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What You should Know About Sleeping Bags

Since 1994 members of the outdoor community have been trying to make sense of temperature ratings for sleeping bags. Their purpose was to help consumers make wise selections when they buy sleeping bags. Static tests were developed but they are of little value to the consumer. It was determined that consumer knowledge and education would provide the best service for the consumer.

This somewhat dry, technical document is cram packed with detailed information about sleeping bags, how they work, what to look for and how to buy them. This is your ticket to a well considered sleeping bag selection and a warm comfortable nights sleep.

Table Of Contents
A) What is a thermal insulator and how does it work? (R. Parry and L. McCullough)

  1. How body regulates heat
  2. Insulation - Dead air space
  3. Conduction
  4. Convection
  5. Radiation
  6. Evaporation

B) Design features influencing bag performance (J. Maggio and G. Foweraker)

  1. Draft tube
  2. Collar
  3. Shifting Insulation, Loft Distribution 60/40, Amount of Loft
  4. Sewn through seams & Baffles
  5. Hoods
  6. Foot box
  7. Air space within a bag/cold spots
  8. Off-set quilting/shingles/etc.
  9. Moisture management
  10. Differential cut
  11. Shell and lining fabric
  12. Insulations
  13. Performance when wet
  14. Mated bags
  15. Size and volume of bag-length and width

C) How to keep warm in a sleeping bag (H. Heyburn and A. Lee)

  1. Human components

a.     a.      Role of metabolism and body mass

b.     b.     Diet (protein, carbohydrates, sugars, hot beverage)

c.      c.      Age/Physical condition

  1. System components/Auxiliary products

a.      .       Ground and ground pad, conductive surfaces

b.     a.      Cots, hammocks and convective areas

c.      b.     Tent

d.    c.      Clothing

  1. Human/Environment Interaction

a.      .       Campsite selection/protection from the elements

b.     a.      Bag conditioning in the field

  1. Manipulating bag features, adjusting insulation, controlling heat loss.
  2. Optimize performance

a.      .       Revitalize in home dryer before trip

b.     a.      Keeping your bag clean

  1. Bags deteriorate over time

D) How to choose a sleeping bag (P. Hickner and S. Klien)

  1. Define intended use

a.       Time of year, range of conditions, environment. Single/Multi-season range of use

b.     Type of camping

c.      How much use/durability

2.      Determine bag requirements

a.                   Size and Shape

b.                Fit

c.                 Weight, compressibility

d.                Price, budget

  1.  Be aware of acceptable levels of variation and construction
    1. Materials
    2. Workmanship and Quality
    3. Warranty

E) How to care for a sleeping bag (F. Dieter and E. Weidler and P. Hickner)

  1. While in use
  2. While in storage (air bag after use)
  3. Cleaning your sleeping bag
  4. Drying in field
  5. General tips

 A) What is a thermal insulator and how does it work?

 

A) What is a thermal insulator and how does it work? (R. Parry and L. McCullough)

INTRODUCTION

What do temperature ratings really mean? People shopping for a sleeping bag must understand that the ratings given by a manufacturer are conditional. They need to understand all the factors involved for keeping warm. The bag is only an insulator, not a heat generator. A manufacturer can only estimate the temperature rating of a bag and some may be more connservative than others. As a shopper you must carefully evaluate your own metabolism and tolerance of cold. You must decide how much margin to leave for nights you are hungry, wet, sick, injured, exhausted, disheartened, or caught without a tent. The thickness of the insulating layer generally determines the warmth of a bag, but different types of insulation require a different thickness.

1) How body regulates heat

Heat Production in the Body The human body generates heat as a by-product of metabolism (the chemical processes necessary for essential body functions). A person’s heat production is primarily dependent upon their basal metabolic rate and activity level. Some muscular activity generates mechanical work, and the remaining activity generates excess heat energy. Therefore, the higher the activity level, the higher the body heat production.

Body Heat Balance A person is most comfortable when the excess heat produced by the body is exactly the same as the heat lost to the environment. When the rate of heat production is equal to the rate of heat loss, the person is said to be in heat balance. Excessive heat loss to a cold environment will cause the body temperature to decline and may lead to hypothermia. Although the body can tolerate a heat deficit for a short period of time, eventually the heat must be regained to stay healthy.

Factors Affecting Thermal Comfort (Heat Balance) There are three major factors that can be adjusted to maintain body heat balance and thermal comfort:

1. human activity level determines amount of metabolic heat production.

