How to stay warm camping: sleeping bag ratings, sleeping mat R-value and condensation

TUREFF• •0

A cold night is the most common reason people give up on camping. It is usually fixable, and rarely by buying the thickest sleeping bag you can find. Warmth at night comes from a bag rated for the conditions, a mat that insulates you from the ground, and a few habits.

Sleeping bag temperature ratings

Many bags sold in Europe and elsewhere are tested to ISO 23537-1, the standard that replaced the older EN 13537. A thermal mannequin is used in a lab to estimate three figures:

Comfort: the temperature at which a typical sleeper can sleep relaxed. This is often treated as the figure for people who tend to feel the cold.
Lower limit (limit of comfort): the lowest temperature at which a typical sleeper, curled up, can sleep through the night without being cold. This is often treated as the figure for people who sleep warm.
Extreme: a survival figure only. At this temperature there is a serious risk of hypothermia. Never choose a bag based on it.

Different brands describe the reference sleepers in slightly different ways, and some bags, such as quilts and bags for very low temperatures, fall outside the standard's scope, so treat these as estimates for comparison rather than promises. Your own warmth also depends on how tired and well-fed you are, what you wear, the mat underneath you, the wind and the humidity. A practical approach: look at the lowest night-time temperature you expect, and choose a bag whose comfort rating is at or below it if you sleep cold, or whose lower limit is at or below it if you sleep warm.

Down or synthetic

Down is the warmest for its weight and packs small, but loses much of its warmth when wet.
Synthetic fill is bulkier but keeps more warmth when damp, and is usually cheaper and easier to wash.

The sleeping mat matters as much as the bag

The fill underneath you is crushed flat by your weight, so the ground takes heat from you through the bag. A mat stops this. Its insulation is described by an R-value: the higher the number, the more it insulates. Many brands now test R-values to a common standard, ASTM F3340, which makes mats from different makers comparable. Under that standard, R-values add up, so a closed-cell foam mat under an inflatable one gives roughly the sum of the two. One maker that helped develop the standard suggests rough ranges of about 1 to 2 for summer, 2 to 4 for three-season use and 4 to 6 for winter, though needs vary. Not every brand uses the standard, so compare with care. An airbed without insulation is one of the coldest things you can sleep on, because the air inside it circulates and carries heat away. If you use one, put a blanket or foam mat on top.

Layering for the night

Wear dry clothes to bed, not the ones you sweated in during the day. A base layer, warm socks and a hat make a big difference.
Do not overdress: wearing your thickest jacket inside a bag can compress the fill and leave you sweaty. Light layers are better.
Fill the gaps: spare clothes stuffed around you remove cold air pockets.
Close the hood and draught collar on cold nights; much heat escapes around the head and shoulders.

Habits that help

Eat a proper evening meal and keep a snack by the bed. Your body needs fuel to make heat.
Warm up before getting into the bag: a short walk or a few minutes of moving about.
Go to the toilet before bed. A full bladder makes you colder, and a trip out at 3am is worse.
A bottle of hot water in a sock, inside the bag, is an old trick that works. Make sure the lid is tight and the bottle is made for hot liquids.

Condensation

Waking to a damp tent is usually condensation, not a leak. You breathe out a surprising amount of moisture overnight, and it settles on the cold inside of the flysheet.

Ventilate: open the vents and, if the weather allows, part of the door, even on cold nights.
Pitch sensibly: hollows by water and damp, still valleys are the worst places. A slight breeze helps.
Keep wet things out of the inner tent, in the porch.
Wipe the walls with a small cloth in the morning, and keep your bag from pressing against them.
Dry everything out during the day or at home.

What keeps you warm on cold nights? Share your tips or ask about your own setup in the comments.