dskoon wrote:Yeah, temps really seem to vary bag to bag, even from the same manufacturer, etc.
Ex. the bag you speak of, my friend also has, Marmot Helium. However, he's complained about it not being quite as warm as the bag he was intending to replace(due to a little less weight), which was/is the Marmot Pinnacle. He claims the Pinnacle, also rated to 15f, is warmer than the Helium. I also have the Pinnacle, and though I haven't quite got it down to 15, I've been warm in it in low 20 temps on an icy night up in Yellowstone.
I think they also vary a lot from person to person, and the only real way to tell if your 15F bag is warm enough to take to 15F is to try it in the field. My Mountain Hardware Phantom 15 keeps me comfortable down to at least 15F, as indicated on my thermometer, if I'm sheltered from the wind and am wearing heavy thermal underwear, but I sleep very warm. I've been in a tent with my girlfriend where she's complaining about the cold in her OF North Face bag and I'm sweating in my 15F bag (not surprising as she always wants at least one more layer on the bed than I do).