The science behind sleeping bag design is at once simple, and yet very complicated. For many years the science of sleeping bag design has changed and evolved to incorporate the latest technological breakthroughs and use the latest innovative fabrics and materials available. Recent advancements in sleeping bag technology include waterproof down insulation, ultralight materials, and breathable vapor barriers. As complicated as the technology might become, the objective of a sleeping bag design is straightforward.
Sleeping bag design boils down to one, ultimate goal: Trapping dead air around the body to keep the user warm and reduce body heat loss. The two main factors in play in sleeping bag design are reducing Heat Transfer (the transfer of thermal energy between objects of differing temperatures) by creating Thermal Insulation (achieved through specially engineered methods or processes, as well as object shapes and materials). Everything else is just marketing.
Body Heat and Thermoregulation…
To understand how heat transfer can be managed through design, we have to understand how the human body produces heat in the first place and the factors that affect thermal comfort (heat balance). Mammals produce body heat through basal metabolic processes and physical activity. Excess heat created through increased activity must be transferred to the environment or our bodies would overheat and shut down as soon as we exerted ourselves. The ideal condition for heat balance is when the amount of excess energy created by the body is the same as the amount lost to the environment. The human body has natural mechanisms to control body heat (thermoregulation) like perspiration, vasodilatation, and shivering. These mechanisms normally don’t kick in until our body is in an extreme state of stress, something we normally want to avoid.
While we sleep, our natural metabolism is the primary contributing factor to maintaining our body heat. In ideal environmental conditions, we would maintain heat balance naturally while we sleep and wouldn’t need any assistance. This doesn’t happen in the natural world, so we look to insulating layering systems (Thermal Insulation) to keep us warm while we sleep. This is where sleeping bag design enters the equation. Sleeping bags should be designed to reduce thermal conduction enough that our natural body heat can maintain an optimal core temperature while we sleep.
Thermal Conduction in Sleeping Bag Design…
Thermal Conduction is the transfer of energy at the atomic level between objects of differing internal energy. This results in heat transfer from the higher energy object to the lower energy object as they seek equilibrium (heat balance). In our case, it is the loss of heat energy from the human body to the colder environment (air). Sleeping and resting individuals lose about 25% of their heat through respiration (a combination of convection and evaporation) and insensible perspiration from the skin (evaporation). The remaining 75% is dry heat loss from the body surface. Sleeping bags slow Thermal Conduction by trapping dead air as a thermal barrier between your body and the environment. This enclosed system also virtually eliminates external convection factors from the equation.
We quickly heat the trapped air through conduction and radiation. As the air gets closer to our core temperature, our bodies have to produce less heat to seek equilibrium with this air layer. Air is a poor energy conductor, so the more air that can be trapped around the body the less conductive heat loss we will experience. The insulating and shell fabrics also serve to slow conductive heat loss but are much less effective than air. The primary function of the thermal layer material is to hold air. The more air volume it can hold, the more effective the material is as an insulator.
For this reason, insulating layers need to be thick and resilient. It is important that the fill material retains its thickness over time and recovers quickly from compression. The fill should also maintain even distribution to avoid “cold spots”. The thicker the fill, the more effective it will be at reducing heat transfer by trapping greater air volume. Density also plays a role. Low-density filling will not allow the air to stabilize allowing greater convection loss and will also be less resistant to radiant heat losses. Other design considerations that will reduce convective heat loss are any elements that assist in trapping a layer of air around the body. Insulated neck baffles, overlapping insulation layers at the stitching and zippers (draft tubes) as well as added fill-in compression areas like the bottom of the bag can all be used to insure a more stable thermal air barrier around the body.
A note about sleeping pads
The cold ground is a much more efficient conductive mass than any air that surrounds you. This means that it will draw heat energy out of your body in an attempt to reach heat balance faster than anything else. It is also a mass that you will never reach equilibrium with, so it will pull heat energy from your body until you are the same temperature as it is. To reduce conductive heat loss through the ground, an insulated sleeping pad should be used to separate you from the ground and create a layer of air between you. Since the sleeping bag insulation will be compressed underneath you reducing it’s effectiveness, the sleeping pad is an essential piece of gear in your sleep system and shouldn’t be ignored.
Materials used in Thermal Insulation…
Traditionally, either Goose Down or Duck Down has been used as a lightweight natural fill insulation material in jackets, quilts, and sleeping bags for many years. Other natural insulation materials are Wool and Animal furs. Out of these natural insulators, wool is the only one that performs well when wet.
Many synthetics have been produced over the years. Some of the more popular materials are Polarguard, Hollofil, and Thermolite. Traditionally heavier and bulkier than natural insulation, these manufactured products are engineered to trap air and reduce thermal conduction just like natural insulators. They are also engineered to function under adverse conditions. As technology advances, these synthetics are getting lighter and more versatile.
