Understanding sleeping pads and R-values

Heat always flows from warm to cold. If you lie directly on the ground, it flows out of your body into the ground through contact (conduction), across the whole area you are lying on. The ground is large and can absorb a lot of heat without warming up itself. That is why people often cool down more on cold ground than in air of the same temperature.

The sleeping bag helps little underneath: your weight squashes its fill so much that hardly any air stays in it. The sleeping pad takes over this job. It holds still air – in the cells of a foam, in air chambers or in a fill inside an air pad. Still air conducts heat poorly and therefore acts as a brake.

The R-value is a measure of this thermal resistance. Higher means the pad slows heat loss more. Because resistances in a row add up, two pads on top of each other give roughly the sum of their R-values. That is especially useful in winter.

In the past, manufacturers measured the R-value with their own methods. The figures from different manufacturers were therefore hard to compare. The ASTM F3340 standard established a uniform test method for camping mattresses, which many manufacturers have adopted. So only compare R-values determined under the same standard.

The R-value is not a temperature rating. It does not say down to what temperature you will sleep warm on a pad, because that also depends on your sleeping bag, your clothing, your body and how tired or hungry you are. People who feel the cold easily need more insulation than others.

The R-value also says nothing about comfort, durability or weight. A very well insulating pad can be heavy or delicate, a comfortable pad can insulate poorly. Choosing remains a trade-off between what matters to you and the conditions you expect.

Foam pads are made of closed-cell foam containing lots of small air bubbles. They are robust, cannot burst and need no care – but they are bulky and less comfortable. They can also be used as a sit pad during breaks or as protection under a more delicate pad.

Air pads are inflated and pack very small. They are often thick and comfortable. Their insulation comes from inner chambers, fills or reflective layers. Their drawback: a thorn, a sharp stone or a spark can make a hole, and then you are suddenly lying on the cold ground. So a patch kit always goes with them.

Self-inflating pads are a middle way: inside is open-cell foam. When you open the valve, the foam expands and draws in air; with a few breaths you make the pad firmer. They are more comfortable than pure foam pads and less prone to punctures than pure air pads, but usually heavier than the latter.

In winter or on snow a combination has proved itself: a foam pad underneath and an air or self-inflating pad on top. The values roughly add up, the foam pad protects the upper pad from sharp things, and if the air pad does get a hole, you are still lying on foam instead of snow.

Some manufacturers advise against inflating air pads by mouth, because the moisture from your breath can condense inside and freeze in frost. They offer pump sacks for this – read the manufacturer's instructions. At home, store air and self-inflating pads as the manufacturer specifies, usually open, dry and not tightly rolled.

Without a pad, everyday things also insulate: cardboard, an empty backpack, a layer of clothing or a thick layer of dry leaves. They are not a full substitute but much better than bare ground. And do not invest only in a warm sleeping bag – a good pad is often the cheaper way to sleep warmer at night.

On snow, ice or frozen ground you can get badly chilled without enough insulation underneath, even in a good sleeping bag. Plan considerably more insulation for such nights than for summer and take a reserve, such as an extra foam pad.

Watch for warning signs of hypothermia: intense shivering, clumsiness, confusion or unusual drowsiness. Then act in time: insulate the person well from the ground, put on dry layers, protect them from wind and get help via the local emergency number (112 in the EU). Children often cool down faster than adults and need a good pad just as much.