In this article, I’m doing a deep dive on camping sleeping pads: how they’re tested, how their different levels of insulation are measured (in R -Value), and how some sleeping pads keep you warmer than others.
A sleeping pad’s main goal isn’t just to make you more comfortable while you’re lying on the ground; it’s to increase thermal resistance between your body and the ground, decreasing heat loss from the cold temperatures transferring from the ground to your body, and thus keeping you warmer overnight (when used in conjunction with other elements of your sleep system)
My Key Takes
- Sleeping pads provide warmth and comfort by increasing thermal resistance between your body and the ground.
- R-Value measures a sleeping pad’s thermal resistance; higher values mean greater warmth.
- Different types of sleeping pads include inflatable air mattresses, self-inflating foam mattresses, and closed-cell foam mats.
- Baffle design in inflatable pads affects thermal resistance; continuous baffles generally offer better insulation.
- Personal experience and individual preferences are important when choosing a sleeping pad.
R-Value: A Simple Explanation
The thermal resistance of any type of sleeping pads is represented by an “R-Value” (i.e. R for “resistance”); a value which is assigned to a sleeping pad after its being subjected to a rigorous standardised testing procedure: ASTM (that’s the American Society for Testing and Materials) F3340-22.
The R-Value scale for sleeping pads runs between measures of around 1 to 9 – and the higher the number, the warmer the pad is going to be.
- R-Value less than 2 would be appropriate for summer nights (temperature > 50F or 10C)
- R-Value between 2 to 3.9 would be appropriate for for three-season use (temperature above 32F or 0C)
- R -Value between 4 and 5.5 would be suitable for winter use (temperature above 0F or -18C)
- R-Value above 5.5 would be for extreme winter use (temperature below 0F or -18C)
F3340-22: ASTM’s Standardized Testing Method to Determine the Warmth Ratings of Sleeping Pads
The warmth ratings of all retail-approved sleeping pads (which correspond to the R-Values described above) are assigned as a result of standardized testing, done on each pad, by ASTM international. The testing method they use has existed in multiple iterations, and was updated most recently in 2022.
The first seven pages of F3340-22 (the most recently-updated version of their standardized test method) can be viewed here – but we’ve summarized the key points for you.
The ASTM states that their test method “covers the measurement of the thermal resistance of camping mattresses in conditions of steady-state heat transfer, using a two-plate apparatus”.
To determine its thermal resistance, “the camping mattress is held under constant compressive force between a guarded hot plate and a cold plate. The primary heat transfer for this method is one dimensional, vertically through the camping mattress thickness. As such this measured thermal insulation value is a repeatable comparative measurement.”
They do, however, state “this value [i.e. the value of thermal insulation provided by the pad] will not always correlate to actual insulation performance as it is feasible that real-world heat transfer will differ slightly because of additional heat losses possibly involved”.
The testing method used by ASTM to determine the thermal resistance of sleeping pads “is applicable to all types of camping mattresses”.
Types of Sleeping Pads Recognized in ASTM F3340-22
Thanks to the multitude of camping situations in which you could find yourself (ranging from a relatively mild night in, say, mid-July on the US East Coast to a shoulder season camping trip in the Cascades), so too are there numerous shapes and sizes of sleeping pads to fit each individual situation.
Those “all types of camping mattresses” officially recognised by ASTM F3340-22 are as follows:
- Inflating air mattress with or without insulation
- Inflating air mattress with reflective materials
- Self-inflating open cell foam mattress with or without coring
- Closed cell non-inflatable foam mats
Inflating air mattress with or without insulation:
Inflating air mattresses (or “inflatable sleeping pads”) work by using multiple air baffles which, when inflated, lift the user off the ground and reduce heat transfer between the two.
The insulation that ASTM F3340-22 refers to in this description is any additional synthetic insulation (similar to the stuff that’s inside synthetic sleeping bags) that’s built into the baffles of the sleeping pad to increase temperature resistance between it and the cold ground.
Any synthetic insulation that’s added into an inflating air mattress is aimed to increase levels of temperature resistance against the cold air from the ground to decrease heat loss – and keep you warmer overnight.
Inflating air mattress with or without reflective materials:
Some inflatable pads designed for use in colder conditions (like the Therm-A-Rest NeoAir XTherm) have extra material on the inside that’s designed to reflect radiative heat from your body and thus keep you warmer. Therm-A-Rest’s version of this reflective material is called “Thermacapture”.
In the XTherm’s case, this reflective material is used in conjunction with the same “triangular core matrix” that’s used inside their NeoAir XLite NXT that’s designed to trap convective heat.
The material arranged to create this “triangular core matrix” (as well as the other internal baffle designs used by other manufacturers to achieve the same result and trap convective heat) is a lot more delicate than you may think. A couple of years ago, I cut a 2015 model of the Therma-A-Rest NeoAir into about ⅔ of its original length, with the goal of then ironing it back together and creating a torso-length sleeping pad (which went surprisingly well – I still use it to this day) – and I was shocked by quite how thin, almost papery, the materials creating this matrix (thereby also joining the top of the pad to the bottom) were.
Self-inflating open cell foam mattress with or without coring:
Instead of using air to fill a number of individual baffles within the sleeping pad, self-inflating open cell foam mattresses use compressible foam. This technology works by pushing out air from tiny interconnected chambers while compressed (with the valve closed to prevent the foam from taking in any unwanted air at the same time as the user is trying to compress it), and pulling air back in when de-compressing the foam (with the valve open, letting air in) and unpacking the sleeping pad for use.
As there’s obviously only so far the foam can compress, a self-inflating open cell foam mattress won’t pack down as small as an inflatable air mattress – and won’t be as light (as, clearly, it’s full of foam and not just air) – but they’re thicker than traditional closed-cell non-inflatable foam mats and generally have a higher R-value.
