Overview
Lightweight backpackers have long complained about how hard it is to compare the warmth of different sleeping pads. Recently, three pad makers and retailers announced that they would use a new standard for measuring sleeping pad insulation. And in 2020, REI and MEC will require all their suppliers to report those values. At first glance, this looks great. But the devil is in the details.
For many years, pad makers like Nemo and Big Agnes have advertised temperature ranges without describing how they came up with those numbers. Others promoted R-values using different protocols. This made it almost impossible to reliably compare sleeping pads between manufacturers.
Then late last year, MEC, REI, and Therm-a-Rest announced that they would use a new sleeping pad R-value standard to measure their own products. And starting in 2020, REI and MEC will only sell pads furnished with standard R-values. Many people were glad that they could soon compare sleeping pads across manufacturers, like the EN/ISO 13537 temperature ratings for sleeping bags. But knowing the R-values of sleeping pads raises new questions.
What’s an R-value and how is it measured?
An R-value measures resistance to heat transfer through an insulating layer. An R 3 sleeping pad provides three times more insulation than an R 1 pad. But as one part of a sleeping system, you won’t sleep three times warmer.
R-values started as a way to measure building insulation. In this post, I’m using United States R-values; the rest of the world uses metric units sometimes called RSI. To convert from the metric values found in many scientific papers to US R-values, multiply by 5.68.
| US R-Value | Metric RSI (K·m2/W) |
| 1 | 0.18 |
| 2 | 0.35 |
| 3 | 0.53 |
| 4 | 0.70 |
| 5 | 0.88 |
While the new R-value standard is behind a paywall, Outside described the basic testing setup: two plates at constant temperatures of 95 F (35 C) and 41 F (5 C) squeeze a pad to simulate human weight. Many other factors are under control, including pad inflation, ambient temperature, wind, and humidity. At the time of their announcement, only MEC, REI, and Therm-a-Rest owned the $50,000 machines required to measure sleeping pads using the new standard.
Sleeping pad tests in a well-controlled lab only go so far. Roger Caffin covers many of the pitfalls of pad testing for BPL in Apparatus for Testing Thermal Insulation Properties. Roger and Will Rietveld also go into more detail on the real-world performance of sleeping pads in Lightweight Inflatable Sleeping Airmats – State of the Market 2011 Part 1Â and Part 2.
Many BPL readers understand R-values to some extent, but most consumers don’t. And they certainly won’t know what R-value of sleeping pad they need.
What R-value should I look for in a new sleeping pad?
If you know the R-value of your existing sleeping pad, soon you can decide if your next pad should be warmer, cooler, or about the same across many brands. But you’ll need more information if you don’t know the R-value of your current pad or you’re new to shopping for sleeping pads.
Unfortunately, the guidance from pad makers, retailers, and gear reviewers is inconsistent and prone to misinterpretation. For example, see:
1. The Therm-a-Rest blog What Is R-Value?
2. Therm-a-Rest Ultralight Adventure pad finder:
- R 2 to 3.3 – Three Season
- R 3.4 and higher – Winter
- R 5.7 and higher – Extreme Cold
3. REI Expert Advice How to Choose Sleeping Pads
“Thicker pads generally offer higher R-values and the average summer camping pad should be around 3 or higher. Campers who sleep cold, often women, may want to choose a pad with an R-value of at least 4.”
4. REI in-store sign next to sleeping pads:
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5. Philip Werner’s Section Hiker blog Sleeping Pad R Values
- “For winter backpacking and camping, an R-value of 5, or higher, is recommended.
- For three-season backpacking and camping, an R-value of 2, or higher, is recommended.
- An additional R value of 1 is usually a good hedge for women and other cold sleepers.”
6. Outdoor Gear Lab How to Choose a Backpacking Sleeping Pad
“We have found that pads with R-values between 2.0 and 3.0 generally work well for three-season use but should be supplemented with a foam pad for use in the winter where an R-value of around 5 or above is ideal. Pads with R-values lower than 2.0 are great for summer, but you may want to supplement them with a closed cell foam pad on colder nights in the spring and fall.”
All this advice shares two serious shortcomings:
- Seasons in coastal Hawaii are very different from northern Minnesota, so “winter” low temperatures could be anywhere from +40 F (4 C) to -40 F (-40 C).
- Poorly-defined seasons don’t match up with sleeping bag EN ratings by temperature, which is where most people start their shopping.
But in one important way, all this guidance is wrong.
