>>1245441Yeah basically. Wear and tear in use, stuffing, moisture, and UV rays breaking down the outer material. Dirt/grime/oils of the skin will reduce loft as well. No real way to avoid that though, that's what they're for. As long as you properly air it out if you're camping for a long time, and dry/hang it when you get home, it shouldn't lose too much loft.
Of course, any bag will lose loft over time even stored in optimal conditions. It would take many years though. The protein in the feathers breaks down over time.
A down bag, moderately used and properly cared for, will probably still lose 20-40% of it's loft over 10 years. It'll still be usable, just not quite as warm.