What is Down “Fill Power”?
“Down is graded for quality according to the number of cubic inches one ounce of down will displace when under a controlled pressure. This displacement is referred to as “fill power”; 400-450 fill down is fair, 500-575 fill is good, 600 and above is excellent. Many factors affect fill power. As a result a laboratory will condition a down sample for several days in order to stabilize it, and still they can expect a 10% variation in their results. Since few consumers are equipped to scientifically test down fill, your best protection is to buy established brand names from reputable retailers.”
From John Francis Maggio – Western Mountain Sports
Thermal Radiation in Sleeping Bag Design…
The human body is constantly giving off heat in the form of electromagnetic waves referred to as Radiant Energy. This energy can be reflected, absorbed, and converted into other forms of heat energy. In a cold environment, there is a constant interchange of radiant energy from the (warmer) body to the (colder) environment. The higher the temperature gradient, the faster radiant energy will be dissipated into the environment. Sleeping bag design can fight radiant heat loss through both reflection and absorption.
The best example of the use of reflection to reduce heat loss is the emergency “space blanket” in most backcountry first aid kits. It uses a highly reflective material as the sole source of insulation to keep a person warm. It’s main function is to use your body’s natural radiant energy to provide heat. It is also an impermeable layer that reduces convective heat loss by blocking any wind that might try to rob you of warmth. There have been several products on the market making use of thermal reflective materials used close to the skin at the inner shell to reduce radiant heat loss.
Using materials to absorb radiant heat is a trick more commonly seen employed in insulating garments instead of sleeping bags, but it is still a factor in the design and material specifications of sleeping bags. Using a material that will absorb and hold the radiant energy transferred from the body will gradually reduce the amount of heat transfer that occurs and, potentially, radiate heat back to your body in the process. Insulation materials with absorptive qualities also increase the rate at which the stable air layer will reach heat balance with your body.
Sleep Science and Sleeping Bag Design…
Sleep Science is an article in and of itself that I won’t get into. The science is still fairly young and much of it inconclusive. What is known is that physical comfort and the ability to achieve the deepest levels of sleep required for mental and physical rejuvenation are hugely subjective and highly personalized. Some people don’t move much in their sleep and can achieve great sleep in a typical, form-fitting mummy bag. Others require a little wiggle room or the ability to spread out to achieve deep, satisfying sleep. Sleeping bag shape design then must address comfort and maneuverability for the individual.
Material specifications must also be considered when addressing individual comfort. Softer, more pliable materials net-to-skin will improve sleep comfort. There are a variety of sleeping bag liners available for this as well. Personally, I find insulation and outer layer materials that are too stiff or make noise when I move to keep me from achieving deep sleep. I’ve also read complaints from some users about bags that don’t “breathe” well creating a high-moisture micro-environment inside the bag that keeps them awake.
What about temperature ratings?
Sleeping bag temperature ratings can be misleading. When manufacturers tell you a bag is rated for -10°, we mean that it will keep you alive at a temperature down to -10°. We don’t mean you’ll be comfortable. You won’t. You will be COLD! But you’ll be alive.
Temperature ratings are really survival ratings. For comfort, most people need a sleeping bag rated about 20° to 25° colder than the expected nighttime temperature on their camping trip. If you are expecting zero degree nights, then buy a -20° to -25° sleeping bag.
In the end, Sleeping Bag Design is primarily about maintaining warmth and reducing heat loss. But there is a human side to design, the side that ventures beyond pure science into intuition. This is the side that addresses creature comfort, security, and aesthetics. The side that transcends the specifications and playfully elicits an emotional response. It takes something we need and elevates it to something we want. That’s when design becomes art. And that’s when you know they’ve done it right…
Tips for staying warm at night in your Sleeping Bag from Teton Sports
You shouldn’t have to freeze your tail off every night just to go camping. First, understand that staying warm in a sleeping bag is all about heating the air in the bag, and then trapping that heated air inside the bag as long as possible. When your sleeping bag is directly on the ground, you mash the air out of the bag’s fill and you let a lot of cold come in from underneath you. The first suggestion is to sleep with a pad under your sleeping bag, not simply a plastic ground cloth.
Do what you can to warm your body before you get in the bag. Twenty jumping jacks before you hop in the bag may keep you warm hours longer, because your body will better warm the air in the bag. Once you’re in the bag, strip off all non-essential clothes so your body heat can be used to warm air, not clothing. And of course, don’t sleep with wet clothes.
Use the shoulder baffle included on all TETON Sports sleeping bags to cinch around your neck and trap the warm air in all night. Buy a wide enough bag of the right temperature rating so you can leave the side fully zipped all night long. Also, if you eat right before bedtime, your body temperature may drop slightly while you metabolize, and you could suffer all night as a result..