This info came with my sleep sacks, thought it might go well with this thread. They do suggest a blanket for very cold conditions but as they also describe the 2.5tog sack as regular weight I wonder if its possible in colder climates to buy a heavy weight one of maybe 3.5tog? This info is for UK climate.
Room temp Which bag? What to wear
25degC + sheet sleeping bag
22-25degC 1.5 tog cotton bodysuit
20-21degC 2.5 tog cotton bodysuit
17-19degC 2.5 tog cotton bodysuit & sleepsuit
14-16degC 2.5 tog cotton bodysuit & sleepsuit & spare blanket*
*I guess as the concern here is the blanket, instead of this, as suggested above extra socks or Tshirt or both would most likely be a good replacement. Warm feet warm everywhere, cold feet freezing everywhere, thats my experience.
newmum
PS. very scary stuff my LO slept in a blanket, wrapped not swaddled for first couple of months then just a muslin for a few months because of heat wave then back to blanket until she was at least 5-6months old before I got her first sleep sack!