Types of Sleeping Pad Construction
There are several main categories of sleeping pads on the market today: closed cell foam, self-inflating, and air construction. Which one you choose will depend on how you prioritize your needs. Let's begin by breaking down these construction types.
Closed Cell Foam
These are the simplest pads made. They are little more than a piece of foam between you and the cold ground. Most are less than half an inch thick and aren't comfortable. They rely on tiny air pockets within the foam to protect you from the ground's heat-zapping convective powers.
Cons: Bulky, uncomfortable, often difficult to brush off snow and dirt
These products use an open celled foam (like a sponge) sandwiched between two pieces of fabric. When the valve is open, the foam expands and inflates the pad. The foam also traps air and retains heat.
Cons: Less durable than foam, less comfortable than air construction, can be fairly bulky
These are the latest craze in the evolution of sleeping pads. They are similar in construction to inflatable pool toys except that they are much warmer, lighter, and more durable. The downside to air construction pads is that they take a long time to inflate compared to self-inflating pads. Once inflated, they provide about 2 inches of cushion between you and the ground. This type of pad relies on either synthetic, down, or baffled structures to provide insulation. Generally, air construction pads are the lightest weight and most compact pads available. We think that air construction pads are the best choice for most outdoor sleepers.
Cons: Not as durable as foam, take longer to inflate, sometimes feel bouncy
Prioritizing Your Needs
Below, we highlight the various qualities that this type of product can offer. As you investigate your next purchase decision, we recommend concentrating on the characteristics most important to you.
Weight & Packed Size
The further you plan to hike in a day, the more important the issue of weight becomes. Consider for a moment that you're working out at the gym. If you're only doing five reps, adding 5 lbs. of weight might not be that big a deal. But if you're going for 50 reps, that 5 lb. increase makes a major difference. Reducing weight is important if you plan on pushing yourself to your limits. That said, not everyone will place the same importance on weight reduction. Comfort, warmth, and price are much more important metrics for many casual outdoor enthusiasts. Reducing weight almost always comes at the expense of one or more of these three things. Figuring out what is most important to you will help you make the best choice. We think that the Therm-a-Rest NeoAir XTherm gives an excellent balance of weight, comfort, and warmth but comes at a high price. If comfort and price are more important to you than weight, the Therm-a-Rest NeoAir Venture is super comfortable and inexpensive, but it isn't very warm or lightweight.
A small packed size helps you use the smallest backpacking backpack possible. This reduces weight and lets you carry a less bulky (and therefore more mobile) backpack. The most packable high scoring pad we tested is the air construction Sea to Summit UltraLight; it packs down to about the size of a soda can or two. Again, keep in mind that smaller packed size can come at the expense of warmth and comfort. For example, the self-inflating REI AirRail 1.5 is the bulkiest inflatable pad we tested but it is very comfortable. Closed cell foam pads don't compress as much as inflatable pads, but their supreme durability allows them to be lashed to the outside of your pack without having to worry. While they are physically quite bulky, CCF pads don't take up any internal volume, in this way, foam pads could be considered the most space efficient pads available, if you don't mind having them strapped on the outside of your pack. The biggest downsides to placing your pad outside your pack occurs when bush whacking through dense vegetation, scrambling through chimneys, or hauling your backpack up short sections of rock.
In addition to warmth, comfort is the biggest reason to buy a sleeping pad. If you camp exclusively in warm climates, comfort is the only reason to buy one. We think that pads with smooth surfaces are more comfortable than those with deep baffles. Pads that are 2 inches thick are really nice because they make sleeping on uneven ground less problematic. Deep in the backcountry, it's really nice to have a pad that can turn a pile of pine cones or grass clumps into a cloud-like mattress. Our testers found that self-inflating pads typically felt more stable than air construction pads but often didn't provide as much cushion. Overall, we think that air construction pads are the more comfortable of the two until you start comparing them to The Best Car Camping Mattress.
In cold climates, warmth is the most important reason to bring a sleeping pad along. You must realize that your pad is just one integral component of a much broader sleep system designed to keep you alive and warm. Understanding this is critical to tailoring your set-up to be as lightweight and as warm as possible. First, think about what kind of shelter you'll sleep in the most. A double walled tent or properly made snow cave is vastly warmer than an open bivy or ultralight shelter. Moving past your shelter choice, the next two things responsible for keeping you warm and comfortable at night are your sleeping pad and backpacking sleeping bag. Often people blame their sleeping bags for cold, sleepless nights when actually their pad was to blame! Obviously you need a good sleeping bag to have a chance at staying warm, but don't think for a second that your pad isn't just as important - the ground can zap your heat faster than the air above you.
Further, you should think about what surfaces you're likely to sleep on (ice, dirt, snow, rock), what the ambient temperatures are, how much of your body will be contacting your sleeping mat (side sleeper or back sleeper), and how inflated your pad is (e.g. firmness vs. softness). If you frequent cold environments, you should buy a pad with as high an R-value as possible. Pads with low R-values combat frigid environments with ease when supplemented with cheap, closed cell foam pads. For many people, the two pad combo is a great option because it is usually cheaper than a high R-value pad and will be lighter in the summer when the foam pad is unnecessary.
Be warned, if you don't like nerding out the ins and outs of sleeping pad warmth, skip to the bottom of this section.
