The Sleeping Pad R-Value Standard
The Sleeping Pad R-Value Industry Standard: ASTM FF3340, defines a rigorous and reliable methodology for measuring the R-Value of all sleeping pads, including air mattresses, self-inflating pads, and foam pads. It was rolled out to consumers in 2020 and all of the major sleeping pad vendors including Thermarest, Exped, REI, Sea-to-Summit, Big Agnes, and NEMO rate their pads based on it.
When it was first introduced, I spoke with Brandon Bowers and Greg Dean, both from Therm-a-Rest, about the R-value standard, and here’s a summary of our conversation. The questions were mine, but I’ve paraphrased their responses below for readability. Any errors in translation are my own.
What problem does the new sleeping pad R-value standard address?
While many manufacturers provide R-Values for their sleeping pads today, there’s never been a consistent way of measuring them so you can reliably compare the insulation value of pads from different manufacturers. The new standard addresses this issue. Those manufacturers who provide R-values for their sleeping pads already will need to test them again to bring their pads into compliance with the new standard. Pad manufacturers that only provide temperature ratings today will probably be required by outdoor retailers to start measuring them and publishing the results. For example, REI and MEC were instrumental in facilitating the widespread adoption of sleeping bag temperature ratings and educating consumers on how to understand what they mean. Both were involved in the definition of the new R-value standard as well.
What are R-Values?
R-Value measures how well an object resists the flow of heat from one side to another. The higher the R-value, the greater the resistance, and the better its insulating properties. When applied to sleeping pads, R-value measures a pad’s ability to keep your body heat from passing through the pad and into the ground. Sleeping pads aren’t designed to be warm, so much as to resist the transfer of heat. Highly insulated pads will usually feel warmer because they keep your body heat close.
What is the new R-Value Testing Process?
The test method measures the thermal resistance of sleeping pads using a two-plate apparatus. The pad is sandwiched between a hot plate and a cold plate and the R-value is measured in three different locations on the pad. The same test is run on three identical pads and the results are averaged together.
Are there any automated testing tools for measuring the new R-values?
Testing hardware for measuring R-values, as defined by the new standard, is already available from a company called Thermetrics. The R-value test methodology and calibration scheme were kept simple to make R-value testing affordable by smaller brands with fewer capital resources to purchase expensive equipment. Hopefully, they will also see the advantage of publishing standardized R-values for their sleeping pads.
How much does a Thermetrics R-value testing machine cost?
Will independent testing labs be responsible for testing sleeping pad R-values or will that be left up to the manufacturers themselves?
It’s too early in the process to know what will be required. The standard was just approved. KSU (Kansas State University), which is the lab that does a lot of the sleeping bag temperature ratings for the outdoor industry, has opted not to do R-value testing for sleeping pads because they want to stay focused on their core competency and not dilute it.
Why are R-values a better way to measure insulation value than temperature ratings?
An R-value is a physical measurement that doesn’t require a lot of qualifiers to interpret. For example, don’t have to know whether a person using the sleeping pad is a man or a woman, whether they’re wearing long underwear, or wearing a hat when the test is performed. This makes it very simple for people to understand and compare two pads.
Are R-values additive? Can you stack multiple pads together to create an insulation surface that is the sum of its parts?
Which will be warmer: an air mattress placed on top of a foam pad or a foam pad placed on top of an air mattress?
Sleeping pad R-values don’t measure pad warmth, but their resistance to heat loss. As long as there isn’t an air gap between the pads, they’ll have the same additive R-value no matter how they’re stacked.
Does the moisture in your breath, when you blow up a pad by mouth, reduce or alter the R-value of a Therm-a-Rest sleeping pad?
It might have an effect over the long term in extreme temperatures, but it won’t have any effect in normal use.
How does letting some of the air out of an inflatable sleeping pad affect its R-Value?
It might reduce it by 5-10%, but that’s a guess.
Are there any other factors that can reduce the R-value of a sleeping pad after extended use?
Synthetic or feather insulation may compress over time.
What are the different design variables that alter the R-values of Therm-a-Rest’s sleeping pads?
The number of internal chambers, radiant layers, and foam thickness.
How well do the R-values of Therm-a-Rest’s existing sleeping pads correlate with measurements under the new R-value standard?
