As temperatures drop all around us here in the Northern Hemisphere, making sure we are warm when camping is vital. In many places, the discussion as winter arrives is about insulation: a warmer sleeping bag, adding layers and so on, all to avoid heat loss from convection and radiation. When the temperatures are low though, we face a very big issue: a cold ground and (often) not enough insulation to protect us from heat loss via conduction. A cold ground for tent sleepers is a major issue as your body heat seeps into the ground throughout the night, getting you very, very cold.
The solution for the conduction problem between the body and the ground is a sleeping mat, or one of the other various names it has out there. Outdoor sleeping mats are first and foremost there to insulate the sleeper from the ground. Comfort is secondary to the insulation, though it is very important for your sanity!
And this is where we encounter to R-values: R-values are (usually) the industry standard for defining how well a sleeping mat will insulate you from the ground. You might know R-value from other places, notably as a housing insulation measurement, and the idea is very similar.
What is R-Value?
R-Value is a measure of thermal resistance, or the reciprocal of thermal conductance (otherwise known as the heat transfer coefficient). In normal human terms, R-Value tells us how well a specific material prevents the transfer of heat. R-value is a function of the thickness and the thermal conductivity value (k) of a material. In order to the determine the R-Value of an item, like a sleeping mat, we need to know the k value of the material that it is made of. Alternatively, a sleeping mat can be tested for in a lab.
Frustratingly, there is no one way to test R-value and so the results can vary (although this may change). Some companies actually bother to test the mats, while others just estimate. Some use the hot plate test, while other use the cold room test. Usually, if a mat is tested in lab conditions, it should be pretty accurate.
R-Value in the Real World
The big problem with R-value testing is that it is done in a lab, where wind, sleeping position, your latest meal or even condensation aren't factored in. Side sleepers tend to get less R-Value from their mattress; having a higher body mass (aka, being heavy) reduces the mat's thickness and so its R-Value.
Before relying on your new mat to keep you warm in any conditions, test it closer to home with your full setup to see how it functions. You might be a cold sleeper...
What Does That Mean for You?
The bottom line for R-Value on a mattress is: the higher the number, the more insulating it is. The recommended R-Values for different seasons are as follows (for the average person):
- Summer - 1 to 2 R-value
- Shoulder seasons - 3-5 R-values
- Winter - 6+ R-values
Picking a R-Value that is too low for the season you are backpacking in will usually result in a bad night's sleep, and in extreme conditions, hypothermia. Not surprisingly, the higher the R-value, the heavier and more expensive the mat is, but you'll sleep better.
R-Value, CLO, TOG, and Temperature - What is the Connection?
To make things complicated, and I mean really complicated, R-Values are not the only rating system out there. I won't go very deeply into each, just a quick explanation and the relationship between them. Caution, this will involve math, physics and function - you have been warned....
Essentially the same as R-value in measurement but used to measure the heat resistance of warm clothing. The units represent the same level resistance but CLO is based on the metric system while R-Value is Imperial, so:
1 CLO = 0.155 K·m²·W⁻¹ = 0.88 R (where R means ft2·°F·hr/Btu)
What the CLO is measuring is the amount of energy needed by a resting person to maintain thermal equilibrium (staying comfortable) at 21°C (70°F).
The British version of CLO, usually used for duvets but might be used for down clothing, it is really the same thing with this relationship:
1 tog = 0.645 CLO = 0.5632 R
This is where the water becomes really murky - stating the temperature of a sleeping pad is pretty problematic as we first need to know: environment temperature or ground temperature? You can see how this easily ruins any chance of comparison, especially as most temperatures are ranges.
Nonetheless, there should be some relationship between the two but there is no real math here, just estimates. As it is, there is no company out there that is willing to make a chart that puts temps to R-values. (Big Agnes had one, but it has disappeared.)
Picking the Right Mat
Picking the right camping mat can be a real challenge, especially when considering the many options of R-Value, thickness, weight, materials, etc. As this post is about R-Values only, I suggest reading this great post by Paul Magnanti: Sleeping Pads – A grounded view. It is informative and gives a true overview about what to look for in a mat.
Personally, I prefer air mats that are wider than usual. I have used the Thermarest NeoAir (1st version) for years but as I'm working on a new mat, I have been using the prototypes. My current prototype is a 3.4 R-value mat that is regular length and wide (25") and it is working great for me in pretty much all conditions, even if it is heavier than the NeoAir. (Update: This prototype has been discontinued.)
Getting More From Your Sleeping Mat - The Stacking Game
One thing that is really great about R-value is that it is aggregated - you can stack 4 mats with R-value 1 to get R-value 4, and that is really convenient.
The real life implication is that you rarely need to have a 4 seasons mat, but actually should have a solid 3 seasons mat (3-4 R-value) and either stack another warm mat (3-4 R-value) or, better, stack a couple of closed cell mats (2-3 R-value) that will offer insulation even if your warmer mat loses air (those tend to be air mats or self inflating mats).
That is it - a quick look into R-Values, how to treat them and how to get the best sleeping mat for your needs. Remember to get a mattress that was tested in a lab and always test it in "close to home" conditions before committing to a night on a cold ground in the middle of nowhere.