>>455428It all depends on how you design the bivy bag. Mine was homemade, like I said, and I didn't really address that issue with my design. Some people attach a little cord to each end that they tie to their tarp to keep the top fabric taught and away from their face. Some people don't use bug netting on top in their design, but rather a flap of breathable nylon ripstop.
In my experience, I've found that even with the bug netting laying against my face, mosquitoes don't bite me through it. In the fall and spring, you really don't even need to have a bivy that seals up like that, though, since there wouldn't be any mosquitoes flying around at that time of year. Cold weather camping is great for this reason. I am mainly just concerned about keeping the creepy crawlies out. I've done my fair share of camping in the Rocky Mountains with just a tarp and no bivy, and sometimes you do find a spider crawling around on your body in the morning, sometimes even inside your clothing. Alpine arachnids are very tame, but if I'm in the desert or in temperate regions where spider bites can cause serious pain or necrosis, I absolutely don't want spiders crawling on my body during the night.
