What you don’t know may not kill you; but it could lead to you being cold and uncomfortable.
Air-filled sleeping mats have become very popular in the last few years; they are lighter and have a more compact packed volume than foam-filled self-inflating mats. (If you’d like to read a little on the evolution of mats, you can do so HERE
But – do they work? And if they do work, how do they work?
There are two principles you need to consider with regards to sleeping warmly and comfortably. They are:
(A number of other factors will affect your choice of a sleeping mat, including ease of use, packed volume and weight, reliability and expected lifespan and of course, warmth. We’ll deal with these topics in a follow-on post entitled ‘Which mat to choose’. For now, we’re just concentrating on what makes an air-filled mat work)
When you lay on an air-filled mat, you warm the surface of the mat immediately underneath your body.
Even in summer, the ground is colder than your body, and of course, in spring and fall or even winter it may be much colder. As a result, the warmth flows from your body into the ground in one of two ways:
One is a circular air current which is created by the warm air from the top side of the mat swapping places with the cold side of the mat; this is called convection. The other is warmth which simply flows from the top side of the mat to the colder bottom side through a process called radiation.
Convection can be prevented by reducing the movement of air within the mat: either by the type of construction used (small chambers allow less air movement than larger chambers, even more so if they do not allow air to pass from the top of the mat to the bottom) or by using an insulating medium such as Primaloft® or Thermolite®. Mats made of large, tubular chambers do not suppress convection well.
Radiation can be prevented by using a reflective barrier. Unfortunately, some of these barriers are noisy. Exkin® Platinum, used in Sea to Summit mats, is a non-woven material (not a film), and is virtually silent.
Loss of warmth in a mat is compounded massively by air being ‘pumped around’ the chambers of the mat by the movement of the sleeper. With some mat types – specifically the ones with large, ‘cushy’ chambers – simply rolling over in your sleep squeezes the warm air from beneath your torso off to the extremities of the mat where it dissipates very effectively, and thus you get cold. Stable, baffled constructions are much better than large tubes in this respect as they reduce the amount of air being moved (which can be significant). Air Sprung Cells™ are better still: press down on one cell and the amount of air which moves is very, very small – and your weight is spread over many, many cells which virtually eliminate internal air movement.
One final factor is that moist air can hold more energy than dry air. If you mouth-inflate a mat, you will inevitably include a lot of moisture with your breath. This moisture will have two effects. Firstly, it will conduct heat away from the upper surface of the mat – inflating a mat with a pump will reduce the amount of moisture inside the mat and thus the potential for losing warmth. Secondly, as the moisture vapor condenses it occupies less volume which contributes to mats ‘going soft’ during the course of the night. Using a pump is therefore a good option (quite apart from avoiding the exertion of mouth inflation, particularly at altitude). Sea to Summit has two excellent pumps available.
The ability of a mat to insulate you from the cold ground is expressed as an R-value equivalent. This can be calculated by placing a warm plate on one side of a mat and measuring how much energy is necessary to raise the temperature of a plate on the other side by a specific amount. It is important to know that not all mats are tested. Some quoted ‘R-values’ are simply estimated, and some mats offered for sale quote a ‘temperature rating’. This rating is nothing more than a manufacturer’s estimate: there is no test involved, and no standard which must be followed.
In contrast, Sea to Summit has its R-values computed by the EMPA Labs in Switzerland – and the Air Sprung Cell™ construction means that these laboratory values are also real-world relevant.
You can read about the R-Value which you will need for your type of use in the blog post ‘Which mat to choose’
Comfort is, of course, subjective: there is no scale, no calculation, no empirical International Comfort Units which can be measured.
However, there are certain construction principles which affect comfort. It’s important to note that ‘cushy’ does not necessarily translate into ‘comfortable’ – a soft and squishy mat will just feel wobbly underneath you, and may result in you laying on the ground next to the mat as the large squishy chambers collapse under you as you roll over. The two most important considerations when it comes to construction are lateral stability and vertical compressibility.
Is the mat stable when you roll over? Longitudinal tubes are less stable than transverse tubes; both are less stable than Air Sprung Cells™. Merely laying on your back when you test a mat will not indicate whether you will sleep well: most mats are fairly comfortable if you remain static. If you feel the mat wobbling beneath you, or if you roll over and the outermost tube squishes flat causing you to roll off of the mat, try another brand.
Do you ‘bottom out’ when you sleep on your side? Tubular constructions allow a large amount of air to move within each chamber, which can result in your hip and shoulder touching the ground. Not only is this uncomfortable, it also results in instant heat loss. Making tubes of a larger diameter is an inexpensive way to reduce ‘bottoming out’, but it also makes the mats more wobbly. Lots of smaller baffles are better than tubes in this respect, Air Sprung Cells™ are better still. In addition, the upper and lower chambers of the Comfort Plus mat can be independently pressurized according to your preference; this is both laterally very stable and almost completely eliminates bottoming out.
Hopefully, this will give you an indication of the physical principles involved in sleeping warmly and comfortably. As always, if you have any specific questions, just ask.