ASTM FF3340-18 R-Values Tell Us Little About A Sleeping Mat's Real-World Performance

Contents

• ASTM's FF3340-18 R-Value Standard
• The ASTM Working Group
• The Dynamic Sleeper
• r/ultralight: R-Value is Dead as a Metric for Sleeping Pad Warmth Evaluation
• Conclusion: The Scramble Experience
• Final Words: Some Useful Advice From On High

Introduction: The ASTM FF3340-18 R-value standard

R-Value is a measure of resistance to the flow of heat through a material of a given thickness, the higher the R-Value, the better the insulation. A major driver of insulation research and testing has come from the construction / home-building sector. Importantly, insulation in homes is housed in a rather static environment largely free of dynamic forces.

In 2020 the outdoor industry adopted a new testing methodology to standardise the measure of insulation (R-Value) for sleeping mats, known as ASTM FF3340-18.  

Ultralight manufacturer Yamatomichi explain some key features of the ASTM F3340-18 methodology:

In a sealed environment at 20°C (room temperature), the pad is placed between two metal plates. The upper plate is subjected to a pressure of 2 kPa to simulate a person lying on it and is maintained at a temperature of 35°C to mimic body heat. The lower plate, representing the ground at a campsite, is kept at 5°C. The energy required to maintain the upper plate at a constant temperature is measured. This energy is then used in a formula to calculate the R-value. Pads with higher insulation require less energy and thus have a higher R-value, while pads with lower insulation need more energy, resulting in a lower R-value.

In our view, the testing protocol greatly overestimates the real-world effect of thick air beds (whether insulated with synthetic or down fill and/or with radiation reflective films and fabrics) and diminishes the real-world efficacy of closed cell foam (CCF) and open cell (memory) foam self-inflating mats. This has created a malincentive to chase ASTM R-Values (of dubious significance) by creating conveniently ultra-expensive, super-comfy, air-mattresses rather then genuine survival tools for sub zero conditions.  
 

The ASTM Working Group

A big red flag is when the industry leaders populate the working group that creates the standard and testing protocol:

We (Cascade Designs / Thermarest) were one of the founding brands of the ASTM (American Society for Testing and Materials) Working Group which involved a variety of camping gear brands and retailers. Therm-a-Rest has always been an advocate for using R-value as the most objective way to measure the insulation of your sleeping pad.

Guess what, if I was in charge of the protocol to test beauty and intelligence, imagine how beautifully intelligent I would be "proven" to be. To be clear, we're not saying there's anything wrong with R-Value as a measure for insulation; we're simply suggesting that the current lab testing protocol is not fit for purpose.  
 

The Dynamic Sleeper

UOG sum up the thermodynamics of sleeping mats:

When you lie down on a sleeping mat, your body heat begins to transfer to the colder ground through three primary mechanisms: conduction, convection, and radiation. A good sleeping mat counteracts all three. It reduces conduction by creating a barrier between you and the ground. It minimises convection by trapping air (in the case of inflatable and Self Inflating Mats), and some mats even reflect radiant heat to your body using unique materials, such as reflective foil.

Below we've included a post made on the Reddit sub r/ultralight by contributor Wandering_Hick regarding potential issues with the ASTM testing protocol. We agree with many of his concerns (but don't agree that subjective "real world testing is the best indicator"); we've highlighted some of the most salient points. However, we would add that a significant failing of the test protocol is that it doesn't account for a moving body. As we mentioned earlier, home insulation resides in a static environment whereas sleeping mats are acted upon by a dynamic force; the contoured sleeper moves in his or her sleep and is not a flat, static plate.

On the convection issue, UK Climbing point out:

The simplest air mattresses hold lots of air, but what you find sleeping on them in cool to cold temperatures is that they do not insulate well. What happens is the temperature difference between your body and the cold ground below produce convection currents inside the chambers of the mat and this circulation soon means heat is being lost from your body. So while open chamber air mattresses are comfy on rough ground they're not good at insulating you from the cold ground. I’ve found pure air mattresses to be unpleasantly cold even in temperatures in single digits but still above freezing. Companies have tried different ways to stop the circulation – either complicated interlocking chambers or insulation in open chambers; down or synthetic.

