Whether you’re a fan of closed-cell foam pads for their ultra-low weight and because they can’t be popped by a thorn or sharp rock, or if your back prefers the luxury of an inflatable air mattress, you should know the R-value system for categorizing sleeping pad insulation properties. If you’re spending lots of time in the backcountry, you probably do, but here’s a refresher. An R-value measures how well, or poorly, a given material resists—that’s where the “R” comes in—the flow of heat. The higher the R-value, the more that material will insulate by trapping heat. This is a measurement used by engineers to rate the insulation of all kinds of things, it’s not specific to sleeping pads.
The range for sleeping pads typically runs from around 1, which would be suitable only for warm summer nights when you just need cushion, to 5 and up for below-freezing temperatures. The higher the number, the more insulation is present in the pad, which also increases the weight. Selecting the right pad for an alpine trip means weighing the amount of warmth you might need against how much weight you want to carry.
Most sleeping pads on the market today display their R-value somewhere. But not every brand actually tests their pad to determine the R-value, some simply estimate the rating based on the material used in the pads. And, to make things even more confusing, there hasn’t been an industry standard that brands all adhere to. The customer must simply trust that the manufacturer has either tested the pad or adequately estimated the R-value.
The American Society for Testing and Materials recently developed, and it’s a mouthful, the “Standard Test Method for Thermal Resistance of Camping Mattresses Using a Guarded Hot Plate Apparatus (ASTM F3340-18)” Complicated name, but a pretty simple test. Ever wondered how a sleeping pad’s insulation ratings are tested? The sleeping pad is sandwiched between a cold plate and a hot plate. How much heat passes between the plate is measured. Three identical model pads from each brand will be measured and the results are averaged together. Et vóila, there’s your R-value.
This new standardized testing system is important because it’s expected that retailers will likely expect pad manufacturers to adhere to the standard and to publish the results of the tests of their pads; REI and Canada’s MEC were involved in the development of the standard, for example. In theory, if you slept on three different R-3 rated pads available today from, say, Therm-a-Rest, Nemo, and Big Agnes, you may experience different levels of heat loss since they’re not necessarily adhering to the same testing standard. Beginning next year, it’s expected, they will.
Brands that already test their pads will have to re-test them according to new standards; brands that don’t test their pads will likely either secure their own machine for testing, or off-site them for testing at another facility. Some brands use temperature ratings rather than R-value, and it’s expected those brands will switch over the R-values to fit the standard.
Lots of factors can affect the R-value of a pad, by the way. Insulation, obviously, but also things like the thickness of foam in closed-cell pads or as insulation layers in inflated pads, the internal chambers of an inflated pad, and any kind of thermal radiant layers used in construction. An easy way to increase the R-value of an inflated pad in cold temps has always been to use one over the top of a closed-cell foam pad (though some backpackers argue for the other way around).
The standard was finalized at the end of last year and it’s expected that the new R-values will be on packaging for pad manufacturers in 2020 as the big dog on the sleeping pad porch, Therm-a-Rest, sets the tone by including the new ratings on their pads.
Since we’re talking sleeping pads, here are some of our favorites
The Nemo Tensor Insulated Pad is as well-rounded as insulated inflatable pads get. It also has a durable feel that doesn’t make it feel like you’re sleeping on a fragile balloon. Quiet too. $160
For the most comfy pad we’ve used, the new Sea to Summit Ether Light XT takes the crown. We’ve used the non-insulated model and with the stuff sack that doubles as a pump sack, it’s a no-brainer choice for a pad. $160
It weighs only 8.8 ounces and for an absurdly light sleeping pad experience, there’s really no other choice as far as inflatable pads go. The Therm-a-Rest NeoAir UberLite is more comfortable and supportive than seems possible at that weight. $180
Nemo’s Switchback pad is of the closed-cell foam variety so no worries about poking it with sharp sticks. Collapses down impressively small too. $50