If you have ever gone camping and woken up in the middle of the night shivering and freezing cold it might be because you have the wrong sleeping pad. Specifically, it might be because you have a sleeping pad with the wrong R-Value. This single value is one of the most important attributes of any sleeping pad yet it also remains poorly misunderstood by many new and experienced campers.
Thankfully, a brief review of R-Values and their implications is all it takes to be brought up to speed. An understanding of R-values, and the role they should play in selecting a sleeping pad, can help save you the worry and risk of body heat loss and even hypothermia. It is simply that important.
What is an R-Value?
An R-Value is a very important measure of the thermal resistance of something and it applies to more than just sleeping pads. Everything from the insulation of your house to the glass on a car can have an R-Value. Engineers pay very close attention to this number and you should too.
R-Values basically tell you how well a substance can stop or slow the movement of heat from one side of that substance to the other. Heat can move in different ways such as radiation, think of the heat you feel from a warm fire, and convection, as happens with warm air rising and cold air sinking. R-values measure resistance to a third form of heat movement called conduction.
Conduction is direct heat transfer through a substance. This is what happens when the handle on a boiling pot of water gets too hot to grab. Heat transfers from the stove to the metal of the pot, and into the handle. This is also how your body rapidly loses heat when sleeping on the cold hard ground during a camping trip. Having a sleeping pad with the appropriate R-Value for the environment you are in can prevent this.
Which R-Value is appropriate?
R-Values for sleeping pads range from 0.5 to about 5.5 and the colder it is outside the higher you are going to want that number to be. Keep in mind that the sleeping pads are rated for how well they insulate your body from the relatively cooler temperatures of the ground. So, when picking one you need to be thinking about the temperature and type of surface you will be sleeping on and not just the ambient air temperature.
If the ground is a nice balmy 60° F (16° C) then an R-Value of 0.5 will suit you just fine. As the ground approaches and dips below freezing temperatures then R-Values of 2.0 or 2.5 become more appropriate, anything lower and your body heat is going to be conducted into the Earth.
R-Values appropriate for negative temperatures kick in around 3.5 or so and anything with an R-Value of 5.5 or higher will provide the best insulation. At that point, it is usually best to get the best-rated equipment you can afford while also verifying its effectiveness through third party reviews.
Choosing a sleeping pad
When you are in the market for a sleeping pad, you will find that not every company lists an R-Value, typically Big Agnes and NEMO. Some manufacturers will just provide a temperature range for which they recommend their product’s effectiveness. Regardless, whether there is an R-Value or not it is important to understand that none of these ratings are standardized or independently verified. This means one company’s 3.0 R-Value might feel more like a 3.5 R-Value from another company.
Also, some company’s sleeping pads will have an R-Value of NA or there just won’t be any information listed about the temperature rating. Usually, this means no testing was performed by the manufacturer so it’s anybody’s guess how well the pad will insulate you.
When selecting a sleeping pad you should consider the R-Value along with the weight, thickness, and type of pad. These all work together to create different degrees of functionality. All these metrics often increase together so a higher R-Value sleeping pad is also thicker and heavier. This isn’t always the case though so if you are planning a long hike and you are trying to save on weight, it is worth doing the research. This is especially important for ultralight backpackers, you can save weight here.
Side sleepers and women must also make special considerations when choosing sleeping pads. If you sleep on your side then you might want to avoid one of the inflatable sleeping pads or go for a higher R-Value than you expect you need. This is because not all of your body will be in contact with the sleeping pad surface so you won’t receive the full effects. Some sleeping pads depend on body heat to warm up and insulate you as well so the lack of body contact will inhibit that.
Women should also choose a sleeping pad with a slightly higher R-Value than needed because they have less body mass than men. The lower body mass means they retain less heat and are more susceptible to hypothermia and other conditions. This also applies to children and generally for anyone who tends to have a cold body when sleeping.
Anyone in need of more insulation from their sleeping pad might also consider buying a second pad. The R-Values add up so the insulation provided by a 1.5 and a 2.0 stacked on top of each other would roughly equal that provided by an R-Value 3.5 sleeping pad.
Overall, you should consider the choices you make with your sleeping pad as part of a bigger picture. Your tent, sleeping bag, clothing and sleeping pad all work together to properly insulate you from the environment. Collectively you have to consider the temperatures you might encounter, the weight you can carry, and the amount you can spend on each item.
You must also seriously consider the environment that you will be in relative to your personal preferences. Namely, what type of ground will you be sleeping on and how sensitive you are to it. For example if you are expecting sharp sticks and rocks the whole way and you know you are a light sleeper then go for a thicker closed cell pad. Alternatively, if you’re looking at warm grassy meadows and you sleep well anywhere you might just get by with the thinnest and lightest pad available.
The dangers of having the wrong sleeping pad
Whenever a warm object comes into contact with something cooler, heat begins to transfer into that cooler object. When that warm object is your sleeping body and the cooler object is the ground it becomes dangerous. At best the heat transfer is minimal and you just wake up tired and sore, at worst hypothermia can set in.
Conduction of body heat into the ground is not the only way our body loses heat. A significant amount of body heat can be lost through radiation and evaporation. When radiation, evaporation, and conduction aren’t kept in check that’s when the real danger occurs. In freezing temperatures the body can quickly lose heat faster than it is produced.
The ground will often be significantly cooler than the air around it and this is even more of a problem if the ground has frozen. Under these conditions laying down to sleep and exposing so much of your relatively warm body mass to the cold becomes a risk. The right sleeping pad can prevent an emergency by trapping your body heat above ground and in you.
Without the right sleeping pad, your body will slowly radiate and conduct heat to the colder surroundings. This might not pose an immediate risk, depending on the degree of cold, but hypothermia can set in slowly over time. It doesn’t have to be quick or extreme.
Taking the time to buy the right sleeping pad with an appropriate R-Value for the environment it will be used in is very worth it. As temperatures outside drop. ground temperatures will be even lower. When you lay down on that cold ground, even from within a tent, all your body heat is at risk of being slowly conducted into the ground. The right sleeping pad can prevent this.
With proper insulation helping you maintain your body heat throughout the night you can wake up refreshed and energized as opposed to weak and drained. You will also stave off the pernicious effects of hypothermia which can set in even when you don’t expect it.
Bonus Tip: Learn the differences between mild and severe hypothermia with Dr. Donner!
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