2001: A Sleeping Mat Odyssey

Through the end of the last century, outdoor enthusiasts relied on one of two types of sleeping mats: thin closed-cell foam mats or slightly thicker self-inflating mats. Both provided some level of comfort and insulation from the cold hard ground, but neither did so with great effect. In 2001, EXPED introduced a revolutionary product - the DownMat – which created an entirely new category of sleeping mats – much warmer, lighter and more compact than previous mats. This new category necessitated a means of measuring and comparing the efficiency of the insulation, so we pioneered the use of R-value testing to determine the “warmth” of a sleeping mat. Almost 20 years later the outdoor gear companies have agreed upon an international standard method for testing the R-value of sleeping mats, based heavily on EXPED’s previous work with EMPA, the Swiss Federal Laboratories for Materials Science and Technology. 

WHY IS R-VALUE IMPORTANT? 

Image: EXPED founders Andi and Heidi Brun on their honeymoon: some cold night’s sleep led to the development of R-value for sleeping mats.

This was the genesis of R-value measurements in the outdoor industry. Why the obsession with warm sleeping mats? For the answer we need to travel back in time to a unique honeymoon in 1981, a chilly, months-long affair in the Canadian Wilderness. Heidi and Andi Brun, founders of EXPED, left Switzerland to seek alone-time and adventure, and they found it. During one of their sojourns away from camp, they spent 10 nights out in minus 40 degree temperatures on thin, 8mm closed cell foam mats. These were uncomfortable nights to say the least, but a seed was planted in Andi’s brain, and 20 years later he was developing his own range of sleeping mats for use in extreme environments. 

But before we go further, let’s define what R-value is and why it’s important. First, R-value is a measure of thermal resistance (that’s the “R” part) and is most commonly associated with the construction industry. When you buy insulation for your house, its efficiency has been tested and is expressed with a number: R-value. This allows insulation to be easily compared. Because it is resistance to heat transfer that is being measured, the higher the number, the more efficient it is at retaining heat. Without getting into the specifics, suffice it to say that R-value is a useful way to compare the insulation efficiency of different materials. So, having successfully patented a down-filled mattress, Andi set about trying to prove his idea to the outdoor world, successfully winning the unofficial “worst product in show”/ Lemon Award at the first trade show where he unveiled the mat (although later winning the official best-of-show Outdoor Award in 2002 at the Outdoor Trade Fair in Friedrichshafen). Andi could have thrown in the towel at that point, but instead he did what any self-respecting Swiss person would do in the face of such skepticism: he contacted the Swiss Federal Laboratories for Materials Science and Technology. At EMPA, they had a device for measuring thermal resistance according to an esoteric German standard that turned out to be perfect for benchmarking the thermal resistance of sleeping mats.

THE FIRST TEST

In 2001, way back before ASTM standards, when we got our results by mail (with a friendly Swiss German “Salü”) our first test result for the DownMat was a whopping 1.7 m2 × K / W or R9.5. In the end we adjusted the down fill for our target R-value of R8. 20 years later, in another lab, we get almost exactly the same result!

Image: a young Andi Brun figures out the R-value of his DownMat at EMPA

EMPA’s location in St. Gallen, Eastern Switzerland, was perfectly placed to develop the R-value measurement technique as it was only an hour from EXPED’s headquarters in Zurich. Since EXPED’s first R-value test, many more brands have developed insulated sleeping mats, and it was clear that some kind of standard test was needed to make it easier for people to choose sleeping mats. Some brands did their own testing, some used EMPA and some simply made it up! If you’ve been reading closely you may find it unsurprising that the new standard test bears more than passing resemblance to the original EMPA test ordered up by EXPED.  

HOW R-VALUE IS TESTED

In the test, the mattress is pressed between two special measuring plates, one mimicking the cold ground and the other a warm human body. What’s left to define are a few parameters like mattress inflation pressure, pressure exerted by the top plate and number of samples to test.

Image: 17 years later independent testing with SGS Hong Kong, the test apparatus looks suspiciously similar! 

To clarify, the new standard only defines how to measure R-value, not the temperature at which it should be used in the outdoors. It’s a great starting point and no mean feat to get an entire industry of competing interests to come to an agreement! The standard also doesn’t define where to test – but Exped has always tested independently and we feel strongly that this is the only way to eliminate the otherwise inevitable sales and marketing bias that prefers higher numbers over lower ones.

With this in mind, we at EXPED are proud to share our R-value test results with the world, independently measured by SGS Hong Kong - see the table at the end of this article.

The following table highlights the differences between the EMPA and new ASTM method: 

THE FUTURE OF R-VALUE

Image: EXPED’s R-values and temperature ratings are based on 20 years of experience in the field and measurements in the lab

So, that’s a little bit about the origin and meaning of R-value and the new outdoor industry standard, but what about the future? As previously mentioned, the new standard only measures the R-value of the sleeping mat itself, and brands are free to interpret this according to their experience with regards to season-of-use, temperature limit, etc. Andi Brun developed his own equation relating R-value and temperature limit over almost 20 years of measurements, observations and feedback from users. We hope to do further research into the complex relationship between sleeping mat R-value, sleeping bag temperature rating and sleep comfort. 

In conclusion, the new ASTM F3340 standard is a huge leap forward for folks buying and sleeping on sleeping mats. EXPED is proud to have played a lead role in its development and we look forward to improving the outdoor sleep experience for another 20 years.