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.
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 lower limit 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.
|Make / Model||R-Value||Min Weight (Oz)||Type|
|Therm-a-Rest NeoAir XLite NXT||4.5||13||Air|
|Therm-a-Rest NeoAir XLite Max NXT||4.5||18||Air|
|Therm-a-Rest NeoAir XTherm NXT||7.3||16||Air|
|Therm-a-Rest NeoAir XTherm NXT Max||7.3||19||Air|
|Therm-a-Rest Trail Lite Women's||4.5||25||SI|
|Therm-a-Rest Trail Pro||4.4||29||SI|
|NEMO Tensor Alpine UL||4.8||17||Air|
|NEMO Tensor UL Insulated||4.2||15||Air|
|Sea-to-Summit Comfort Plus Insulated Air||4||29.8||Air|
|Sea-to-Summit Comfort Plus SI||4.1||34||SI|
|Sea-to-Summit Comfort Plus Women's SI||5.1||33.5||SI|
|Sea-to-Summit Camp SI||4.2||27||SI|
|Sea-to-Summit Ether Light XT Extreme Insulated Air||6.2||25.4||Air|
|Sea-to-Summit Ether Light XT Extreme Insulated Women's Air||6.3||24.2||Air|
|Sea-to-Summit Camp Plus SI||4.3||31||SI|
|Big Agnes Insulated Q Core Deluxe||4.3||25||Air|
|Big Agnes Insulated Air Core Ultra||4.5||22||Air|
|Big Agnes Hinman||5||34||SI|
|Big Agnes Rapide SL||4.2||19||Air|
|REI Helix Insulated Air||4.9||21||Air|
|REI Trailmade SI||5.5||39||SI|
|REI Helix Insulated||4.9||21||Air|
|Exped Ultra 5R||4.8||20||Air|
|Exped Ultra 7R||7.1||22||Air|
|Exped Dura 5R||4.8||30||Air|
|Exped Dura 8R||7.8||33||Air|
For a complete list of all the pads with ASTM F3340-18 R-values, see our article Sleeping Pads R-Values of 2023. 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.
Phil, what is the significance of 5.38? Does it correspond to a particular air temp, i.e 0? Or something else?
It’s the R-value of the pad used in the sleeping bag temperature rating standard apparatus. Every EN/ISO sleeping bag temperature rating is based on using a pad with that R-value. But they use it to test all bags, those rated for warm temperatures and those rated for cold ones. Why that value? I have some hunches, but I don’t know for certain. I suspect it has to do with statistical stability.
Perhaps some of the folks involved in establishing the ISO/EN standard can comment on its evolution.
Is this ASTM R 5.38 value the stated value in the ISO test spec? Other descriptions of this I’ve read (possibly including here), have variously said, “standard self inflating pad” (somewhere), “a basic foam mat” (Themarest en-iso-sleeping-bag-ratings article),” a R 4.2 pad” (here I think but I could be wrong), “an R 4.8 pad” (some apparently knowledgeable forum discussion) and now ASTM R 5.38 (here) which is a new one. Presumably these statements/values came from somewhere at the time. The spec cost money of course and I’m probably not buying it just out of vague curiosity.
Just go to any sleeping bag listing at REI
“Many sleeping bags are rated for temperature by an independent testing protocol. REI and the broader industry now use the International Organization for Standardization (ISO) 23537 protocol, which improves on the older European Norm (EN) 13537 standard; you may still see older items in our assortment that were only EN tested. With either protocol, each bag gets a Comfort (or T-Comfort) rating and Lower Limit (or T-Limit) rating. The Comfort rating is the lowest temperature at which the bag will keep the average “cold sleeper” comfortable, and the Lower Limit rating is the lowest temperature at which the bag will keep the average “warm sleeper” comfortable. Ratings are based on a person wearing one long underwear layer and a pair of socks, sleeping on an insulated surface with a minimum R-value of 5.38. Many brands use the Comfort rating for women’s bags and the Lower Limit rating for men’s and unisex bags. Everyone’s body and sleep comfort differ, and conditions of use (posture, clothing, sleeping pad R-value, wind, humidity, etc.) affect total insulation, so EN and ISO ratings are intended as a guideline to help you compare products, rather than a guarantee of warmth.”
REI was the leading force between the ASTM r-value standard and all non-outlet sleeping pads listed on their website use it.
Interesting. Thanks. Guess I haven’t had a reason to read that deeply into an REI sleeping bag listing recently. It is reasonable to suppose REI got it from the spec. Odd that the value has been a bit if a moving target amongst commentators over the years but maybe it’s a standards convergence thing now we have the ASTM R ratings.
The spec (which I have) provides a formula for computing that thermal resistance value which is based on another ISO standard: 11092. These standards documents are really technical publications, so I’m not surprised that REI made the effort to translate it into something that laypeople can understand.
Lots of different information about the R-Value used to rate sleeping bags. ISO 23537-1:2022 states sleeping bag measurements are made using “a foam mattress with a material specific thermal resistance R = (0,85 ± 0,06) m K/W when tested in accordance with ISO 11092 and placed on an artificial ground.” Multiply by 5.68 to convert from the metric value to US R-value. In this case, 0.85 x 5.68 = 4.828 or 4.8 when rounding.
Mike, Is your question why such a strange value as 5.38 rather than 5.4 — or even “cleaner”: 5.5? I don’t have any connection with the actual development of the standard and so can’t give you a definite answer. But as a retired engineer I can easily come up with an example of how kind of number can arise. Here’s my hypothetical chain of thought:
1) I need a cheap and readily available material with high quality control. (So that it will have the same R value ten years from now; “statistical stability”.)