Before we start jumping into some of the physics behind sleeping pads, we need to first remember that cold is nothing more than the absence of heat and that heat involves the transfer of energy from warmer objects to colder objects. Got it? Great! Now let's move on to three of the primary ways we lose heat when we're sleeping: conduction, convection, and radiation. In case you don't remember thermodynamics from school, know that heat transfer via conduction has to do with you touching something colder (ex: touching metal in the snow). Convection has to do with a fluid or gas moving over you (ex: getting chilled from the wind). Radiation is heat transferred via electromagnetic waves to objects around you. The temperature difference between you and your surroundings dictates how much heat you lose by radiation. At temperatures below -20º F, heat lost through radiation becomes significant.
Why Sleeping Mats are So Important
The job of a pad is to give you a comfortable surface to sleep on and insulate your heat from the cold ground. Each one of us is different, but let's assume that when you lay on your back in your sleeping bag (no pad) that 40% of you is in contact with the ground. No big deal right? Wrong! The air above you has an average thermal conductivity value of .024 while the ground can range anywhere from .05 (dry snow) to 3.98 (granite). This means that the ground can conduct heat away from you anywhere from 2 to 165 times as fast as air. The products in this review are designed to minimize convective heat loss. Often, you'd be better off with a lighter weight sleeping bag and a very warm sleeping mat than you would with the opposite combo.
Most manufacturers give their pads a number called an R-value that relates how well the pads resist the flow of heat. The higher the R-value, the warmer the pad will feel. The equation is linear, meaning that a pad with an R-value of 3 is 3 times better at resisting heat loss than a pad with an R-value of 1. Unfortunately, getting an accurate R-value for sleeping pads is a very complicated issue and there are no industry standards governing how manufacturers report this data. Laboratory tests often fail to include many real world variables such as significant internal convection heat loss caused when you toss and turn. Inflating your pad to full capacity ensures that you take advantage of its full R-value. Now that we have the basics out of the way, let's jump into some of the details.
The surface you're sleeping on can play a big part in how warm you are. You may not guess it, but in the winter, it is much better to sleep on snow than on dirt or rock. Granite can conduct heat 165 times more efficiently than does air and up to 80 times better than snow does. When temperatures drop below freezing, you should always try to camp on snow rather than rock or ice. Snow is a very poor conductor of heat. Ice conducts around 6 to 44 times more effectively than snow, depending on the amount of air in the snow. Granite conducts heat away from you 6 to 80 times more effectively than does snow. So, if temperatures have constantly been below freezing, sleeping on snow is a wise choice compared to sleeping on ice or granite. For three season backpacking, you probably won't be as concerned with what surface you're sleeping on, but dirt and pine duff will feel much warmer than rock. We wouldn't worry too much about conductive heat loss when temperatures are above freezing and would always choose to sleep on durable surfaces (like rock or sand).
Although it's dependent on your individual physique, roughly 40% of your body contacts the ground when sleeping on your back. This suggests that back sleepers will benefit more from increasing the R-value of their pads than side sleepers. But not so fast! There's much more to the story. Side sleepers will compress more of the pad resulting in a decreased R-value around the hips. To counter this, you could inflate your pad more, which leads us to the next topic of discussion.
According to an experiment performed by Backpacking Light, the level of inflation greatly affects the real world R-value of a sleeping pad. Simply, the more air you have in the pad, the warmer it is. This also means you'll be sleeping on a very firm mat, which many people will find uncomfortable.
Supplementing an Air Mattress with a Foam Pad
If you read through our Best Sleeping Pad review, you'll probably notice that we often recommend adding a foam pad to increase warmth. Since R-values are fairly linear, you can add a foam pad with an R-value of 2 to an inflatable pad with an R-value of 3 to achieve a total R-value of 5 that is suitable for winter use! But not so fast, the question remains should you put the foam pad underneath or on top of the air mattress? The answer, unfortunately depends on the specific foam pad in question and the specific inflatable pad in question, but in general, it's probably best to put the foam pad on top especially if you are using a foam pad with a dimpled surface like the Therm-a-Rest Z Lite SOL. When directly on snow, the dimples increase the surface area of the pad in contact with the snow, which conducts more heat away than flat-faced pads. This means that the real world R-value of dimpled pads when used on snow is likely less than the manufacturer's claim. Putting the pad on top minimizes this problem.
At the end of the day, our experience has taught us a few simple rules of thumb when it comes to choosing a sleeping pad for warmth. First, you need to think of your pad as part of your sleeping system. If you skimp on your pad's warmth, you should definitely use a warmer sleeping bag and vice versa. Second, we have found that pads with R-values between 2.0 and 3.0 generally work well for three-season use but should be supplemented with a foam pad for use in the winter where an R-value of around 5 or above is ideal. Pads with R-values lower than 2.0 are great for summer, but you may want to supplement them with a closed cell foam pad on colder nights in the spring and fall. Third, know how your own body works! If you know you're a cold sleeper, do yourself a favor and opt for a warmer sleeping pad.
No surprise, self-inflating mats are the easiest to inflate; they are filled with open celled foam that does most of the work for you. After letting the pad sit for a few minutes, you'll just need to top it off with a couple puffs. Air construction pads take a lot more work to inflate typically taking anywhere from 30 seconds to a minute or two. Never fear though, Big Agnes, Sea to Summit, Nemo, Therm-a-Rest, and Exped are just a few of the companies that make stuff sacks that double as sleeping pad pumps. These allow you to save your breath and save your pad from moisture build-up that can be especially problematic in winter. While this is one of the last things we'd consider when buying a pad, the convenience factor of self-inflating pads is a nice bonus.