How do sleeping bag temperature ratings relate to R-values?
Most people want their entire sleep system – clothing, sleeping bag, sleeping pad, and shelter – to keep them reasonably warm at night. And many consumers focus on sleeping bag temperature ratings. For example, a warm sleeper might decide that for a trip with forecast low temperatures in the 40-50 F (4-10 C) range, a sleeping bag with an EN/ISO lower limit rating of 40 F (4 C) should work well. The EN/ISO testing protocol requires a zipped up sleeping bag with the hood cinched along with several other requirements including using an R 4.8 sleeping pad.
If your sleeping pad is substantially less than R 4.8, you’ll lose more heat to the ground, and you might not be warm enough in an EN 40 F (4 C) sleeping bag on a 45 F (7 C) night. I wrote “might not” because how warm you sleep on any given night depends on many other variables. One paper claims (PDF) that using an R 1.3 or lower pad could cut 10 F (6 C) from the effective rating of a sleeping bag.
Relying on EN/ISO sleeping bag temperature ratings argues for always using an approximately R 5 pad. This is more-or-less the approach taken by Zenbivy, who states: “only mattresses with R value 5 or greater will deliver full temperature rating performance from EN rated sleeping bags.”
Conclusion
R-values alone are not enough for many consumers choosing new sleeping pads, and much of the current guidance is inconsistent. Manufacturers and retailers who suggest lower R value pads for a wide range of temperatures could be seriously misleading buyers.
You can’t look at a sleeping pad’s R-value in a bubble. Consider your pad part of an integrated system that includes the clothing you wear to bed, your sleeping bag or quilt, the type of shelter you’re in, the nature of the ground surface (snow, moisture, etc.), ambient temperatures, and of course, your own physiology. Photo: Ryan Jordan
Discussion
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The material you’re sleeping on is a factor
If you sleep on organic material it adds insulation
If you sleep on mineral material it provides a little insulation but not as much.
I sometimes measure the temperature underneath my pad in the morning just before I get up. It can be 20 F warmer. Like one trip the air temperature was 42 F, it was 62 F under the pad. Soil that was a mix of mineral and organic. It takes a few hours to warm up.
If you sleep on snow it will be 32 F. Then you need an extra CCF pad.
“IN REI WE TRUST” …well mostly, especially for R ratings of REI brand sleeping pads and mattresses.
My REI FLASH All Season insulated air mattress has a “5.3” R rating so that means “5” to me. Maybe OK for down to -5 F. but below that I’ll take along my Therm-a-Rest Ridge Rest pad to put beneath it. I’ll be sleeping in a solo tent on ski-compacted snow using my -20 F. down bag.
In fact I’m going to test a thin (3/16″) mummy shaped pad of floor underlayment that I cut from a roll of left-over that I have. One night with it and one night without (if temps stay similar) may tell the story. It will go beneath the REI mattress, natch.
Finally, published R values for mattresses and sleeping bags may be more accurate for fit young to middle aged backpackers but geezers like me absolutely have a lower basal metabolism so we will most likely need more insulation everywhere and at all times for winter camping. (As you may have noted, I use a -20 F. down bag and I also carry a -30 F. down parka for camp use.) “Belt & suspenders”
I note the reference to our article on the testing of airmats: that is good. But a vital part of the conclusions from that review have not even been mentioned here (afaik).
If you blow up your air mat hard, it may well have the rated R-value. But you will sleep very poorly as the bed will be rock-hard.
If you let some air out of the mat you will sleep much more comfortably, but your hips and shoulders will be much closer to the ground, and the effective R-value will be much lower. You will be colder.
I dare say every mat mfr will want to claim te highest R-values they can. I understand that. But I have yet see one vendor mention how hard they blew up their mat for the test.
Bottom line: if you are a fit 25 yrs old you may not care, but if you are of ‘mature age’ (like me) you might want to err slightly upwards in selecting a mat for R-value.
Cheers
I just use an Xtherm year-round and add closed cell foam when on snow. Â It also allows me to get away with a lighter quilt/bag and I can leave it a bit squishy to coddle my hips and shoulders while having adequate insulation,
If I am camping by the car it’s a 4” thick self inflating pad – with a pillow from my bed. đź‘Ť
Philip Werner just posted a long list of new ASTM-standard R-values on SectionHiker.com:
https://sectionhiker.com/the-new-sleeping-pad-r-value-standard-has-arrived/
And he attempts to explain the massive confusion some manufacturers have created trying to distinguish between old R-values, new R-values, “old” stock, and “new” stock. Buyer beware until the old stock, often discounted, is gone.
