My factor isn't so much where I use my insulation as much as it is where I use my money and how many items I have. I own three sleeping bags. One was purchased for "camping out" in my parents' living room when I go to visit. It is a heavy, rectangular bag that may or may not keep me warm down to +25 or so. I will probably never take it out, unless it's an overnight car camp. The second bag is a North Face down-filled which I got from a friend. I haven't cleaned it or tried it out yet, but it is a long, and I am short, so I am not overly hopeful.

The third bag is a Coleman +20 (which means +30-35, right?), which I got on clearance for $5. With synthetic longjohns, in a hammock but no tarp, I got it down to +15 with only cold toes. I think that, were I to add a layer of insulation below the bag (between bag and hammock), my tarp, a pair of socks, and a toboggan, I could get it down to +10 or perhaps even +5. Crazy, I know, but I can't deny what the thermometer said that morning. I've had it out in +25-30 temps with no problems, either.

The thing is, even though I know that there are some really great down bags out there, I cannot justify plunking down the Benjamins for one, when I have one that will do what I want. Also my usual haunts are rarely that cold. In summer it *might* get down to 85 at night, which means I won't even be taking a bag. Just a light blanket and a sheet. If I think I need to get it down colder, I can either take an additional layer of clothing, or maybe a bag liner (which serves the dual purpose of being a warm-weather bag -- a win-win for me).

From what I've read, bag liners are more hassle than help. So I may buy one for a warm-weather bag, and experiment with it in my back yard come winter.

Just my two cents. That an a couple dollars will buy you a mocha latte.