I experience a variety of weather, a third of year it rains getting a total of 40 inches of rain annually. The summers for the most part are quite mild, but I've seen nights get down to 45 degrees. Our winters are also quite mild and dry as well, we've only just gotten our first snow a week ago.
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I don't think bivy sacks, used on their own, make any sense unless you are in the western high mountains and dealing with passing afternoon rain. More specifically, a bivy on its own is a lousy way to deal with an all-day/night rain or with horrible bugs.
Based on where you live and on the assumption of day long rains and bugs, I would suggest 1 of 3 solutions. All of them are going to be in the 2-3 lb range, minimally, more if you want more space or protection. They are:
Bivy + Tarp or Single walled tarp/tent: The tarp/fly keeps of the majority of the rain. The bivy protects the bag from run off under you or condensation under the tarp. A bivy with integrated bug net gives some (stuffy) protection from bugs. Alternatively, just pack a head net. Not the best solution for creepy-crawliies like scorpions, ticks or snakes. But it's nice and close to nature. The military system is of this kind.
Ultra-light tent: This will have bathtub floor to keep out run off and creepies, a net inner tent and a rain fly. You can use these in full winter conditions just fine, so long as you aren't above treeline. Non-free standing tents will lower weight.
Hammock + Tarp: This is useful if you deal with rough terrain where finding a spot to pitch a tent or even lay down a bivy is hard. These are getting popular here in New England for camping up high in the Boreal, where the ground is too steep, rough and fragile to make even bivy/tarp combinations feasible. I could this approach making sense for swamp camping.
The tent approach is the most traditional and the most psychologically safe feeling. The tarp experience is more open and exposed feeling. I like tarps so there you have it. I consider a bivy to be an essential part of my tarp system.