Also, it seems Therm-a-Rest’s claim that the Z Lite Sol metallized coating increases insulation – doesn’t actually show up in ASTM R-value testing.
Every silver lining (R-value standardization) includes one or more dark clouds. I’m expecting more nonsense as the industry and retailers stumble into the future.
— Rex
if I’m reading that correct, there is no difference between old stock and new stock as far as the actual “guts” of the pad- just a “recalibrated” way to measure r values
thus my “old” Xtherm and “old” Uberlite should be the same as the “new” ones as far as warmth
I should have known that BPL’s own Richard Nisley had covered much of this topic in earlier posts and his contributions to a Wirecutter review of sleeping pads, especially the “Understanding Sleeping Pads” section:
https://thewirecutter.com/reviews/best-sleeping-pads-for-backpacking-and-car-camping/#understanding-sleeping-pads
Wirecutter isn’t the first place I look for backpacking gear reviews or advice, but they are increasing their coverage and consulting the right experts.
Note that my blog post, the Wirecutter story, and basically any sleeping pad review published before mid-January 2020 will need updates as the new ASTM R-values roll out.
— Rex
Two weeks ago Therm-a-Rest posted a video that explains how they measure ASTM R-values, doubles down on their confusing seasons-based R-value recommendations, adds “variance zones” for more muddle, mentions then dismisses the R 4.8 pad used for EN sleeping bag ratings, then ultimately says the equivalent of “figure it out for yourself.”
Too bad.
Thanks to PMags for the pointer.
— Rex
Screen cap with Therm-a-Rest’s latest R-value versus season recommendation:
— Rex
I dunno, Rex, I thought that was a pretty good video. Nothing confusing about needing higher R-values for colder nights. Nothing confusing or wrong about different people sleeping warmer/colder and hence needing different R-values. And I wouldn’t characterize what they said about the R-4.8 EN value as dismissive, but simply pointing out that the fact the EN test uses it doesn’t mean it’s the right value for you and me all the time.
You’re a tough crowd! :-)
Section Hiker website just published the new “industry standards” for rating sleeping mattress R values.
So I called REI to ask them about the ratings for my FLASH 3 Season mattress (3.7 R) and the similar All Season mattress (5.2 R) and they said, after some consultation among Customer Service staff, these REI brand mattresses ARE rated by the new standard. Â Nice to know.
it certainly seems ThermaRest did pretty well on the new evaluation; all three inflatable pads I own were bumped up
wondering if that’s the case for all pad manufacturers?
Adventure Alan posted a new video comparing sleeping pads, with some really interesting details near the end on how different Therm-a-Rest pad R-values changed under the new ASTM standard – and why.
He still uses seasons for recommendations, but at least defines the temperature range more than others. And he generally recommends R 4+ pads.
Worth watching the whole thing.
Tip: Want the new R-value in your inflatable pad? Inflate it hard!
— Rex
Want the new R-value in your inflatable pad? Inflate it hard!
True, but that is like sleeping on a bare wood floor. Not very helpful, plus a greater chance of pad failure.
Cheers
like sleeping on a bare wood floor.
It depends.
On my last trip, I deliberately inflated a Nemo Tensor Insulated sleeping pad as hard as I could due to unexpectedly low temperatures. The pump sack kept popping out the inflation valve, limiting pressure. Maybe that’s a feature?
Slept fine. And I don’t normally sleep well on hard or even sort-of-hard surfaces. No failures, yet, but very short data set :-)
— Rex
I do not know the Nemo Tensor mat, and I do not know the valve they use, so I am just guessing. I do know the Exped valves, and they do not look very different.
However, it does sound (to me) as though the valve was malfunctioning. Certainly, I do not think it popping out was a design feature. That could go horribly wrong!
If so, I wonder just how hard the mat was inflated?
Cheers
“it certainly seems ThermaRest did pretty well on the new evaluation; all three inflatable pads I own were bumped up
wondering if that’s the case for all pad manufacturers?”
According to the article in SectionHiker, many R values went down.
I “suspect” one of the reasons why the R values got better for TRest is the pads are now tested at a higher ambient temperature than before. One would think that temperature doesn’t affect R values, but it does. Especially in air-only mattresses..
Sad truth is, the pad’s R value is probably most important to be accurate when the air and ground temps are colder, not warmer.