Everything will change a little bit because our existing R-value ratings were done in a cold room, which is a different testing environment. But we’ve always used R-values to rate our sleeping pads and we already own a Thermetrics R-value testing machine.
How will the R-values or temperature ratings of other sleeping pad manufacturers be affected by the new R-value standards?
Some manufacturers will have to make major adjustments to their sleeping pad ratings.
Is there agreement on how R-values will be communicated to consumers in manufacturer or retailer product listings? For example, the EN13537 sleeping bag temperature ratings are documented in a standard way across manufacturers and retailers.
Not at this time. Therm-a-Rest will be undertaking a major consumer education campaign in 2019 to help consumers understand R-values and the new R-Value standard.
Philip: Thanks guys. I appreciate the background information and think my readers will find it informative.
Excellent writeup, Phillip!
Good information! Thank you for doing the work, Philip.
Thanks guys. I’ve been a big proponent of Sleeping Pad R-values for the reasons cited in this post. I’m just glad all the manufacturers will be using the same rating system now. It should also spur a little more innovation (and less BS), which is always a good thing for consumers.
Excellent update Philip! Many thanks!
The answer on the foam/inflatable pad order is wrong, at least for how it works in the field. Nearly everyone replicates the same experience, that foam on top is warmer. This is probably because when hikers are doubling up on pads, it is very cold, enough for cold air to affect the pad from the sides, and for a pad like the neoair xlite, it is not constructed to account for this. Maybe the effect is lessened for pads that have insulation in them. Anywho, it doesn’t sound like the new testing method accounts for exposure to cold air on the sides.
I always put my foam on the bottom…
The obvious reason for putting the foam on the bottom is durability, not warmth. If your air pad is punctured its R-value will be rather diminished, to put it mildly, until it’s repaired.
Mordecai is right that “R-values will simply add, and addition is commutative” is an oversimplification. But it’s a very good first approximation, and while I can think of physical reasons why putting the closed-cell pad on top would be warmer, I can also think of reasons for the reverse. Without more testing, I’m not sure which effects are larger. Both kinds of effects should be small corrections compared to the overall added R-value.
Everybody does it differently, so they’re wrong.
Sounds like a backpacking gear myth in the making. Your evidence, Mordecai? Some factual basis?
The best attribute of a foam pad is durability. Using foam on the bottom to protect your air pad is sensible. The majority of hikers are practical people.
I had no idea this take was even in dispute. Let alone myth making.
Are you saying the only way heat is lost through a pad is directly into the ground?
(Seems I can’t reply directly to Doug, no reply tab next to his comment, anyway…)
My current Neoair xlite has about 5k miles on it. No foam pad would do that without becoming compressed and losing it’s R-value.
Put me firmly in the camp of CCF pad on top when stacked with an inflatable pad. While I agree that the equations for my Heat Transfer classes 30 years ago would show that the order of the pads shouldn’t matter, there must be some other real world phenomena that the idealized equations don’t capture.
Every camper I know puts the foam pad on the bottom…
This is correct. The difference is how foam vs. air pads transfer energy – conduction vs. convection. Foam pad always goes on top, though the test apparatus as described wouldn’t tell you this.
This also brings up another question: this test would seem to skew results for large inflatable pads as the metal plates are much smaller than pads tested.
The test doesn’t skew the results that way. You take measurements at 3 locations on each pad, that are all the same size across all the pads.
I’m barred by a very strict copyright (they’re all strict) from sharing the text or diagrams of the standard with you…but one can buy it and read it for $54 using the link in the post.
I don’t think so. With foam on top you can not benefit from the heat radiance from the reflectors built in mats like the TAR Xtherm and STS Comfort Light.
Agree. Foam on top of air mattress is warmer. I’ve tired it both ways. I wanted the foam on the bottom to protect my air mattress, but I was freezing my ass off. Switching it made a huge difference.
I always put my foam on the bottom. Contact with very cold ground (e.g. snow) on the entire surface of the pad will cool the air in the pad significantly, and has a lot more suface area than the sides, so that would seem to be a bigger problem.
I have no evidence it actually makes a difference in practice, but that’s the logic behind my choice
With no options for third-party testing on the horizon, it’s $50K to stay in the game with major retailers like REI and MEC.
