I like the pyramid floorless shelters for winters.
You can get a shelter that will sleep up to six that
weighs less than 3 pounds (not including the pole).

Disclosure -- (I make some kinds of them for sale to schools)

Here is a discussion on their use at another site.

http://www.backpackinglight.com/cgi-bin/...ost=80229#80229

Having used Wiggy's bags instructing at Outward Bound, we found
they were not worth the extra money or weight. They lost loft just
as fast as the the $60 slumberjacks, and proved no warmer.

Hot
temperatures in particular are hard on the silicone coating on the
high lofting synthetic fibers. So wash em in cool water and do not
use heat when drying.

My wife owns an REI down bag from the 50's that still has
extraordinary loft. Down lasts a long time.