The tent is a windbreak, not a heater
A tent traps a little warm air and blocks wind, but it has no insulation worth counting on. On a clear night the inside of a fly can be as cold as the outside air. Everything that keeps you warm comes from your pad, your bag and your body. If you are cold at 2 a.m., the tent is rarely the problem.
That is why two people in the same tent can have completely different nights. One has a pad rated for snow and a dry base layer; the other is on a thin foam mat in a cotton T-shirt.
Fix the ground before the bag
You lose heat in two directions: up into the air and down into the ground. The ground is usually colder. A bag rated to -7 degrees on a summer pad will leave your back cold all night.
Look for an R-value of at least 3 for three-season camping and 5 for snow. If you sleep cold, add 1 to that number. A cheap closed-cell foam pad under an inflatable adds about 2 R-value for very little money.
- R-value 2: summer only.
- R-value 3 to 4: three-season, most camping.
- R-value 5 and up: snow and winter.
- Add a foam mat underneath for a cheap boost.
Dry clothes, warm body
Change out of the clothes you hiked in, even if they only feel slightly damp. Sweat cools fast once you stop moving. Put on a dry base layer and dry socks before you get into the bag.
Wear a hat and keep the hood of the bag snug. Your head and neck lose a lot of heat, and a beanie is lighter than a warmer bag. Do not wear so many layers that you sweat inside the bag; damp insulation stops working.
Ventilate to stay dry
A fully sealed tent collects condensation. By morning your bag footbox is damp and your sleeping pad is slick. Open the top vent and leave a small gap at the door, even when it is cold.
Pitch with the door away from the wind and use both guylines. A tent that flaps all night loses heat and keeps you awake, and a taut pitch is warmer than a loose one.
Eat, drink and pre-warm
Have a hot drink and a small snack before bed. Digestion generates heat, and a body that is low on fuel shivers sooner. Avoid a large meal right before lying down, though, or you will be uncomfortable.
Fill a 1-liter bottle with hot water, wrap it in a sock, and put it at the foot of the bag for 20 minutes. It takes the edge off the first hour, which is when most people feel coldest. Do not use a bottle that cannot take boiling water.
The morning routine
Leave the bag only when you have your puffy and socks ready beside you. Never cook inside the tent or the vestibule: burning fuel produces carbon monoxide, which is colourless, odourless and has killed experienced campers in sealed tents. Cook outside in a sheltered spot, and get a hot drink into your hands within 5 minutes of waking.
Pack the bag last so it stays lofted until you are ready to leave. A bag compressed in a stuff sack overnight loses warmth, and the morning is when you need it most.
