After far too long I finally managed to break away for an overnight trip. It came up quickly so I threw a bunch of stuff in my pack and got outta Dodge to beat the morning traffic. At the trailhead I picked what I really needed for the next 24 hours and stuffed it into my day pack. It only weighted a few pounds and I realized that I'd gladly trade ounces for cubic inches to make the pack even smaller or have the option of extending the trip. For example, I wished I'd brought my old sil-nylon tarp because it packs smaller than my DCF model even though it weighs a little more. It's harder to quantify gear volume in a meaningful way because shape matters but I've started to think of a way to put together a tiny 3-day kit. I just wish someone would invent an inflatable sleeping bag that packed down to half the size even if it came out at twice the weight.

thoughts?

Down is extremely compressible, warm, and much lighter to boot. Of course a good down bag will definitely lighten your bank account!

I think it would be possible to create an inflatable sleeping bag lighter than a down bag, but the problem is it wouldn't breath. You'd be drenched in sweat by the morning.

My down sleeping bag packs smaller than my matress pad. Sure the matress pad needs to support itself with me on top of it, but it does need to be made more robust simply because the fabric needs to be air tight. My sleeping bag can be made with extremely lightweight fabrics because its only purpose is to keep the down in place.

As mentioned above, having a breathable sleeping has comfort advantages, too. What you are proposing would create a vapor barier layer (VBL). Lots of people use them (particularily in extreme situation extra ultralight backpacking). They are very efficient at keeping heat in your body but are quite challenging to manage body moisture. (You'll make it through the night, but it won't be fun unless you have everything nailed down just right.)

I think down will still (almost always) be a more compact system (you can add a vbl to down, or just make a bag out of waterproof fabrics).

You guys are right, down seems optimal. My bag was bigger than it needed to be this last trip because it was rated for colder than needed and I was probably too conservative about compression.