I’m guessing that sleeping pad prices will go up, except for those made by REI, Therm-A-Rest, and MEC, who already own Thermetrics R-value testing machines. Or smaller pad makers that can’t afford $50K will stop selling through REI and MEC.
Everything has side effects.
There are no smaller sleeping pad manufacturers selling through REI and MEC, so it’s a moot point. And at $50,000, there’s no reason why multiple small manufacturers can’t buy a R-value machine and share it. Of course REI and MEC could also volunteer to test any pads they sell, because they also own Thermetrics machines. They make money if they sell their own pads or other peoples. My guess is that they’re more focused on their members and giving them a wide selection, rather than pimping their own brand. Everything has side effects, but there are ways to make this work and benefit consumers.
REI’s web site carries 113 sleeping pads from 15 vendors, including a few that could be described as small businesses: ALPS Mountaineering (58 employees), Big Agnes (“We’re a small company”), Klymit (27 employees), and NEMO (23 employees).
Those four companies represent 43% of the pads carried by REI.
I hope they find a way to get their pads tested!
This is exciting news and a very good write-up. You asked all the right questions.
You know vastly more about statistics and sampling than I do, but I used to do that sort of thing in an industrial setting and I’m curious so I’ll bare my ignorance and ask something else.
The sample size for the test seems small to me. Wouldn’t a minimum of 30 samples rather than 3 give you a better chance to reduce variation or at least be able to more accurately measure variance? Or will testing be 3 pads but multiple tests on each pad? How much of the variation is due to the testing procedure versus variations in the pads themselves? I don’t see how that could be determined without a LOT more samples.
I’d really geek out to know the model to model variation versus pad to pad variation among the same model versus spot to spot variation on the same physical pad. But I guess I’d have to pay for the test to know that, huh?
I don’t know the answer to your question about the variance. It’s arguable that there won’t be much, since the pads being tested are manufactured rather than hand-sewn. The variance is probably worked out upstream in the manufacturing process, or it will be for companies that operate at scale if R-values are important.
There’s also the question of what gets reported. Companies such as Therm-a-Rest tend to conservative in their ratings and underreport them, so consumers may never know. In other words you take the floor, rather than the average of the test samples.
The more interesting question (I think) is the variance between testing machines. The standard requires that it remain at -3% and +1% of the calibration samples. That’s pretty tight, which may indicate that commercial pad variance is very low.
Heat loss to the ground? You folks need to get off the ground and get a hammock and UQ. You’ll sleep better too!
Seriously though, thanks Phillip for the great article.
To clarify what the guy means by,” provided there is no air gap between them.” He is referring to a complete gap in which in can pass completely through between pads. Not air pockets trapped between pads. Between a closed cell pad and an air mattress for eg. This would actually result in a slight increase in thermal efficiency as trapped air is insulative, as with a down jacket for eg.
Probably air gaps *between* sleeping pads are most relevant (if at all) at the margin all around the pads, which might increase with decreasing temps and increasing airflow.
One more hitch: ASTM sleeping pad R-values are in U.S. units. Getting European pad makers, standards bodies, or testing organizations to measure and report values in U.S. units probably won’t happen.
Maybe they’ll agree to test in metric R-values (aka “RSI-values”) and we’ll see two sets of values on many pads. A pad with 5 R-value will be 0.88 RSI-value.
Or they could use another unit like tog (1 tog = 0.1 m2K/W = 0.1 RSI-value), so a pad with 5 R-value would be 8.8 tog.
Will be interesting to see how this plays out.
Non issue. Exped is Swiss and a huge proponent of R values. They’ve been using them for years.
Excellent and informative Philip!!
As a new manufacturer of camping and sleeping pads, where could we get some of our products tested for the new standards?
I’m sure there are 3rd party testing facilities available to anyone without having to fork out $50k before even making the first sale.
THANKS again for a well-written article!
Ask REI. Maybe they’ll test it for you. Since the guy at REI was the lead on the standard, I bet he knows who all the outsourcers.
You asked this question of the Thermarest representatives, but I wanted to get your take on it. Have you found air pads to lose some of their perceived warmth when you deflate them a bit? I’m a side sleeper, and when I get the pad deflated enough to be comfortable, my hip is probably 1/4-1/2 inch off the ground. I’m wondering if in doing so I’m losing a lot of the insulating capacities of the pad. Also, have you found deflation to affect air pads’ insulation capabilities regardless of the construction they use to keep you warm (synthetic or down insulation, reflective aluminum layers, or open cell foam)? Thanks for your help!
