Sleeping mats: understanding R-value so you stop feeling cold at night

What R-value measures

Midnight, beside a lake. The bag is a good one, the tent is properly pitched, and yet the cold seeps up through your back and hips, wherever your body presses down. The bag can do nothing about it: squashed under your weight, its down no longer insulates. The whole night comes down to the few centimetres between you and the ground.

That is what R-value measures: a material’s thermal resistance, in other words how well it slows the passage of heat. The higher the figure, the better the mat insulates [1]. There is nothing exotic about it: engineers use the same measure for windows and fibreglass [1]. Lying directly on the ground, you lose heat by conduction, and compressed down no longer forms an insulating layer: hence the importance of the mat [1].

The ASTM F3340 standard: comparable figures at last

For years, manufacturers did not all measure R-value with the same test, and some rated their mats with temperature ranges or other subjective measures, which led to some fanciful claims. In 2016, Therm-a-Rest and a group of other mat makers began developing a common method, which became the ASTM F3340-18 standard: it allows the R-values of two mats to be compared directly [1].

The principle of the test is simple. The mat is placed between two metal plates, one of which is kept at a constant temperature by a measured electric current. The better the mat insulates, the less energy it takes to keep that plate warm, and that energy is converted into an R-value. At Therm-a-Rest, the test is carried out in a cold chamber at 4 °C [1].

Enlarge diagram

Foam or inflatable: the rough figures

Foam insulates at roughly R 3 per inch of thickness, according to Therm-a-Rest [1], or about R 1.2 per centimetre: a 2 cm foam mat therefore comes out at around R 2.4. Older inflatable mats, designed before 2000 for comfort rather than insulation, mostly stay below R 2 [1]. Recent inflatables, with built-in insulation or reflective films, go much higher: Therm-a-Rest quotes R 4.5 for its NeoAir XLite and R 7.3 for its NeoAir XTherm [1].

Why the mat matters as much as the bag

Sleeping bag temperatures are measured in a laboratory, on a manikin lying on a mat. As Exped points out, ISO-rated bags are tested on mats with an R-value of 7, and the brand recommends at least 6.5 for your bag to deliver the warmth it is rated for [2]. A good bag on a mediocre mat therefore falls short of its promise, as our guide to sleeping bag temperature ratings explains.

The reverse is also true: Therm-a-Rest notes that with a warmer mat, you can sometimes keep a three-season bag for colder nights rather than carrying a bulky winter bag [1]. And the weight penalty is small: at Therm-a-Rest, moving up from the NeoAir XLite to the NeoAir XTherm adds about 70 grams, far less than a bulky winter bag [1].

Stacking two mats for winter

Two insulating layers laid one on top of the other add their thermal resistances together: that is physics, and the best news in this guide. An R 2 foam mat under an R 4.5 inflatable gives about R 6.5, the level Exped recommends. For the sum to work, the two mats need to lie flat against each other, with no air pocket shifting about during the night.

The pairing has another advantage. The foam protects the inflatable from stones and thorns, and if the inflatable punctures in the middle of the night, there is still an insulating layer between you and the snow. In winter, that is lightweight insurance.

A good habit to get into: choose your mat together with your sleeping bag, never afterwards, and look at its R-value before its weight.

Worth watching

Therm-a-Rest, one of the brands behind the ASTM F3340 standard, explains what R-value measures, how it is tested and how to use the figure to choose a mat.

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Guillaume Hoarau

Camping, wild camping, bushcraft and hiking

Guillaume Hoarau writes for Outfarer about camping, wild camping, bushcraft and hiking. A survival-skills enthusiast and a collector of old field manuals, he likes clear explanations, lists that earn their place and other people’s stories, which he tells well.

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