home and had the bug bivy sent to me when I was done with the mountains and before the aggressive and plentiful mosquitoes of Louisiana. Still my current bivy (Borah side zip) is only 7 ounces and my bug bivy (Ti Goat Ptramigan Bug Bivy) is only 5.3 ounces. So taking both would be lighter than the REI Minimalist bivy I used on the ST.

I find making the time to air both the sleeping bag and bivy works well for me.
Usually stop for a lunch break near some tree's and find a half hour hanging takes care of any moisture.
If theres no trees or handy fence or if theres rain, I'll then make a point of either an afternoon break of similar duration or finishing riding a little earlier than usual in order to attempt to air the pair.
I find it an easy adjustment to get used to and make part of my touring rituals.
Groundsheet also tends to help with condensation according to many.
Utilising a tarp in frosty weather will help with the bivy breathing as frozen condensation on the outside of the bivy will exasperate the internal moisture issue.
I'm yet to try my new bivy since my old one "passed away" a couple of seasons ago from hard use.
The replacement has a torrent wear "underneath" and e-vent upper so I've high hopes for its christening.

Some good points from staehpj1 above.

Vapor barrier liner, next to your skin will reduce the condensation in the insulation of your sweat ,
the source of most of the moisture that does condense (and cause the Down to lose Loft)..

Unless your night includes the Dew Point , dropping the temperature through that , overnight.

(had dew frost jamming a Zipper of my tent , once , in SW Ireland in end of Feb/1st week of March.)

My comments are specific to tents and tarps, I have never used a bivy sack.

Not really any problem for me until I get below freezing. At that point,some of the body moisture that gets into the down freezes into ice crystals in the down and stays there because only part of the down is close enough to your body to remain above freezing. But when the weather is above freezing, moisture in the bag can readily evaporate. If it is warm and dry in an afternoon after I set up camp, I often put my bag on top of the tent to air out in the breeze. As noted above, the foot section is often a damp area of a bag after use.

I have heard of amateur campers that bring damp things into the bag at night to try to dry them out, that is a good way to accumulate too much moisture in a bag.

Below freezing is where you can have problems. I did a week long snowshoeing trip and when I got back into civilization, I took my down bag out of the stuff sack and after the bag warmed up, it was quite damp from the accumulated moisture. When Steger and Shurke led an dog sled expedition to the North Pole in the 1980s, some of their sleeping bags weighed something like 40 pounds at the end due to accumulated moisture in the bags, but that is an extreme case.

I do a week to a week and a half long trip every year in October on the Canadian border where it occasionally falls below freezing at that time of year, have not had any problem yet.

I tried vapor barrier sacks for mountaineering eons ago and hated it. The i stumbled upon vbl clothing. Quite an improvement!

I have used vapor barrier clothing from Stephenson's Warmlite for 25 years to prevent moisture from accumulating in my down bags, quilts and clothes. The material feels very comfortable like flannel against the skin. It is aluminized to reflect back some heat too. And it is very inexpensive and durable at $25 for the shirt. They offer pants, gloves and vbl socks as well. I have and use them all on every trip be it bike tour, back pack or kayak tour. Most everyone who sees me using the shirt comments skeptically until I let them try one. They are usually surprised in a good way by the results. Did I mention that the vbl clothing is light weight? (6 to 8 oz for the shirt.)

Warning: Zealot pontificating about vbl benefits follows: Just my consumer experience. You can read the vbl treatise on the Warmlite site.

Aside from the benefits listed above I find that I can take fewer and lighter clothes to keep warm even when on the Divide Ride or kayaking for 30 days circumnavigating the perimeter of Glacier Bay NP in Alaska. I lived in the shirt for the 30 days of kayaking in cold wet rainy conditions. Contrary to all advice i took a down bag on that kayak trip. A much lighter bag than recommended because the vbl limits heat loss and keeps the down lofting and the bag clean. Almost no need to dry out my bag or quilt at all even though others needed to do so every day.

On bike tour at the end of the day I remove my merino wool t-shirt and after a water bottle shower i put on the vbl shirt and am quite comfortable over a wide temperature range sitting around before sleep. After i wash the wool T, I put it on over the vbl to dry it before i go to bed and it works for the thin T's I use. Another pleasant surprise.

There are newer more high tech vbl clothing available now but quite expensive. I have had no need to try them. They may be even better than those i use.

Ok thanks every one I think I will stick with my tent then.

Theres a lot to be said for the new tents available now.
Many offer similar weights to the once advantageous bivy, where once they would have been twice the weight.
Even taking into consideration a groundsheet and tarp, though this advantage is mostly a thing of the past.
I believe the one advantage left of the bivy is just where you can utilise it.
Many is the stealth camp site I've had where I'd not have gotten away with a tent.
Also with the bivy I need a much smaller space to "put down" and can do it under much lower scrub compared to a tent.
Roadside (just into the trees/scrub) is very possible with a bivy many more places than where you could pitch some thing larger and you don't have to go as far to be away from prying eyes.
I've both and appreciate their varies strengths and weaknesses.
Theres a place for both.

I have to disagree on the weight difference. My bivy weighs 7 ounces and if rain is likely I take a 7 ounce tarp and 2 ounces of stakes and cords. So for about a pound, I have a very nice shelter system. This without going for high tech high $$$ stuff like cuben fiber which could cut the weight a good bit more.

The cost of my bivy setup is a good bit less than tents at the lightest end of the weight range.

There is also a lot of flexibility with a bivy. I sleep on top of it when it is warm, but as it cools I can climb inside. If I expect bad weather I pitch the tarp but otherwise don't bother. If rain comes up unexpectedly I just pull the tarp out and pull it over me and my gear.

I still might take a light tent for some trips depending on the expected conditions, but the bivy has been my choice for my last several bike tours and backpacking trips and it has worked out very well.

Wow, that is light.
Seems you turn your head and sneeze these days a newer lighter version of something has snuck past you.
I think my bivy is over a pound just for the bivy.
I think my previous one was close to two pounds.
I'm impressed.
Of course I concur with your informed assessment about weight.
As for sleeping on top it, I tend to worry a bit about the spiders and snakes hereabouts and inside my bivy I sleep better feeling a little more secure.

I spotted a tent that impressed me recently with its stuffable low bulk and weight (26oz) in the
Tarp Tent Pro Trail:
Tarptent Ultralight Shelters

Although by the time I wear out my current two tents and bivy I imagine the weights will have halved yet again.
Very happy with my current gear but was pleasantly surprised with the obvious advances exampled by your bivy.
Thanks for the heads up

Great thread, i hope it's okay for me to steal it with a question.

I have been using a Bivysack from Carinthias, but have had a bit of condensation issues. Therefore I'm thinking of switching over to a hooped bivy with a mosquito net, and just wanted to know if anyone has any experience with one?

Recently i stumbled upon the Uber bivy, and think it looks good, though maybe a little easy to break? MilesGear - Uber Bivy
Otherwise maybe someone tried the Dutch army one? Dutch Bivi Bag Hooped Bivvy Type Army One 1 Men Tent Shelter MVP L XL | eBay

Thank you in advance