Jun 19, 2019

What Is an R Value? | The New Standard Test for Sleeping Mats

It’s an exciting time for sleeping mats: for the first time there is a recognised Standard test for determining how warm they are. This means consumers can be sure of what they are buying and can compare brands’ models side-by-side with much greater certainty. Mountain Equipment are an early adopter of the Standard and our Aerostat mats are tested according to the new method.

WHAT IS AN R VALUE RATING?

Before the new Standard there was no one method of assessing how warm a sleeping mat was. A sleeping mat’s warmth – its thermal resistance – is defined by an R value, where a higher number means a warmer mat, but R values were determined in numerous different ways. Many manufacturers had their own method, making accurate comparisons between brands’ mats difficult, if not impossible. The project of creating a unified Standard for sleeping mats was a three year project involving numerous sleeping mat brands and some large retailers. Mountain Equipment was involved in some of the writing and consultation on the Standard. The Standard was finished in 2018.

ASTM F3340-18

The new Standard is known as ASTM F3340-18, “Standard Test Method for Thermal Resistance of Camping Mattresses Using a Guarded Hot Plate Apparatus”, and from Spring-Summer 2019 Mountain Equipment’s Aerostat mats bear an R value which are tested according to this method. Our self-inflating mats will also have R values determined by ASTM F3340. Most other manufacturers will certify their mats in time for 2020.

THE TEST

The test method itself is a lab test, which reduces much of the error associated with testing products on real people. A sleeping mat is held under pressure between an upper hot plate (representative of a person lying on top of the mat) and a cold plate under the mat (representative of cold ground). The test measures the energy required to keep the hot plate at a steady temperature, and by a series of calculations works out the R value and thus how insulating the mat is. Compared to testing, for example, a sleeping bag, it is a relatively simple test, but it still requires specialist and expensive equipment to carry out.

WHAT DOES THE TEST MEAN FOR ME?

The big breakthrough is that sleeping mats from different manufacturers can be compared side-by-side. It doesn’t mean that existing mats have inaccurate R values; just that they were tested using different methods and so their R values might be different.

ANY DOWNSIDES?

The Standard gives a measure of warmth while the mat is in an ideal position but it doesn’t account for factors like compression of the mat or sleeping posture which may impact on warmth. These factors will be minor for most mats, but if a mat is too small for you then it’ll clearly not be as warm as one that fits you. There may also be some types of mats which over- or under-perform in the test versus a user’s real perception, but this won’t be clear until plenty of mats have been tested.

What is an R Value? | Mountain Equipment

WHAT DOESN’T IT TEST?

It’s important to remember that ASTM F3340 only tells you a mat’s R value: its warmth. It doesn’t test how comfortable a mat is, how durable it will be, if it’s easy to inflate, nice to use, or pleasant to sleep on. In short, a high R value does not always equate to a good product, and for users in warm conditions they might not always want the very warmest mat. Also, ASTM F3340 only tests new sleeping mats, which may offer different R values to mats which have been used for a few weeks, as while the insulation in some mats is very durable, other mats’ insulation will degrade with time.

IN A NUTSHELL

Comparisons between sleeping mats should now be a lot easier, and if you remember that the test only measures how warm the mat is then it’s a really useful buying tool that should help when purchasing an important product.