Staying warm at night: the pad, the bag and the air between them

You lose heat in a tent by three routes, and a different piece of kit handles each. Conduction takes it into the ground, and the pad is the only thing that stops it. Convection takes it into moving air, and the bag stops that by trapping air in its loft. Evaporation takes it whenever moisture leaves your skin or clothing, and the tent — with what you wear — decides how much of that happens.

Knowing which route is doing the damage is most of the skill. Cold people blame the bag, because the bag is the thing touching them. Usually the bag is working hardest and the pad is failing, and a warmer bag is the most expensive way to not solve the problem.

A sleeping bag holds still air in its loft. Underneath you, your weight crushes that loft flat, so whatever the bag is rated for, the half beneath you has been switched off. The pad is the whole insulating layer on that side, and the ground is the better thief: it touches you directly, and cold damp earth pulls for as long as you lie on it.

Pad insulation is measured as an R-value under ASTM F3340, which is why figures from different brands compare at all. The pads here run from R 0.5 to R 8.5, median 3.4, and the inflating ones reach 10 cm thick; the thinnest are closed-cell foam at 0.3 cm. The rule of thumb is roughly R 1–2 for mild summer nights, R 2–3 down to freezing, R 3–5 for frost and R 5 upwards for snow; the detail is in sleeping pad R-values .

Two things follow. R-values add, so a foam mat of R 2 under an air pad of R 3 gives you R 5 — the cheapest way to push a pad you own into colder nights, and something to sleep on when the air pad punctures. And your bag's rating assumes a pad under you: ISO 23537 ratings are measured on a heated manikin, on an insulating mat, in a laboratory. That number is not a survival guarantee, but an estimate for a fed, dry, rested sleeper.

Side sleepers press hip and shoulder into a pad far harder than lying flat does, so one that is fine on your back can be cold on your side. Check yours against the pad pool .

Shutting every vent and zip feels right when it is cold, and it is the most counter-intuitive mistake in the subject. You put out water vapour all night through breath and skin, and so does anything damp in there with you. In a sealed tent it finds the coldest surface — the inside of the flysheet — and condenses. Then it drips, or you brush the fabric and it rains on you. Insulation that does not loft does not insulate.

Open the vents, and both doors a crack if you have two. You are trading warm wet air for cold dry air, and you feel that as a draught for a few minutes. Over a night you come out ahead, because a dry bag is a warm bag. The exception is wind chill straight onto you: vent away from the wind. Two doors make that easy, which is part of why the second earns its weight — see choosing a tent .

Two habits follow. Do not cook inside the tent, quite apart from the carbon monoxide risk, and do not breathe into your bag: both put the night's water vapour exactly where you least want it.

The clothes you walked in are the wrong ones to sleep in, even when they feel dry. They hold sweat, and evaporation from damp fabric takes heat out of you for as long as it lasts — inside a bag, all night. You lose heat steadily and cannot say why, because nothing feels wet.

Carry a dedicated sleep set: dry base layer top and bottom, dry socks, a hat. Keep it in a dry bag and never walk in it. It is the highest-return item in the pack for a cold sleeper.

Your body keeps releasing moisture while you sleep, and some passes into the bag and condenses in the outer insulation where the fabric is coldest. Down absorbs it and the clusters collapse, so the third night is colder than the first in the same bag. Synthetic fill suffers less and dries faster, which is the real argument in down or synthetic . The fix is cheap: each morning the weather allows, turn the bag inside out in sun or moving air for twenty minutes. Looking after your kit has the rest.

Cold air is denser than warm air, so it sinks and flows downhill like water, pooling in hollows, valley floors and the ground beside a river. These frost hollows can be several degrees colder than ground a short walk up the slope. The flat, sheltered, obvious spot at the bottom is often the coldest one available.

Move up. A bench a few metres above the low point, out of the drainage line, is usually the warmest ground within reach — but not so far that you end up on an exposed ridge, where wind costs more than the degrees you gained. On a clear night you also radiate heat to the open sky, which is why calm nights are the coldest.

• Avoid the lowest point in sight, and the ground beside water.
• Avoid hollows, and the uphill side of a wall where cold air backs up.
• Prefer dry ground to damp: wet soil conducts heat away faster.
• Take partial tree cover over open sky — but check what is overhead.
• Then pitch tight. A slack fly flaps, touches the inner, and moves air over you.

Most cold nights are decided before you get into the bag. This routine costs half an hour and fixes more than a kit upgrade.

1. 1

   Eat properly, and late

   Digestion generates heat for hours. A real meal with fat and slow carbohydrate in it, eaten near bedtime rather than at five in the afternoon, is the most effective single thing you can do. Keep a snack by your head for later.

2. 2

   Drink

   Dehydration impairs circulation and makes hands and feet harder to keep warm. A hot drink is pleasant; the water helps more.

3. 3

   Change into the dry set

   All of it, socks and hat included. Put the day's clothes in a stuff sack, not loose in the tent where they damp the air.

4. 4

   Loft the bag early

   Shake it out as soon as the tent is up. Compressed insulation takes time to recover, and a bag that goes under you flat has lost part of its rating.

5. 5

   Go before you get in

   Your body spends energy keeping a full bladder warm, and you will lie awake resisting the walk outside.

6. 6

   Warm the bag, do not just enter it

   A few minutes of star jumps — enough to feel warm, not to sweat — puts heat in for the bag to hold. Getting in cold means waiting an hour.

7. 7

   Close the neck baffle, then the hood

   Most of what a mummy bag loses goes out around the shoulders and the face opening. Leave a hole to breathe out of.

Do not lie there hoping. Shivering burns energy you need.

1. Check you are still on the pad. Sliding half off it is the commonest cause of waking up cold, and the cheapest to fix.
2. Eat the snack you left beside you. Fuel first — everything else works better afterwards.
3. Put a hat on, and dry socks if yours are damp.
4. Move. Thirty seconds of hard isometric tensing inside the bag generates real heat. Stop before sweating.
5. Add a layer, but a thin one. A puffy jacket inside a snug bag compresses the bag's own loft and can leave you colder.
6. Put something under your torso: the empty pack, a foam mat, spare clothing. The fastest fix when the pad is the problem, and it usually is.
7. A hot water bottle at the feet lasts hours — but only a sealing, heat-rated one, checked upside down outside.
8. If you cannot get warm, are shivering uncontrollably, or start to lose coordination or clear thinking, stop treating it as comfort: insulation, calories, warm fluid, and be willing to end the trip. Hypothermia does not announce itself politely.

If cold nights are a pattern, the kit is wrong for the trips you do. Work out what you need with the finder , or read sleeping bag temperature ratings for how much margin your bag has.