Monday, October 7, 2013

Some thoughts on R-Value

R-value: putting a number on warmth

When buying a sleeping mat, most people shop around a bit and compare the different brands and models available. Important points such as price, quality, comfort, ease of use, reliability, compressibility and warmth all enter into the equation. But it's that last one, warmth, that's a bit sticky. With all the other parameters it's pretty straightforward to compare mats. Price and quality go together and info about comfort, ease of use, reliability, and compressibility can all be culled from online reviews,  in-store poking and prodding, and manufacturer websites. But this warmth rating, R-value, isn't as easy to accurately verify across brands and deserves an explanation, so we thought we'd clarify a few points.

R-value. Huh?

First, R-value is a measure of thermal resistance and is most commonly associated with the construction industry. When you buy insulation for your house, it's efficiency has been tested and is expressed with a number, it's R-value. This allows you to compare insulation easily. Because it's resistance to heat transfer that's being measured, the higher the number, the more efficient it is at retaining heat. Without getting into the specifics ( R = \Delta T/\dot Q_A), suffice it to say that R-value is a useful way to compare the insulation efficiency of different materials.

Ten years ago, Exped unveiled the patented DownMat. It was revolutionary in many ways (down insulation, 7cm thick and super comfortable, stuff sack inflated) and at that time Exped introduced R-value to sleeping mats. It makes perfect sense to express warmth in this scientifically accurate way.

Independent testing

So, as we just said, R-value is, theoretically, a good way to compare insulation. What? Theoretically? It sounds like double speak. Sorry. But here's why we say "theoretically." Like so many measurements used by various industries (and customers!), the usefulness of the number is based upon whether it can be readily compared across brands. Our mats are independently tested by EMPA, the Swiss Federal Laboratories for Materials Testing and Research. In other words, the R-value we state on our mats does not originate from the Exped offices. Rather, it is arrived at by a standard R-value test conducted by a neutral party. Being a Swiss company, we are naturally big fans of tests like this and would love to see standardized tests used by all mat manufacturers. We're not there yet, but the topic is being discussed within the outdoor industry, and that's a start. But certainly, within the Exped line, you can compare mats accurately.

Exped mats offer a wide range of R-values and there is a mat for every need. The following list shows the R-value of our mat line, low to high:

·         All AirMats                          0.7
·         MultiMat                              1.2
·         SynMat UL 7                       3.1
      DownMat Lite 5                   4.1
      SynMat 7                             4.9
·         DownMat 7                         5.9
·         DownMat UL 7                   5.9
       SynMat UL 9                      6.0
·         SynMat 9                            6.0
·         SIM Comfort 7.5                 6.4
·         DownMat 9                         8.0
      DownMat XP 9                   8.0
      SIM Comfort 10                  9.5
·         MegaMat 10                       9.5

R-value and warmth

The whole idea of expressing mat warmth with an R-value is to make buying a mat easier. Remember, the higher the number, the warmer the mat. But not every situation requires the very warmest mat. There's a big difference in nighttime temperatures between Florida and Alberta. Very warm summer nights often require little or no insulation and warm sleepers will be especially happy that they are not overheating. On the other hand, anyone doing any sort of snow camping will always be happy with a warm mat. And as always, cold sleepers should always look for the warmest mat they can afford. Pretty obvious why R-value is a useful tool, eh?

Here's another thing to consider. The warmer the mat (higher R-value) you use the more impressed you will be with your old sleeping bag. You know, the one that's rated to 20° F. but never seems warm enough below 32°F. We've come to understand this not just from our personal experience, but from countless emails and phone calls from our customers who report it to us. It turns out that most of the heat loss you suffer during the night is going into the cold, hard ground - not through your sleeping bag into the atmosphere. By improving your ground insulation you will be substantially warmer and you'll like your sleeping bag again!

So R-value can be very helpful when buying a sleeping mat. Just remember that not all measurements are created equally.

Sleep well!