Closed cell non-inflatable foam mats:
Closed cell non-inflatable foam mats work by creating a physical barrier between your body and the ground when camping, in much the same process as a self-inflating open cell foam mattress (though they’re not compressible at all – once they’re rolled/folded up, that’s the best packed size you’re going to get).
As temperature passes more easily through physical surfaces than through the air, and no additional insulation is built into it (unlike in some inflating mats, as we discussed earlier), a closed-cell foam pad won’t be as warm as a self-inflating open cell mattress or an inflating air mattress.
Much like how different manufacturers of inflatable air mattresses choose to employ different baffle designs, not all closed-cell foam mats are created equal.
The ASTM F3340-22 document doesn’t outline the different types specifically; but what you’ll mostly find are ones that either just look like a rolled up length of foam (used for decades by Scouts the world over, and aren’t the most comfortable piece of equipment you’ll ever own; but have got enough people through a night’s sleep to ensure that they remain a mainstay of the campsite) or, in slightly higher tech, ones that fold up into segments with little divots in them and are orange on one side and silver on the other.
The second I describe is the Therm-A-Rest Z-Lite Sol; an incredibly popular mattress for long-distance hikers. Though I don’t usually use a foam pad, I did end up on one of these at the end of my Pacific Crest Trail thru-hike in 2024 after my air mattress (another Therm-A-Rest, the NeoAir XLite NXT) failed quite dramatically – and actually really quite liked it. Though it’s obviously not quite as comfy as an inflating air mattress, I definitely found myself just about warm and comfortable enough on the ground, even out in early fall in the North Cascades.
How baffle design affects thermal resistance in inflatable pads
Baffles are the air pockets found in an inflating air mattress. Having been inflated, sleeping pad baffles serve two main purposes: to help the pad maintain a defined structure for a person to lie on, and to insulate the user from colder temperatures on the ground below it.
Without baffles, an inflating air mattress wouldn’t have anything to maintain a flat structure, and would just end up like a massive, very expensive, balloon. Baffle designs are either continuous or non-continuous (i.e. the baffles are either interrupted, or not, by extra material in between them), with either option providing certain methods aimed to increase temperature resistance and decrease heat loss.
Continuous Baffles
Continuous baffles are long, continuous pockets of air uninterrupted by design-intentional gaps between the insulation. They’re either vertical or horizontal, and will run continuously from one side (or end, if vertical) of the pad to the other.
If you’re unsure about whether your existing pad has continuous baffles or not, shine a flashlight into it and take a look from the other side; you’ll see whether the baffles are continuous or whether there are little gaps built in between the baffles.
Because continuous baffles don’t have any gaps interrupting the insulation (be that either just the baffles alone or the baffles as well as any other additional insulative/heat reflective materials inside them), there’s less chance for unintentional heat loss.
As a long-time proponent of the ever-reliable Therm-A-Rest NeoAir XLite (including both the 2023 and 2024 NXT models, as well as the ancient 2015 version I cut into ⅔ length which I mentioned earlier – which still works), I’m firmly in the continuous baffle camp.
I’ve never personally found my night’s sleep suffering as a result of what’s allegedly the main drawback of an inflating pad with continuous baffles; namely, that it’s less comfortable than one with non-continuous baffles (with the argument that there’s significantly less scope for manufacturers to create original, comfort-focused baffle design outside of just being arranged horizontally or vertically).
In fact, I’ve always found the Therm-A-Rest NeoAir (which, again, does have continuous baffles) to be pretty versatile. One thing I’d really recommend when setting it up for your night’s sleep is not to inflate it the whole way – leave a little give, a little bounce, in it. It’ll still function just as it’s intended, but you’ll mould more comfortably into it – rather than just lying straight down on what really is quite a taught surface when it’s fully inflated.
Non-Continuous Baffles
If the air baffles inside an inflating air mattress are interrupted by a join or a seam, they’re non-continuous.
Klymit’s Static V pad is a striking example of an inflating air mattress with non-continuous baffles. Klymit makes a variety of models of this particular pad (ranging from the regular Static V to the Insulated Static V Luxe), but each model deploys some intriguing, downward-V-shaped baffles – with the goal of maintaining decent levels of thermal resistance while also giving users higher levels of comfort than one might achieve on a pad with more traditional horizontal baffles.
In Klymit’s words, the Static V “conforms to the shape of your body while maintaining ample, lofted air pockets. The result: side, back, and stomach sleepers can all rest comfortably on a generous bed of air”.
This is a valiant effort, and one that I’m fully behind; before switching to a quilt that attaches directly to my sleeping pad (and before discovering the not-fully-inflating-your-inflatable-pad trick I mentioned a few paragraphs ago), I spent numerous hours lying essentially half-off an old, very noisy model of the Thermarest NeoAir after slipping off it in the middle of the night – so the idea of having a pad that performs well while tackling both discomfort and heat loss sounds very appealing.
A problem that’s been discovered on these particular pads, however, is a distinct lack of any kind of thermal resistance at all between the baffles – leading to noticeable levels of heat loss which, in turn, means a colder night.
Not all users experience noticeable problems, though. In fact, our CEO Chris has owned a Klymit Static V for a while now, and reports no problems at all. It’s important to note, though, that some people do sleep warmer than others – so what holds true for one sleeping pad for one person might not work for another.
References
American Society for Testing and Materials (2022) ‘F3340-22”, accessed December 2024, available from: https://cdn.standards.iteh.ai/samples/112900/775659741b374013a61d2927880be384/ASTM-F3340-22.pdf