2. environmental conditions determine amount of body heat loss.

3. insulation systems including clothing and sleeping bags determine amount of heat loss.

In addition, the amount of time that the person is exposed to environmental conditions, while covered with a given amount of insulation is important. You may be able to tolerate a cold environment for a short period of time (allowing for a small amount of heat debt), but eventually the thermal imbalance would cause discomfort and health problems.

Heat Flow From the Body to the Environment Heat is dissipated from the skin surface by conduction, convection, and radiation. These mechanisms of heat energy movement are collectively referred to as sensible or dry modes of heat transfer. Heat is also removed by the evaporation of sweat from the skin surface, called evaporative or latent heat transfer. 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.

Heat always flows from an area of high temperature to an area of low temperature. Therefore, heat is usually transferred from a warm body to a cooler environment. The rate at which this heat flows depends upon the differential between the two temperatures; the greater the temperature gradient, the faster the heat flow. The heat transferred from the body decreases as the amount of insulation between the body and the environment increases. The heat flow also increases as the amount of surface area increases.

Vasoconstriction of blood vessels near the skin reduces heat loss from warm blood and reduces the temperature gradient between the skin surface and the cold environment. Blood vessels in the head however, do not constrict. Shivering is a mechanism the body uses to warm itself.

2) Insulation - Dead air space

Thermal Insulators and How They Work A thermal insulator can be defined as a device or condition that regulates or resists the transfer of heat energy from one body or environment to another. A good insulation system must be effective at stopping body heat loss via conduction, convection, and radiation.

Most insulation systems that are used to reduce heat loss from the body are made of textile structures that are mixtures of fiber and air. Fiber dominates by weight and visibility, whereas air dominates by volume. The fibers obstruct the flow of radiant heat and stabilize the air in the textile structure to minimize convection heat losses. The still air also reduces conduction losses since air has a lower conductivity than fibers.

The simplest sleeping bag insulation system is usually a multi-component structure consisting of an outer shell fabric, an insulating material, and liner fabric.

Shell Fabrics. The shell fabrics should be wind resistant to prevent cold moving air from penetrating the bag and increasing convection heat loss. Shells should be water resistant to prevent precipitation from penetrating the bag, displacing still air, and increasing conduction heat loss. Shell fabrics should also be strong, durable, and soil resistant. To achieve these properties, the fabrics are generally tightly woven in a thin structure made of nylon or other fibers. These high density structures help to block radiation heat loss from the body also. These materials should allow water vapor (i.e., the evaporated sweat from your body) to escape to the environment.

Insulation Materials. The typical insulation materials that are used in sleeping bags include natural fills (down and feathers) and battings of synthetic fibers (polyester fiberfill and polyester/olefin fiberfills). These insulating materials are sandwiched between the shell and lining fabrics using a variety of methods (enclosed channels, layers, baffles). Fiber manufacturers can vary certain fiber characteristics to improve the thermal performance of a batting (increase its insulation per unit weight). They can vary the fiber diameter ( denier), fiber length (from short staple lengths to long, continuous filaments), the shape of the fiber cross section, fiber crimp, and the size and shape of holes or voids inside the fiber. Batting producers can vary the thickness, density, and weight of the fiberfill also.

Effective insulators need to be thick and resilient. The filling materials need to maintain their thickness over time and recover from compression during use. The thicker the insulation, the more resistance to heat transfer it will provide. This is because thicker structures trap more still air and lower conduction heat losses. If the density of the filling material is too low the air will not be stabilized and convection losses will increase. Low density filling materials also allow radiation losses to occur. Battings made with microfibers ( fibers with a denier less than 1) tend to reflect, scatter, absorb and re-emit more radiant energy as compared to the mid-denier and larger fibers.

3) Conduction

This mode of heat transfer involves direct physical contact. A body or material can conduct heat energy to another substance when there is a temperature difference between the two. When in contact, the transfer of thermal energy involves the motion and collision of the atoms and molecules of these materials. For example, when you sit on a cold surface like a rock, heat flows from your body surface to the rock until equilibrium is reached. Another example of conduction involves the transfer of heat energy from your skin surface to the still air layer on the body surface, and to the still air trapped within fabric layers and between fabric layers in your clothing. However, air has a very low thermal capacity and is a much better insulator (poorer conductor) than the fibers and other materials that make up clothing, bedding, and sleeping bag systems. Water, on the other hand, conducts heat much faster than fibers or air. For example, if you were immersed in 50°F water instead of 50°F air, you would lose heat much faster in the water due to conduction.

