When is a bivy sack necessary?

"The solution is to get out a lot and figure out what works for each of us."

The beauty of BPL forums is we get to build knowledge from the experience of others that do get a chance to get out a lot. I am a perfect example of someone that doesn't get out a lot (maybe 10 nights a year), and for that I am very thankful for the knowledge/experience of many on this site like Nick so I have a much better shot at getting the right gear for me the first time.

Reading this I was reminded of the Thomas Edison quote, "I have not failed. I've just found 10,000 ways that won't work."

Start testing in the back yard, then the mountainside :)

Bivy seems to add about 5 degrees more warmth for me. Keeps thugs in place. Light bivy like a ZPacks bivy at 4 oz is very useful.

I would have said a bit more than 5 degrees are added by the bivy, but it probably depends on how windy it is and how well sheltered you are. I do know that when cowboy camping on top of the bivy, if it turns cold and windy, climbing into the bivy and zipping up makes a big difference.

I have found that for me the sleeping bag and bivy combo I use is lighter than the heavier bag that I would probably take without the bivy, so the bivy actually saves rather than adds weight for me with my particular gear. At least that is the case for warmish weather where a cold night (15-35 F) is possible/likely. That description seems to match most of the trips I have done.