I think it would, but I haven’t tested it rigorously.
Paul if you have an X-therm or X-lite pad, and your hip is compressing it enough that the radiant barrier foil (i.e. ‘reflective’) layers are touching each other, that would be a thermal bypass in that spot which would increase heat transfer to the ground. Radiant barriers aka low-emissive layers only function when there is an air gap on one or both sides, the idea is not really about reflecting heat, but more about not emitting (radiating) heat from the layer above.
I beleive the Xlite has two ‘reflective’ layers and the X-therm has three, so this would be more likely to happen with the X-therm, but if you’re compressing it down to half an inch or less, I imagine that even with the X-lite, those layers are probably touching.
But I’d love to hear from someone at Therm-a-rest to confirm if this is correct.. :)
Re “Does the moisture in your breath, when you blow up a pad by mouth, reduce or alter the R-value of a Therm-a-Rest sleeping pad?
It might have an effect over the long term in extreme temperatures, but it won’t have any effect in normal use.”
I find this questionable. The moisture content in exhaled breath is quite large, and high humidity air has different thermal properties than dry air. In a pad that consists almost entirely of air (with convection currents) and baffles, and maybe a small amount of fibrous insulation (which is known to be affected by moisture), I find it hard to believe that the water vapor would not have a measurable difference on heat flow.
This is the position that Sea To Summit describes on their blog. I’m no physicist so I’m not 100% sure, but I know a thing or two about insulation and heat flow, and this was what I suspected before reading this:
“One final factor: moist air holds more energy than dry air. Mouth-inflating introduces a lot of breath moisture to the mat which has two effects. Firstly, it conducts heat away from the upper surface of the mat. Secondly, as the moisture vapor condenses it occupies less volume which contributes to mats ‘going soft’ during the course of the night. Inflating a mat with a pump reduces the amount of moisture inside the mat and thus the potential for losing warmth and pressure and avoids the exertion of mouth inflation, (particularly at altitude).”
Now it’s possible that the difference is relatively small. But considering how much difference for example the argon filling in a double-paned window makes, even though that gap is much thinner than an air pad, and the heat capacity of water, it doesn’t make sense to me that a significant change in the makeup of the gas that fills your sleeping pad wouldn’t have a noticeable effect on it’s R-value. I’d love to see some testing of pads filled both ways.
Finally, the cynic in me has to point out that all Sea To Summit air pads ship with a pump sack, and they are usually inflated that way, while most Therm-a-rest pads are inflated by mouth (in part because their current valves don’t work well with pump sacks). So there is an incentive for one company to emphasize the impact of moisture from mouth inflation, and an incentive to minimize it for the other.. I do know that Therm-a-rest plans to upgrade their valves to flat one-way valves (like most of the competition) in the near future, and I’ll be surprised if they don’t start including pump sacks as well. So perhaps we’ll start hearing a slightly different answer from them in the future..
My Thermarest Xtherm came with a pump sack.
I know the X-therms do, though I’m not sure for how long that his been included, I think that might be a relatively recent change. I don’t believe the X-lite pads come with a pump sack.
Whatever the reasons, my impression was that most users of these pads don’t use a pump sack. In any case, it would be great to get some clarification on this!
Already there are compelling reasons to use a pump sack unrelated to insulation- ease and speed of inflation, and minimizing moisture which accelerates the rate of the polyerathane lining degrading (leading to earlier / more frequent pad failures), and also causes mold growth. Also the loss in pressure as the water vapor condenses from a gas to a liquid. But a reduced R-value would be another compelling reason to bring a pump sack instead of mouth inflation.
All their pads have an antifungal inside to prevent mold.
We use a Vaude pumpsack for the Xtherm, works imho better then the sack of TAR.
I have an REI Flash 3 Season insulated (R 3.2) mattress and an REI All Season insulated (R 5.3) air mattress. Both use the Sea to Summit pump dry sack and I NEVER blow into the pump sack to fill it. That way I can keep bacteria out of the mattresses and avoid mold. (And at 9,000+ altitude avoid getting dizzy blowing up a mattress with my lungs!