Originally Posted by MassiveD

They sell 3 season bags that normally are designed for about 20 degrees to summer night temperatures in non-tropical regions. I find the numbers reasonable, though I would not use one to sleep at consistent temperatures of 20 F. If I hit those temps I would be dressed in my warmest clothes. However, if you follow Rowan's advice and you do what many people do, which is basically use a 3 season bag for the 6 months centred on mid summer, your bag will be too hot in most cases, and sleeping on top of it is a big drop in comfort and temperature protection. You are too hot, and are now dropping 2 inches of loft. Not much tapering there.

I am not saying rowan in wrong, because if you have to prepare for a month sleeping at 20, or a month sleeping at 75, you would be smarter to choose the warmer bag. But if you live in an area where the heating bill is greater than the cooling bill, and you are camping six months of the year centred on summer, go for the warmer weather bag.