Restless bodies only increase the movement of air inside a sleeping mat's chambers (small or large, and such chambers are massive compared to the tiny spaces in closed cell or open cell foam). Manufacturers like Sea to Summit have attempted to mitigate this effect by creating small welded chambers (to reduce heat-loss via convection), then layering a reflective non-woven textile (Exkin Platinum) through the centre of the chambers to prevent radiative heat-loss to the (colder) underside of the mat. But, as we've seen, the test is conducted at room temperature (!!!), which is not exactly stress-testing such mitigation strategies (more on this later).

The test protocol would ideally be conducted in a much colder chamber environment, with a ground plate at or below freezing with a heated copper mannequin that roles from side to side, like a pit-roast pig on a skewer, but pressed into the mat ... then, how quickly does the mannequin's temperature fall to X degrees or how much energy is required to keep the mannequin at X temperature?  Two static flat plates at room temperature on a 5°C ground tells us little about what happens when we're out on snow covered, freezing ground, lying in a fetal position trying desperately to stay warm, because we bought a lilo pool lounger with an R-Value of 7.0.

Here is the post from r/Ultralight (on Reddit) by Wandering_Hick (simply add our concerns regarding the lack of consideration for any dynamic forces on the mat's surface to his list of concerns):

r/ultralight: R-Value is Dead as a Metric for Sleeping Pad Warmth Evaluation (we should stop referencing it)

Edit 2: I don't think I was clear enough with the point of my original post. I am not saying the R-value numbers are wrong. The test does a great job measuring how well a sleeping pad resists heat transfer between two plates under specific conditions. But the intent of the test is to provide a prediction of how warm a pad will sleep in the real world. Because of how inaccurate the test is in its predictions, I think its usefulness is diminished to such a degree that it should be ignored when trying to determine how warm a sleeping pad will sleep in the field.

It was exciting when the new ASTM R-Value testing standard came out in 2019/2020, but the last 6 years of testing multiple dozens of sleeping pads has led me to the conclusion that the standard is basically useless for determining TRUE sleeping pad warmth.

Pretty much every sleeping pad manufacturer, including the mainstream ones, have at least one pad where the real-world performance of the pad does not line up with what would be expected based on the r-value.

I don't have access to an r-value testing machine, but my theory as to why the testing standard results in pads with a high tested r-value sleeping cold in the field has two parts. I am assuming a basic understanding of the r-value testing process. Thermarest has a video on it if you're not familiar.

The testing happens at room temperature. When convective heat transfer mechanisms inside a sleeping pad are minimal (the air is well connected), that leads to the ~20C room temperature air influencing the pad in a way that might inflate the r-value. I've seen this effect the most with pads that have a lot of reflective film inside of them for reducing radiative heat transfer, but minimal convective heat transfer minimization mechanisms. Thick pads will also be influenced more by this mechanism because of more surface area exposed to the room temperature air.

The testing machine measures an average of heat transfer across an area of the pad. Pads that have "holes" through the insulation might end up with a high average, but I find that the holes act as channels of heat loss that lead to a pad feeling colder. I'm sure there is a thermodynamics explanation for this, but that's above my knowledge level.

So how do we know if a sleeping pad is warm or not? R-value can provide an indication of a pad's potential warmth, but real world testing is the best indicator. I'm always skeptical of thick pads. I also think it is a positive indicator for a pad when it is difficult to deflate (when equipped with a wide dump valve); this suggests the pad has good mechanisms inside it for minimizing convective heat transfer.

Ideally, companies would be doing real world testing themselves and not trying to pass >6 R-value pads off as winter pads if they know the pads can't handle frozen ground/snow and temperatures well below freezing. It'd be great to have the ASTM standard revised in a way that gives an accurate indication of sleeping pad warmth.

