Posted by: willowleaf on Mar-14-13 9:48 AM (EST)
Thinking back to my long ago days selling gear at an outfitter in the heyday of backpacking in the US (the 1970's), another lightweight option a few manufacturers offered for tandem camping was top-over-bottom square bags where the two components were differing wieghts. You could have a 40 degree and a 20 degree bag and place whichever over you that suited the ambient temp. You could even get a bottom component that was uninsulated, basically a ground sheet to keep the top bag in place and eliminate drafts. Obviously your ground pad provided the insulation underneath. Some companies even made top components that zipped directly to the ground pad.
For mild weather camping I've also used the liner and fleece combo somebody else mentioned. I have a poly blend liner (like a folded over sheet stitched halfway up the side and with a pillow pocket) that I tuck inside a lightweight Polartec zippered bag -- it's quite comfy down to near freezing. Adding the Goretex bivy bag takes it down another 5 degrees, even more if you lay clothing like a fleece jacket and pants between the fleece bag and bivy for more insulation atop. The fleece bag compresses well and the liner is so tiny it fits in a daypack side pocket -- in fact you could probably stash it in a Nalgene bottle.
I've also found for packing that changing the stuffsack shape can make it easier to stash bulky sleeping bags. It is just as easy to stuff a sleeping bag in a long narrow tent sack as it is the usual short fat bags they sell with them (which are designed to be strapped to a backpack frame.) A long skinny flexible "wiener" is easier to snake into a kayak hatch or wrap around other gear in a portage bag.
Anyway, it's been interesting to hear the various strategies for camp bedding people are sharing.