Someone who slept badly on their first camping trip almost always blames the bag. They were cold, the bag said three season, therefore the bag is a liar. Occasionally that is true. Far more often they were lying on a thin foam mat on cold ground, and the bag never had a chance.
Air is a poor conductor of heat, which is why a bag full of trapped air keeps you warm. The ground is a much better one, and it is pressed against you, and the insulation underneath you is crushed flat by your own weight. Crushed insulation does not insulate. So everything your bag does on top of you, the ground quietly undoes underneath.
R-value, and how much you need
A mat's R-value describes how well it resists heat flowing through it. Higher is warmer, and since the industry settled on a common test standard the numbers are broadly comparable between makers, which they were not before. The other useful property is that R-values stack: put two mats together and you get roughly the sum of the two.
| Around R2 | Warm summer nights on grass. |
|---|---|
| Around R3 to R4 | Three-season use for most people in most places. |
| R5 and above | Below freezing, frozen ground, or sleeping on snow. |
| Two mats stacked | A cheap closed-cell foam mat under an inflatable adds R and survives a puncture. |
That last row is the trick worth knowing. A thin foam mat costs very little, weighs a few ounces, and does two jobs: it lifts your winter warmth into a range an inflatable alone will not reach, and it means a torn air mat at midnight is an uncomfortable night rather than a dangerous one.
Do not inflate an air mat rock hard. A firm mat gives you pressure points at hip and shoulder, and it is why people conclude they cannot sleep on the ground. Blow it up fully, then let a little out until your hip just fails to touch the floor when you lie on your side.
What a temperature rating is promising
Bags tested to the international standard carry more than one number, and they mean quite different things. Comfort is the temperature at which a standard cold sleeper should sleep comfortably. Limit is the temperature at which a standard warm sleeper is curled up and just holding on. Extreme is a survival figure, six hours without dying, with a real risk of hypothermia and frostbite in the process.
Buy on comfort. The extreme rating is not a rating of anything you would choose to experience, and a bag advertised on that number is being marketed at you rather than described to you.
The test also assumes things that people then do not do. It assumes you are on an insulated mat, wearing a base layer, with the hood up and the neck baffle closed. Sleep in a t-shirt on a thin mat with the hood flopped back and you will not see the temperature on the label, and the bag is not the reason.
Fit matters in both directions. Too much space and you spend the night heating air you do not need; too little and your shoulders and feet compress the insulation against the fabric, making cold spots exactly where you feel them most. A bag you can wear a light layer inside without pressing on the walls is about right.
Down or synthetic
Down is warmer for its weight, packs far smaller, and lasts longer if you look after it. It also loses most of its loft when wet, and takes a long time to dry in the field. Synthetic is heavier and bulkier for the same warmth, but it keeps working damp and costs less. If your camping happens somewhere reliably wet, or in a tent that gets condensation, synthetic removes a whole category of bad night.
The ten minutes before you get in
A sleeping bag is not a heater. It only holds the heat you bring, so getting into one already cold is a decision to stay cold, sometimes for hours.
- Eat something before bed. Digestion generates heat, and a long gap between dinner and sleep is a common reason for waking cold at two in the morning.
- Get warm first. Walk around camp, do something brisk for a minute, or have a hot drink. Get in warm, not hopeful.
- Change into dry clothes you have not walked in, including dry socks. Damp base layers turn a good bag into a bad one.
- Put a hat on. It is the easiest temperature control you have once you are horizontal.
- A leakproof bottle of hot water at your feet works remarkably well. Use one actually rated for boiling water, and check the lid twice.
- Empty your bladder before you commit. The physiological argument for this is overstated; the argument about lying awake for an hour negotiating with yourself is not.
Condensation, which is not a leak
You breathe out a surprising amount of water overnight. In a tent, most of it condenses on the coldest surface it can find, which is the inside of the flysheet, and in the morning it drips. This is normal and is not a fault in the tent.
The fix is airflow, even when it feels wrong. Leave vents open, keep the door cracked, and do not seal yourself in. Do not breathe down inside the bag, which puts that moisture straight into your insulation, and keep wet clothes out of the bag rather than trying to dry them with your body.
The reasons that have nothing to do with warmth
- Pitch on ground you have actually looked at. Clear the stones, find flat, and sleep with your head slightly uphill rather than your feet.
- Avoid hollows, where cold air pools overnight, and avoid the crest of a rise, where the wind will flap the fly next to your ear all night.
- Bring a real pillow or a stuff sack packed with clothing. Improvising with a wadded jacket is why people wake with a stiff neck.
- Earplugs and an eye mask are not soft. Nylon in wind is loud, and in summer it gets light hours before you want it to.
- Expect the first night to be poor. Sleeping lightly somewhere unfamiliar is normal, and it usually settles by the second or third night.
If you camp a few times a year and one thing is going to change, make it the mat. A better bag improves a night you were already having. A better mat removes the reason the night was bad.