When people complain to me that their sleeping bag isn’t warm enough, the first thing I ask them is to tell me the R-value of their sleeping pad. Because, if it’s under R=5.38 and your sleeping bag has an EN/ISO-certified temperature rating, you’re unlikely to realize the bag’s full insulation potential. If you’re using a sleeping pad with a lower R-value to save gear weight, for instance, upgrading to a pad with a substantially higher R-value should increase your comfort level.
The Bottom Line. Your sleeping bag’s temperature rating assumes that you’re using a sleeping pad with an R-value of 5.38 or more. If the pad’s R-value is lower, you’ll sleep colder and your sleeping bag won’t feel as warm when temperatures drop.
Sleeping Pad R-Values: 101
An R-Value measures the insulation level of a sleeping pad, or how much it protects you from heat loss as temperatures get colder. Pads with higher R-Values will keep you warmer in colder weather than those with lower R-values.
Many sleeping pad manufacturers send their products to independent testing labs to have their R-values tested, using an industry-standard called ASTM F3340-18 that was rolled out in 2020. The testing apparatus and methodology is standardized so consumers can compare the R-values of different sleeping pads or brands to one another. In other words, so everyone is singing from the same sheet of music. Before the standard, there was no good way to compare sleeping pads because everyone used a different way of rating them if they were measured at all.
Sleeping Bag Temperature Ratings: 101
Many sleeping bag manufacturers (and a few quilt makers) also send their products to test labs, so consumers can compare the temperature ratings of bags from different manufacturers. For instance, if your sleeping bag has a Comfort or Lower Limit temperature rating, it’s probably been tested using the European Norm 13537 standard or the ISO 23537 sleeping bag rating standard which subsumed it. These are often abbreviated as EN/ISO on labels and they’re essentially equivalent.
The testing apparatus used to generate these ratings requires a sleeping pad with an R-value of 5.38 and assumes you’re also wearing long underwear (top and bottom) and a warm hat.
What do these Sleeping Bag Temperature Ratings Mean?
There are three different temperatures measured by the EN/ISO tests: comfort, transition, and risk.
- The comfort rating measures the lowest temperature where the “average” woman will be comfortable, relaxed, and not feeling cold.
- The transition rating measures the lowest temperature where the “average” man will sleep comfortably, without shivering. Men have more body mass than women and generate more body heat, so they can tolerate lower temperatures.
- The extreme rating measures the lowest temperature a woman can survive using the sleeping bag. Any lower and they’re at a high risk of hypothermia and death.
In this example of the Big Agnes Torchlight 30 (men’s/unisex) sleeping bag, the bag temperature rating for a woman using the unisex Torchlight 30 would be 35 (F)/ 2 (C), and the men’s rating would be 25 (F)/-4 (C). Most people don’t pay attention to the extreme rating much because it not going to be comfortable.
Note that the bag is labeled a “Torchlight 30” and not a “Torchlight 25”, which is the men’s rating. Many sleeping bag manufacturers are not transparent in how they label their bags, often “rounding” up or down numbers to fit some customer profile. It’s marketing. Read the labels.
Who gets their sleeping pads and bags rated by these standards?
If you shop at REI, they and NEMO, Therm-a-Rest, Big Agnes, Exped, Klymit, Sea-to-Summit, Rab, The North Face, Sierra Designs, Mountain Hardwear, Marmot, Kelty, Coleman, and Mountain Equipment, all send their products to test labs to be rated using these standards so customers can compare them and pick the ones that match their needs.
But, many companies still don’t abide by these standards, including many smaller manufacturers and off-shore brands that resell low-cost sleeping bags and pads on Amazon, eBay, or AliExpress. They rely on reputation or customer reviews instead of sending their products out to be tested.
What about backpacking quilts?
Some testing labs have adapted the EN/ISO standard to test backpacking quilts and I’ve heard of some companies that send their quilts out to be rated. But it’s pretty rare. You need to rely on vendor reputation and customer reviews instead.
So What is the Effect of R-Value on Temperature Ratings?
Since EN/ISO sleeping bag temperature ratings are based on the use of a sleeping pad with an ASTM R-value of 5.38, long underwear, and a hat, it would stand to reason that your sleeping bag will not keep you as warm if you use a pad with a lower R-value or don’t wear sleeping clothes.
How much colder, is really going to depend on the design of the sleeping bag and the external temperature when it is used. Unless every permutation is tested, there’s no formula or graph that will tell the minimum comfort or lower limit temperature of your sleeping bag on top of a given pad at different air temperature. It will also vary for every sleeping bag because they’re all designed differently with different “ingredients.” There are also individual differences than can affect your warmth level, whether you’re a warm sleeper or a cold sleeper.
But knowing about the interdependency between sleeping pad R-values and sleeping bag temperature ratings calls into doubt the use of low-weight sleeping pads with low R-values when the external temperature is close to the Comfort or Lower Limit of your sleeping bag.
You might even ask:
- Why anyone would bother to get a sleeping pad with an R-value below 5.38, to begin with, when it’s an easy way to ensure that you’re getting the best insulation value out of your sleeping bag?
- Why do sleeping pad manufacturers make pads that have a lower R-value than 5.38 when sleeping bag temperature ratings depend on that minimum?
- Why is this so complicated to figure out?
- Why don’t companies sell integrated sleep systems that include a sleeping bag and a sleeping pad together to take the guesswork out of all this?
If you’re thinking about upgrading your current sleeping pad to one with a higher R-value, here are the ones currently available with ASTM F3340-18 R-values between 4 and 8. I’ve included that range to give you a sense of the weight and warmth tradeoffs available.
For a complete list of all the pads with ASTM F3340-18 R-values, see our article Sleeping Pads R-Values of 2022. Since R-values are additive, you may want to carry two pads on trips when you anticipate the need for extra warmth, instead of one pad.
See Also
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