Supposedly Down (and most other sleeping bag insulations) lose a lot of their thermal insulating properties when they are compressed.
When I'm inside a sleeping bag, obviously whichever side is on the ground is greatly compressed by my body weight.
Any ideas of how to reduce this?
I don't think it is a huge problem for most people. In a certain sense it is flawed by design; at least if you are concerned by weight saving.
You could use a top quilt. If you want extra insulation, you could also get a down-filled sleeping pad such as the DownMat UL.
Some bags will have variable filling with less down underneath for that exact reason.
If you always use a sleeping pad, some sleeping bags (e.g. Spoonbill UL) will simply have no bottom down fill based on that assumption.
Agree with ppl, don't think it is a big problem, but even for variable filling bags it could mean that some of that expensive down may not be used the way you would prefer.
A quilt may be a bit draugthy if you are not careful. I find the bottomless bags work well for 3 seasons.
An(other) option there is a Rab "Backless" bag; have tried the predecessor, the Q-Top. Alternatively one could sew in a piece of thin nylon fabric under a quilt fairly easily.
What I really wanted to add was that if you don't squirm around too much at night, the best all-rounder might be a sleeping bag without baffles in the rings of fabric you lie inside, ie. only baffles in the head-toe direction (Western Mountaineering calls it "continuous baffle" design).
With those you are free to turn it upside down and shake all or as much of the down as you like to the top part of the bag. So the same bag could be bottomless or have slightly more insulation underneath, depending on what you need.