I felt like I needed to make this post after continuing to see a lot of references to R-value as the best indicator of warmth in posts, reviews, and from companies. I also think it is sending the industry in the wrong direction and wasting R&D potential. Companies are designing pads for the ASTM standard instead of designing pads for actual warmth. It's a race for the highest useless number and its sad to see.

Edit 1: I'm not sure what the best alternative is. The only thing that I have brainstormed is using a modified sleeping bag test system.

• Have a chamber where you can control the air and ground temperature and humidity.
• Place a sleeping bag testing dummy inside a quilt so the bottom is exposed (maybe a standard temp? sleeping bags have a standard temp pad for its tests).
• Measure how much energy it takes to keep the dummy at human body temperature. Then drop the air and ground temp continuously and see how the energy needs change to maintain body temp.
   

Conclusion: The Scramble Experience

We've come to a number of conclusions over the years of testing sleeping mats:

• Take the current ASTM FF3340-18 R-Value with a large pinch of salt, especially for thick, high R-Value "insulated" air mats, and an even larger pinch of salt when comparing different types of sleeping mat's R-Values.
• Don't put all your eggs in one expensive inflatable basket.
• Since using Tyvek sleeping mat protectors, failures have come from a) weld points, b) delamination (glue), and 3) slow leaks from poorly implimented valves NONE from punctures.
• When considering cost, view air-filled sleeping pads as consumables (they fail) not long term investments.  
• If you do go with an air mat, use one by Exped, Thermarest or Sea To Summit (perhaps Nemo), and do not trust the lesser brands (valve let-downs have all come from brands other than these)
• Closed Cell Foam (CCF) has long been the staple of high altitude mountaineers for a reason (see below).
• Many use air-mats as the primary insulator with CCF mats as additional insulation but mainly as a protective layer against punctures.  We've increasingly moved toward the inverse position: to use non-inflatable CCF as the primary insulator with a modular approach to add insulation and comfort via small self-inflating "sit-mats" (by brands like Exped and Sea To Summit) under the CCF.

We would love to have Multimat's 38mm Expedition Summit Compact 38S mat tested with the latest ASTM setup. Personally, I've been toasty with this mat down to -12°C (10.4°F) and others in the Scramble team, quite a bit below that. But my guess is, that its current ASTM R-Value would be in the 4s (at best), indicating it's more of a warm 3-season mat. Just a guess, but that's the difference people are discovering between the real world vs. the fantasy brought to you by ASTM FF3340-18.

Finally, we've written this so readers will understand why the outdoor industry has failed to create a distinct product that, in good conscience we can recommend as a replacement for Multimat's excellent Expedition Summit Compact 38S Self-Inflating Sleeping Mat and why we're testing an approach which is antithetical to the industry's current dogma; a dogma and direction driven in our view by a test that is providing near-meaningless measures.

Now, if we were good little consumers, we'd simply shut up and shell out the £190 (RRP) for a 9cm thick, 357g (ex bag) Exped Ultra 6.5R M Mummy Sleeping Mat with an R-Value of  6.9 and sing our praises to Exped for the extra warmth, comfort and appoximate 150g saving compared to Multimat's dicontinued Expedition Summit we once admired.

But here's a thought ...
 

Final Words:  Some Useful Advice From On High

Interestingly, we've come to a strikingly similar conclusion to this group of high altitude mountaineers in their post on High Altitude Alpine Style Sleeping Systems (perhaps for slightly different reasons, but what do they know that Exped and Thermarest aren't telling us?):

Use A Smaller Mat

Don’t be the climber messing about trying to inflate a full sized sleeping mat by mouth at 7500m whilst everyone waits in the cold, it’s irresponsible and dangerous. Redundancy demands you carry at least half your sleeping mat in puncture-proof foam, so combining that with a small self-inflating (i.e.  foam cored) mat – with the closed cell foam on top, takes just a few breaths and gets you in the tent and out of the weather fast.

The lower half of you gets insulated with your pack – now empty since your (down) suit, (sleeping) bag and food has been taken out.

Last Updated: 16/07/26