“I “suspect” one of the reasons why the R values got better for TRest is the pads are now tested at a higher ambient temperature than before. ”
Per Adventure Alan’s video (above), in the past ThermaRest pads were intentionally tested at 80% inflation as they felt that’s were most people would be for the sake of comfort. The new R-values reflect testing fully inflated pads.
R-value will not change as a function of ambient temperatures, but heat flow will.
“R-value will not change as a function of ambient temperatures, but heat flow will.”
I respectfully disagree:
https://www.buildingscience.com/documents/information-sheets/info-502-temperature-dependent-r-value
Although some types of insulation actually show an increase in R value as the temperature goes down, here’s what it has to say about polyisocyanurate insulation:
“All of the samples show a decrease in R-value as “outside” temperatures go below freezing.10 It appears that the “peak” R-value for all samples occurs when outdoor temperatures are closer to the indoor temperature (i.e. between 36°F or 2.2°C and 108°F or 42.2°C). Winter temperatures (i.e. less than 32°F or 0°C) and solar heated roof temperatures (i.e. greater than 113°F or 45°C) result in lower R-values.”
Given that moisture content may have to do with this dependency, I’d suspect that an fully inflated air-only mattress would probably have a different r value measurement if tested in a sub-zero/humid freezer than testing in a 70 degree, dry environment, especially based on the above article.
In other words: (assuming one doesn’t use their own breath) if the pad were inflated with cold-humid ambient air (like what could actually happen on a winter trip), I’d suspect that it would perform differently than if it were inflated with warmer/dryer air on a balmy spring/summer trip. And given that humidity tends to make things behave poorer than normal, I’d wager that an fully inflated air mattress would have a reduced R value in sub zero temps than the ASTM certified value.
But who is actually testing for this? ASTM isn’t, that’s for sure.
I always inflate my mattresses hard, but believe me, it’s nothing like a bare wood floor. I’m a rotisserie side sleeper FWIW.
Thanks Matt. I was wrong, and shocked –
The good news seems to be, since I use “batt-like” insulation versus polyisocyanurate, that fiberglass and stonewool insulation improves as the temperature fall –
Who wudda thunk?
No worries, Greg. It shocked the whole building industry when the news broke a handful of years ago, especially for the commercial jobs that had spec’d polyisocyanurate insulation. And yeah, fiberglass and mineral fiber insulation do just fine, even if their R value measurements also change with temperature.
Makes one wonder if our currently accepted formulas for heat flow through “assemblies” is over simplified (especially when moisture is involved…)
Interesting that you categorize sub-zero air as being humid,. Up here the air is very dry in winter, much less than summer.
How would the insulation provided by the sleeping bag on top of the mat affect the R Value? A longer and wider pad would be more exposed to the air so would it be colder than a small pad?
REI posted a new chart of overnight low temperatures, R-values, and sleeping bag temperature ratings from their “lab team”:
https://www.rei.com/blog/camp/just-how-warm-is-that-sleeping-pad
This chart seems to imply that you need an R 5.5 sleeping pad to achieve the rated temperature of a sleeping bag, vs the R 4.8 used in standard tests.
“This new sleep system data suggests that it’s worth looking at a more-insulated pad if you want your bag to live up to its temperature rating.”
It’s also a rough guide to how much warmer your sleeping bag must be to make up for a pad with low R-value.
Interesting! Progress!
REI also published this very good video demonstrating R-value testing:
— Rex
According to this article in The Trek, the old Therm-a-Rest sleeping pad test setup held ambient air at the same temperature as the lower cold plate: 4 C / 39 F; while the 2018 ASTM standard calls for 20 C / 68 F room air and a 5 C / 41 F cold plate.
And there’s this quote from Therm-a-Rest:
My opinions:
• On first glance, 20 C / 68 F ambient temperature is not realistic for most conditions.
• However, it’s much more realistic given that torso and “women’s” sized pads are mostly covered by sleeping bags or quilts in the real world. Longer or wider pads, not so much.
• Warmer ambient air means pads lose less heat during the test, since the top plate exposes more than half of a pad’s upper surface and all of the sides.
• Therm-a-Rest’s previous practice of inflating to only 80% thickness to reflect real world use (vs ASTM ~100%), plus the ambient air change, helps explain significant R-value increases for their pads under ASTM testing.
• For new backpackers or people replacing pads without ASTM R-values, consistent and accurate industry guidance for choosing sleeping pads using R-values is still MIA.
— Rex
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