4) Convection

The convective mode of heat flow transfers heat by moving large aggregates of molecules. The motion of these large aggregates, usually gases or liquids, is driven by a temperature gradient. For example, pouring cold water into a hot cup of coffee will transfer heat by convection as the two liquids mix. There are two types of convection: natural convection and forced convection.

In most cases, gravity is the mixing force in natural convection. The interaction of hot and cold air around the human body is a good example. Cold air is at a lower chaotic level (lower temperature) than hot air. Thus, there are more cold air molecules per unit area than hot air molecules at the same pressure. So the cold air will weigh more than the hot air. The still air next to your body will be first warmed by conduction. Then this hot air will rise, and the cold air from the cool surrounding environment will fall. As your body warms the next layer of air, the process repeats itself.

Forced convection can be mechanical (a fan, or forced air heating system can be the mixing force), or it can be caused in nature by differences in pressure (wind). Forced convection can be an important factor when a person is outdoors. The blowing of cold air against your body can accelerate the heat transfer process making you feel colder faster. Convection losses can also be caused by body movement in a large fitting sleeping bag or in loose fitting clothing. Cold air mixes with the warm air inside the bag or garment layers, causing an increase in body heat loss.

5) Radiation

This mode of heat transfer is the transfer of heat energy by means of electromagnetic waves. The human body is constantly giving off heat in the form of radiant energy. This energy can be reflected, absorbed, and converted into other forms of heat energy, or transmitted unchanged to objects in the environment. This transfer does not require a physical medium like air, so it can take place across a vacuum. In a cold environment, where the temperature is lower than that of your body, there will be a continuous interchange of radiant energy. The warmer body will radiate more energy than it will absorb, and the environment will absorb more than it radiates.

6) Evaporation

The evaporation of perspiration from your skin surface to the environment is a major mechanism for heat loss from the body, particularly in warm environments and/or at high activity levels. Heat energy is taken from your body as the liquid water is converted to water vapor.

Perspiration generated by your body needs to be evaporated from the skin surface and transferred through garment layers (and sleeping bag layers) to the environment. If the water vapor cannot escape, the relative humidity near your skin will increase, and you will experience a sensation of clamminess. If a clothing or sleeping bag system is ineffective at allowing the moisture to escape, or if the relative humidity of the environment is high, the "microclimate" near the skin will become saturated and eventually the moisture will condense in the textile layers and on the skin. Under these conditions, you will become very uncomfortable. In extreme conditions, where a considerable amount of moisture condenses and builds up in the insulating layers, conduction heat transfer through the layers will increase. This is because water is a much better conductor of heat than the fibers of the insulating layers.

In other cases, the use of a vapor barier liner (VBL) against the skin prevents evaporative cooling and also prevents moisture from getting into a sleeping bags insulation. VBL's are used in extreme cold situations when there is a premium on survival!

 

B) Design features influencing bag performance

 

B) Design features influencing bag performance (J. Maggio and G. Foweraker)

INTRODUCTION

Before searching for a sleeping bag it is important to define the exact needs you expect the bag to meet. These needs must be realistic so the help of a trained sales person is helpful. Setting expectations that are impractical will only lead to disappointment when the wrong bag is chosen. Remember not one sleeping bag will do everything. So don't be tempted to get a bag that is too warm (ie. heavier, bulkier, hotter), when most of your use may be in warmer summer conditions.

It is important to understand the function of the features that can be built into a bag. Not all bags need to be full featured. By understanding how these features work then you can decide which ones are desirable for the sleeping bag you need. Buy only those bag features you need to meet your intended purpose. This could save you money and weight. Features that are useful for colder temperatures may have no place in a summer weight bag. Bags that are "versatile" compromise in other areas.

Be sure though, to get all the features you need for your bag to preform to your expectations.

1) Draft tube

This is an insulated tube running parallel to and alongside the zipper that blocks heat loss thru the teeth of the zipper. Otherwise, this area would have a long uninsulated seam running the length of the bag. Ideally, the tube is sewn only to the lining material, since sewing through the bag creates a cold spot.

