Sleeping well when camping in spring and autumn

At 8 °C you do not sleep at 8 °C: you sleep at the temperature of whatever is underneath you, and that is nearly always colder. The short answer is three points. The mat needs an R-value of at least 3 for nights close to 5 °C; the bag needs a comfort rating near the forecast low, not near its limit; and what you wear to bed must be dry. The rest of this article explains why, and what can be put right without buying anything new.

Why 8 °C bites harder than the number

Five things add up, and none of them shows on the thermometer.

The ground. Under the sleeper the bag’s filling is crushed flat and insulates almost nothing; only the mat protects you. In spring the soil is often colder than the air early in the night, and damper than it was in summer.

Damp. Air close to saturation, wet grass, dew on the fabric: water carries heat away far better than dry air does. A bag that has taken on moisture insulates less, especially a synthetic fill that was not dried properly, or down that has begun to collapse.

Duration. In late October or early March the night lasts twelve hours or more, against seven or eight in July. The body has time to draw on its reserves, and the lowest point comes just before dawn, when sleep is lightest.

Tiredness. After a day’s walking or driving, a body at rest produces less heat, especially if you went to bed without eating. A hot meal and a snack before sleep belong on the kit list.

Condensation. A tent shut like a box sends the vapour from your breath back onto the walls — several hundred millilitres a night for two people. The detail is in the winter camping guide; in the shoulder seasons the trouble is not frost but the drop that falls on your bag.

Reading the standard honestly

Bags are tested to EN 13537, now replaced by EN ISO 23537, which keeps the same three figures.

The values are measured on a mannequin wearing base layers, on a mat of stated insulation. A sleeper who feels the cold, a damp night or a thinner mat will give a different result. For a forecast of 8 °C, a bag with a 10 °C comfort rating has no margin; one rated 5 to 8 °C does. The full reading of these figures is in how to choose a sleeping bag.

R-value: which number for which night

R-value measures how much a mat resists the passage of heat; the higher it is, the less the ground takes from you. Since ASTM F3340, makers who follow it measure R-value the same way, so their data sheets can be compared with one another; an older mat, or one not tested to that method, compares less well. Values roughly add up: two layers of R 2 give close to R 4.

• Around 12 °C, dry ground: R 2 to 2.5 is enough for most sleepers.
• Around 8 °C: R 3 is a reasonable floor.
• Around 5 °C, or wet ground: R 3.5 to 4, more so for anyone who feels the cold easily.

A self-inflating or air mat of R 1.5 is perfectly good in July; in October it, far more than the bag, will make the bad night. The differences between mats and camp beds, including the way a raised bed lets cold air circulate underneath, are covered in air mattress or camp bed.

Fix what you own before you buy

A new bag is a big outlay for three or four nights a year. Four things come first.

1. Add a closed-cell foam pad under the existing mat: R-values add up, and foam does not puncture.
2. Slip in a liner: a few extra degrees depending on the fabric, for modest weight and bulk.
3. Wear dedicated, dry night clothes, never the day’s. A hat and clean socks weigh little and hold heat where it escapes most.
4. Put the spare blanket or jacket over the bag, not under it: beneath you it is crushed flat; on top, it traps a layer of air.

What goes wrong

Summer bag and thin mat. The commonest combination: everything works in August, and nothing at 7 °C. The cold arrives through your back around two in the morning, and no extra layer on your upper body changes much.

Sleeping in damp clothes. A T-shirt that has half-dried on your skin chills the torso for hours. Dry underwear alone beats being fully dressed and wet.

The airtight tent. Every opening shut “to keep the warmth in”: you gain a degree or two and lose it again to moisture. Leave two vents open, one high and one low, and pitch the fly so it does not touch the inner tent. Ventilation criteria are in how to choose a camping tent.

A child with a different thermostat. Children cool faster than adults and do not always say so; the subject has its own guide, children’s sleep when camping.

When a second bag is not worth it

If you camp five or six nights a year below 12 °C, the better spend is almost always a more insulating mat, perhaps a liner, then a bag chosen for the comfort range you meet most often. A second bag makes sense if nights under 5 °C become regular, or if you often find that a summer bag pushed to its limit would make you cancel a trip. The other way round: if your current bag is comfortable down to 10 °C, a pad, a liner and dry clothes will handle most nights at 5-8 °C without buying anything.

Before you leave, one last check: spend a night at home on the mat, in the bag, with the window open. What you say about it in the morning is worth more than any data sheet.