2) Dow’s extruded polystyrene insulation (“blue board”) would be good. At R-5 per inch, one-inch board would be better than 1.5″ or 2″. It’s closer to the R-values of available commercial mats.
3) Dow’s engineering specs say that the statistical R-value range is between 5-1/4 and 5-1/2. That’s a fairly narrow range. It “should” be small enough to have little effect on our results. (“statistical stability” again)
… Develop the detailed procedure. …
4) Verify that the procedure gives substantially the same results with either 5-1/4 or 5-1/2.
5) In the official description, specify that the process requires a Styrofoam mat with an R-value between 5.25 and 5.50. Any mat used must have its R-value measured and recorded to the nearest 0.05. (This can be done with the apparatus used for the sleeping bag rating.)
6) In the public summary just give the average: 5.38.
So now those of you who haven’t worked with engineers before know why we’re a little crazy about precise wording.
Doug – as you can attest. The temperature rating standard was clearly devised by engineers in the absence of anyone who had to communicate its rationale to laymen. That usually falls on the shoulder of the marketing teams and they must have had a fit when they had this handed to them, “cooked”.
I think your theory about the blue board is good. By statistical stability, I meant something like “if we use R=5.38, all the sleeping bag ratings remain consistent (with low variability) when we do them over and over and across different test labs, but if we use a different R-values (they probably tested quite a lot) the variance becomes too high.
Doug, great info, thanks!
Pretty much this, complicated by the fact that the iso is published in SI units, but few of the manufacturers and writers will use SI units when speaking to the public (at least, the US public).
Also complicated by the fact I’ve never seen a manufacturer quote which units they’re using, but not too much trouble seeing as inch-pound units are about five and a half times greater than SI.
My brother pointed out to me years ago that Therm-a-Rest made “Women’s” sleeping pads that were lighter and cheaper than men’s. Since I’m 5-8″ and can fit just fine on a 5′-9″ pad, I bought the women’s version and later found out it had a higher R value as well.
I’ve used the Therm-a-Rest Neo Air X-Lite Women’s in temps just below zero F and slept just fine. I was using a 30F bag and was also wearing long johns, hiking pants and shirt, hat, gloves and wool socks. Had I been Native American, my name would probably be: “Idiot who sleeps in the back yard when it’s cold”.
2023 xlite NXT will have higher r4.5 and 3″ height. also 8x less noisy.
Wow, my pad has a woefully low r-value (2), according to the manufacturer. For winter camping, I was taught to put a wool blanket between my sleeping pad and sleeping bag. I’m anecdotally convinced this helps. It makes me curious how much that increases r-value, and how I might measure it?
KSU – Kansas state university has a test apparatus. Contact them and ask how much it costs.
My idea is to put a Mylar blanket reflective side up under my sleeping pad. Or perhaps on top with a fleece blanket over that due to the lack of breathability. Anyone have experience doing that. I think most all backpackers learn to be weight conscious and few options add less weight that a couple of emergency Mylar blankets.
This works as r values are effectively additive (although if you put it over you, it’s adding to the r value of the compressed sleeping bag it’s in contact with and not the pad).
Reflective insulation works much better if placed as close as possible to the radiant heat source (i.e., you) so underneath you on top of your pad and if you can stomach it, next to your skin).
Plenty of UK campers do it, but most do put the reflective layer on the bottom and it’s markedly less effective.
I assume manufacturers make sleeping mats with r-values lower than 5.38 for a range of cost and weight choices for their customers. Not everyone needs high r-value mats, especially for warmer weather camping.
Exactly – I carry a 40 degree hoodless sleeping bag during the warmer months when its 60-90 degrees out, but I don’t need all 40 degrees. So if my pad provides less than ideal ground insulation, my sleeping bag can compensate. But I know this from experience.
Thanks for this enlightening report. It is a “no brainer” topic that, strangely, many never think about. I have seen people shiver all night in winter camping because they unthinkingly used a summer mattress.
While my REI Flash All Season insulated air mattress is rated at 5.2 R I’ve found it works fine at 10F. with a 20 F. rated Western Mountaineering overstuffed Megalite bag INSIDE my solo Tarptent Moment DW tent which likely raised the temperature by 10 F. Plus I wore heavy weight long johns and a balaclava. This illustrates one of the advantages of tents over tarps in winter camping.
For colder temps I have a 1/8″ thick closed cell pad of floor underlayment foam to lay beneath my REI “winter” air mattress and an LL Bean -20 F. down mummy bag. At -20 F. or lower I would use my Ridge Rest foam pad under the REI air mattress AND down insulated pants and jacket inside my -20 F. bag.
Thanks for an overlooked topic. Very much needed information here.
My “sleep system”:
1.) 3 SEASON-> WM overstuffed 20 F. Megalite down bag and REI Flash 3.2 air mattress
For temps above 55 degrees I use the bag like a quilt, foot of mattress tucked into foot of bag.
2.) WINTER-> LL Bean -20 F. down bag and REI Flash All Season 5.2 air mattress W/ 1/8″ closed cell foam pad under
BELOW -20F. Ridge Rest under the REI winter mattress AND down pants and jacket over long johns and heavy balaclava while inside solo TT Moment DW tent.
I have the Neo Lite XLite. I have found that the addition of the Military Surplus Light Weight Foam mat a great addition to sleeping on cold ground. And that is Cold ground at 11,000 feet Just two weeks ago dropping to 20 degrees at night. I sleep in the Western Mountaineering Utralite and have for some 20 years now. Wow, how time flies by… No problems with the Bag or Temperature rating. Though I am lucky to have a Dry Cleaning establishment who knows how properly clean Goose Down Bags and Wool blankets and clothing…