The other night, I slept outside in the backyard while testing some gear. Temps hovered around 32 F. What I noticed is that putting on a light down jacket (Mountain Hardwear Ghost Whisperer) in a down bag (Zpacks 20) seemed to make no perceivable difference in comfort/warmth. I was wondering if this was due to compression of the down bag due to bulk, or perhaps because the thickness of the jacket was negligent relative to the down bag. This led me to think carefully about down loft and how one stays warm. Is warmth simply a function of the inches of down on top of the sleeper? In that case, I'm wondering whether I would have been better off layering the jacket on top of me instead of wearing it, or perhaps not wearing it at all. It seems that perhaps there is a fine line between too much room in one's sleeping bag and too little: too much means more air to heat up; too little means compression of down. I'm just curious how much the compression factor plays a role here…
