Man, it looks like you have some incredible rides near you! Good stuff on your blog.

Presumably completely non-breathable? Don't you get a bit sweaty in there?

When it's colder, not too much, but otherwise it does get stuffy. A wicking base layer helps for comfort and the bivy opens up to about halfway down, like a normal mummy bag. Primaloft quilt might be a good option as well.

Would you happen to have a route mapped that we could see? This sounds like fun.

I think it's being a bit optimistic, too, or else others would probably be doing it already, but it'd be a killer light-weight kit. What got me thinking it MIGHT work, is that Sea to Summit makes a few different liners, one of which claims to add up to an additional 25* while still packing in a 5"x7" stuff sack.

Now if only I'd quit riding so much and go and make some money, I could try this out 

After using my Thermo-lite 2.0 Bivvy (http://www.rei.com/product/750938) on a few outings, I know it'll keep me comfortably warm to about 50* with a sleeping pad. I've been toying with the idea of layering that outside of something like a Sea to Summit Reactor liner bag (http://www.rei.com/product/797113) to be comfortable around freezing temps, and the breathable properties of the Sea to Summit bag would probably be a lot more comfortable than having just the Thermo-lite against my skin. It's rare for temps to drop too much below freezing here in Florida, and I'm thinking this setup would make a comfortable 3-season setup for most anywhere else in the SE.

The idea of packing my entire shelter and sleeping system in a smaller stuff sack than my 30* bag alone sounds really appealing, as does not relying on down to keep me warm if conditions are a bit wet (as usual down here). Anyone ever try anything like this?