The one idea that runs through this whole topic#
Warmth at night is a heat balance. Your body produces roughly 60 to 90 watts while asleep. Everything in your kit exists to slow the rate at which that heat leaves, and it leaves by four routes: conduction into the ground, convection through drafts and moving air, radiation to the sky and cold surfaces, and evaporation from skin and breath.
Almost every cold night has one dominant cause, and it is usually not the bag. Conduction into the ground is the route people under-insure, because a pad looks like a comfort item and its label is a number most buyers have never had explained. Evaporation is the one that creeps up over consecutive nights, as fill absorbs moisture and quietly loses loft.
Read the pages below in the order they are listed and you get the full system. Read one page and make it the ratings guide, because it explains why the number on your bag probably describes a different person in different conditions.
The four decisions, in the order that matters#
- Pad R-value first. It sets the floor under everything else and cannot be compensated for with a warmer bag. See sleeping pad R-value.
- Comfort rating second, with a margin. Convert whatever number is on the label into a comfort figure, then leave about 3 degrees C of headroom. See sleeping bag temperature ratings.
- Fill type third, decided by how wet the trip can get and how easily you can dry things. See down or synthetic.
- Everything else fourth, which is where liners, sleep clothing, hot drinks and site choice live. Those are worth several degrees combined and cost almost nothing. See why you sleep cold.
If you are buying rather than diagnosing, how to choose a sleeping bag runs the same decisions as a purchase sequence, and quilts against sleeping bags covers the one substitution that changes the arithmetic.
Frequently asked questions#
What temperature rating do I need?#
Start from the forecast low for the coldest night you plan to camp on, subtract 3 degrees C of margin, and buy a bag whose comfort rating meets that figure. Then check the pad separately. Our sleep system calculator does this conversion, including the adjustment from a limit rating to a comfort rating.
Is an expensive sleeping bag worth it?#
Up to a point, and the point is loft per gram. Higher fill power and box baffle construction buy warmth for less weight and less packed volume, which matters when you carry the bag. If it lives in a car boot, a heavier bag with the same comfort rating does exactly the same job for less money.
Do I need a different bag for summer and winter?#
Most people are better served by one three-season bag plus adjustments: a liner, sleep clothing and a warmer pad extend a single bag by 5 to 8 degrees C. A second bag becomes worthwhile when your trips genuinely span 25 degrees of night temperature, at which point a summer quilt and a winter bag beat one compromise.
How long should a sleeping bag last?#
A down bag that is stored uncompressed, washed when it needs it and kept out of long-term damp will keep most of its loft for a decade or more. Synthetic fill loses loft measurably faster under compression. In both cases, storage does more damage than use, which is covered in storing gear between trips.