Originally Posted by
niknak
Sorry to play Devil's advocate as I know you're an experienced tourist, but I can't imagine where you'd be able to use a bivy unless at a campground, in which case it sounds like you'll have a bed. The terrain is so rocky/volcanic, trying to tuck into a bivy and sleeping bag on the side of the road sounds like torture.
Also, the only bugs you'll see are around Myvatn. The island does a great job of blowing away almost all insects. The birds, however, are a different story. You'll need eyes in the back of your head!
A tarp or tent would be even more difficult to use away from campsites. My take is that I'd rather get into the bivy and zip up than wrap a siltarp around me. Putting a tarp (or tent) up on volcanic lava would be difficult, but if I have to rest it will be easy to get behind a rock and zip up the bivy. Say a nasty fog comes down and I have to stop riding in the middle of nowhere, then it will be nice to wait it out in the bivy. Probably not fantastically comfortable, but it would keep the wind and wet out well. Bugs are something that I hate, so I always have DEET and knowing my luck if I had to spend a night out it would be in the middle of a lot of bugs which is another reason for the bivy.
It really is an emergency thing and at 1lb and similar volume to the Thermolite I think it's the way to go if I don't have a tent and sleeping pad.