R-VALUE: THE MEANING BEHIND THE NUMBER
KEITH ERPS | NOVEMBER 7, 2017
Unless you’re a homeowner, builder, engineer or gear nut, you might be overlooking a key piece of information in your search for the right mattress. Let’s take a look at what R-value is, how we measure it, and how to factor it into how you pack for your next trip.
R-Value and how we test it.
Technically, R-value is a measure of thermal resistance; the higher the R-Value, the more thermally –resistant or, better an insulator, a given material is. We measure every mattress design in our own cold chamber, kept at a cool 4C (39F). Inside this converted shipping container, a mattress is placed between two metal plates. A section of the bottom plate has been turned into a large sensor and is kept at a steady temperature with a measured electrical current. A mattress that provides a lot of insulation will help keep the bottom plate warm and, as a result, the sensor will require less electricity (energy) to maintain its temperature. A mattress with less insulation will require more energy. This measurement–the amount of energy required to maintain the plate’s temperature– plugs into an equation to provide an R-Value for each mattress–the less energy required to keep the plate warm, the higher the R-value and vice versa.
The Therm-a-Rest Cold Lab
What It Means to You
R-value is useful in comparing mattresses, but it’s important to note that while R-value is a standard measurement, there is no standard methodology for testing the R-value of a camping mattress. So, be careful comparing R-values between brands. Some companies actually test their mattresses while others “estimate”; differences in how they test can make apple-to-apple comparisons a challenge. Keep that in mind as you consider what mattress is right for you.
Another useful bit of knowledge is that you can generally draw the line between three-season and winter mattresses at an approximate R-value of 3.0. We also think that if a mattress is to supply any reliable amount of warmth, it should have an R-value of at least 1.5. Any less and you could basically be sleeping on a heat sink, which can continually drain warmth from your body, making for a pretty grim night.
And finally, one of the more useful ways to use R-value is to use it when building your sleep system. Let’s say you’re going into the High Sierra in September. You’ll be expecting brisk nights, but there’s real potential for downright frigid ones too. Do you bring the giant 0F winter sleeping bag to be safe, or your three-season bag that’s rated closer to 15F degrees, and risk a few cold nights? Most people will make a decision based on bag warmth alone, but by factoring in mattress warmth, you suddenly have more options for making your sleeping bag perform better on the margins of its comfort rating. For instance, on a Regular NeoAir® XLite® (R-value:3.8, Weight: 12 oz.), you should get the comfort your bag is rated for. However, jump up to NeoAir® XTherm™ (R-value: 5.7, Weight: 15 oz.) and you could get by on those coldest nights with a greater comfort margin for only an extra 3 ounces – a lot better than jumping to a winter bag that could add up to a pound of extra weight.
Of course, there are so many variables that affect your sleep system’s warmth that this is just a small piece to a very complicated puzzle (more on that to come), but that’s what shaving weight is all about. No single gear substitute is going to make you fly down the trail, but collectively, fine-tuning everything in your pack, and making sure you sleep well every night, will make a big difference in the way you feel on your next adventure.