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Most people shopping for a sleeping pad look at thickness and weight first. The number that decides whether you sleep warm or shiver through the night is the R-value. It is printed on nearly every pad sold today, and once you understand it, choosing a pad gets much easier. This guide explains what R-value means, how it is tested, how to use it to pick a pad and how to combine pads for winter.
Why the ground is the problem
Lie directly on cold ground and the ground pulls heat out of your body. Your sleeping bag does not help much underneath you, because your weight crushes the insulation flat. Therm-a-Rest explains in its guide to R-value that without an insulated pad you quickly get cold through conductive heat loss between your warm body and the cold ground, since the compressed down fails to create an insulating layer. That is why the pad matters as much as the bag. It is the insulation under you.
What R-value means
R-value is a measure of thermal resistance. The higher the number, the more a material resists the flow of heat. Therm-a-Rest points out that the same measure is used by engineers for things like windows and fiberglass insulation. For a sleeping pad, a higher R-value means it does a better job of slowing the heat leaving your body, which means you stay warmer.
How R-value is tested
Therm-a-Rest describes its in house test like this. Pads are tested in a cold chamber kept at 4°C (39°F). Inside, the pad sits between two metal plates. Part of the bottom plate acts as a sensor and is held at a steady temperature using a measured electrical current. A well insulated pad helps keep that plate warm, so less energy is needed to hold its temperature. The less energy required, the higher the R-value.
The ASTM standard
For years, brands did not all test the same way, and some used temperature ratings instead. Therm-a-Rest says this led to wild claims and confused buyers. In 2016, Therm-a-Rest and a group of other sleeping pad makers began developing a common test method that became the ASTM F3340-18 standard, which was finalized and published in 2019.
The point of the standard is a fair comparison. Therm-a-Rest compares it to the EN and ISO ratings used for sleeping bags. With every brand testing the same way, a 4.5 from one company means the same thing as a 4.5 from another. Therm-a-Rest also notes that major outdoor retailers now require pads to publish ratings under the standard. When you compare pads, look for an ASTM R-value on the box or product page.
How to read the numbers
Therm-a-Rest's guide to choosing a sleeping pad tells you to think about where and when you camp, including altitude and latitude, and to remember your own tendencies. Do you sleep warm or cold? Here is how three popular Therm-a-Rest pads compare:
- Z Lite SOL: R-value 2.0. A closed cell foam pad that cannot pop and doubles as a camp seat.
- NeoAir XLite NXT: R-value 4.5. An ultralight air pad that Therm-a-Rest describes as an all season do it all pad.
- NeoAir XTherm NXT: R-value 7.3. Therm-a-Rest notes that cold sleepers use it all year for assured warmth.
Therm-a-Rest also says a warm sleeper might get away with the XLite NXT in snowy conditions, while a cold sleeper might want the XTherm NXT all year. The number is a guide, not a guarantee, so be honest about how you sleep.
R-values add up
Here is the most useful trick. When you stack pads, you can add their R-values together. Therm-a-Rest gives the example of a NeoAir XLite NXT at 4.5 on top of a Z Lite SOL at 2.0 for a combined R-value of 6.5, which it calls enough for a cold winter night. It suggests that year round campers keep one dedicated three season pad and add a closed cell foam pad under it when temperatures drop. The foam pad also protects the air pad from sharp ground and gives you somewhere to sit at camp.
Think in systems
Your pad is one part of a sleep system that includes your sleeping bag or quilt, the clothes you sleep in and anything else under you. Therm-a-Rest notes that you can have a plenty warm sleeping bag and still feel the chill without an appropriately warm pad. Pairing the right pad with your bag can also save weight. In its R-value guide, Therm-a-Rest describes choosing a warmer pad instead of hauling a heavy winter bag on a trip with cold nights.
For the bag side of the equation, see our guide to down fill power. For what to wear to bed, our merino wool guide covers base layers that keep you warm without trapping sweat.
How to choose
- Summer and warm nights: a lower R-value pad like the Z Lite SOL keeps weight down.
- Three season backpacking: a pad around the XLite NXT's 4.5 covers a wide range of trips.
- Winter or cold sleepers: go higher, like the XTherm NXT, or stack a foam pad under an air pad.
Planning more of your kit? Our MSR PocketRocket 2 stove is a great match for any of these pads. For more, browse our buying guides.
