Sleeping Warm Is a Question of the Mat

Lie down on cold ground in a thick sleeping bag and you will be cold, because the insulation underneath you is compressed to nothing by your own weight. This is the single most common misunderstanding in camping, and it explains almost every uncomfortable night that was blamed on the bag.

The mat is the first decision

Sleeping mats are rated by R-value, a measure of thermal resistance, and the numbers are now standardised across manufacturers so they can be compared honestly. As a working guide: R-value 2 for summer, 3 to 4 for three seasons in Europe, 5 to 6 for freezing conditions and above 6 for snow and serious winter. R-values add up, which is why a closed cell foam mat under an inflatable one is the classic winter solution: cheap, indestructible, and it keeps you off the ground if the inflatable fails.

Width and length matter more than people expect. A mat that leaves your shoulders or feet on the ground has a cold spot exactly where you will notice it, and a mat that is too narrow means waking up whenever you roll.

Then the bag

European bags carry three temperature figures. Comfort is the temperature at which an average adult woman sleeps comfortably; limit is where an average adult man is curled up and just about coping; extreme is a survival figure that should be ignored entirely for planning. Use the comfort rating, and treat it as optimistic if you sleep cold, are tired, or have not eaten properly.

Down gives the best warmth for weight and packs smaller, but it is useless when wet and needs care. Synthetic is heavier and bulkier, insulates when damp and costs less, which makes it the sensible choice for humid climates, for boats and for anybody who will not keep it dry. For a vehicle, where weight and space are less critical, a generously rated synthetic bag is often the more practical answer.

The ground takes heat from you all night, without stopping, and the sleeping bag underneath you is doing nothing at all.

The things that add warmth for almost nothing

• A liner: silk or merino, adds a few degrees, keeps the bag clean and washes easily.
• A hat: the head is the one part you cannot pull inside the bag. A thin beanie is worth more than a whole grade of bag rating.
• Dry socks and base layer, kept only for sleeping: damp clothing costs you warmth all night.
• A hot bottle: a litre of hot water in a robust bottle, in the bag half an hour before you get in, warms a bag that you will then keep warm.
• Food: a proper meal before sleeping is fuel for the body's own heating. Going to bed hungry is going to bed cold.

Condensation, the invisible thief

Every night you release around half a litre of water into the tent as vapour. If it cannot escape it condenses on the inside of the flysheet or, worse, into the insulation of your bag, which loses loft and warmth night by night. Ventilate even when it is cold, keep the bag off the tent wall, and air the bag every morning in the sun or the wind for twenty minutes. On a long trip this single habit keeps a down bag working for months.

In a vehicle or a rooftop tent

Sleeping in a vehicle solves wind and insects but adds a metal box that conducts heat and collects condensation. Insulate the windows with fitted panels, ventilate at both ends, and put the same R-value discipline into the mattress that you would into a mat, because a thin foam mattress on a metal floor is exactly the cold ground problem in a different costume.

Rooftop tents are warmer than the ground in cold weather because the air underneath them is not conducting heat away, and colder in wind because they sit up in it. The built-in mattresses are usually inadequate below about five degrees; adding a proper mat on top is the standard fix.

Questions we get asked

What R-value do I need?

Three to four for spring to autumn in most of Europe, five or more once you expect frost, and six or above on snow. Two mats stacked add their values together, which is the cheapest route to a high number.

Quilt or sleeping bag?

Quilts save weight and work well for warm sleepers who do not move much, since the insulation under you does nothing anyway. Bags are more forgiving in wind, in cold, and for people who turn over all night.

How do I stop waking up cold at three in the morning?

It is usually the mat, the hat or the meal. Check the R-value first, wear a hat, eat properly, and use a hot bottle. If it persists, the bag is rated at its limit rather than its comfort figure.