Depending upon the temperature requirements of the bag, this tube may be thickly insulated, a simple flap of fabric, or absent altogether. For cold weather bag, two thickly filled draft tubes may be necessary. In some cases the tube may be sewn to the bottom half of the zipper, this could be less desirable if the tube is scrunched down during the night exposing the cold zipper. A tube hanging from the top won't do this. You may find draft tubes with a stiffener to help prevent zipper snags. The draft tube may be attached to the bag with a single sewn through seam, or it may be attached with two seams so that there is no cold seam. The choice of construction depends on the temperature requirements of the bag. Any one of these constructions is acceptable as long as they are not mismatched to the temperature requirements of the bag.

2) Collar, Draft Collar, Neck Yoke

The collar is an insulated tube covering your throat and shoulders. It is intended to either lay across the top of your neck or to completely encircle it. It may be a flap of fabric or an insulated tube. And its variations in construction are similar to those of the draft tube. The purpose of the collar is to prevent heat loss from around the neck. Some of this heat loss is due to the "bellows effect" that is caused while moving around inside the bag. Warm air can be pushed out and cold air sucked in as the volume inside the bag changes. The collar acts as a dam to help slow this loss. Collars are more important on colder weather bags.

3) Shifting Insulation, Loft Distribution 60/40, Amount of Loft

Down bags may be built with continuous baffles These are uninterrupted tunnels that run from the top of the bag to the bottom. They allow you to fine tune the warmth of the bag by redistributing insulation from top to bottom and vise versa. However, continuous baffles can allow down to shift on its own, so periodic shake-ups may be needed. Other bags are built with a side-block baffle. This is a netting wall running the length of the bag on the side opposite the zipper to hold the down in place. This baffle helps to preserve the generally 60%-40% fill ratio of top to bottom of a sleeping bag. More fill is generally placed into the top of a bag since heat rises and since the insulation value in the bottom of the bag is reduced due to compression. Winter bags should have a side-block baffle.

4) Sewn through seams and Baffles

Sewn-through seams are the easiest, and lowest cost construction for a sleeping bag. The insulation is sandwiched between the inner liner and the outer shell and the seams sewn through all three layers. Since each seam line is a thin, unprotected, uninsulated cold seam, this method should only be used for warm weather bags. Not only will these sewn together seams help lose excess heat, but the bag can be constructed much lighter. However, in colder temperatures sewn thru seams become cold spots and that’s why "two" and "three" season bags bags are constructed with sewn in baffles. In this case the inner and outer shells are connected by a piece of baffling material that separates the two shells. In that way the insulation keeps the two shells apart throughout the entire length of the bag.

5) Hoods

Up to 50% of your body's heat can be lost through your head, an effective hood is a weight efficient way of improving a bag's warmth. Better hoods are tailored and can be subtly adjusted to move with you in your sleep. Hoods should be able to draw up and close to a 3" opening, provide room for wearing a hat, and should feel comfortable against the face. Semi-rectangular bags may not have hoods and depending upon their intended use this is not necessarily a drawback.

6) Foot box

Protruding at right angles to the rest of the body, your feet will tend to crush the upper insulation of the sleeping bag. Better designed mummy bags are flared and boxed, and have more insulation in the foot area. A less tailored equivalent feature in a barrel bag is called a foot circle.

7) Air space within a bag/cold spots

Other things being equal, the more snugly a bag fits, the more thermally efficient it is, since your body will heat a smaller volume between you and your bag. However, the bag must be roomy enough to allow a good night's sleep. The amount of space needed is a personal judgment.

8) Off-set quilting/shingles/etc.

Off-set quilting is essentially two layers of sewn through construction (see above) arranged so that the tubes fill in the uninsulated valleys in one another. Many synthetic insulations are made in sheets (called batts) which can be attached directly to the bag fabric in overlapping layers, like the shingles on a roof.

9) Moisture management

Even at rest, the human body gives off moisture which can degrade the warmth of your sleeping bag. Bag shells may be made of water resistant or waterproof fabrics but the fabrics must remain breathable. Bags may feature insulations which retain a greater degree of warmth when damp, or which may dispurse moisture so it can evaporate more easily.

10) Differential cut

The inner shell fabric of a bag should be cut narrower than the outer shell fabric to allow the insulation to fluff up to its maximum volume and to prevent the inner shell from contacting the outer shell which would create a cold spot. This construction is common in mummy bags, but not so with rectangular bags which are often required to open up and lie flat.

11) Shell and lining fabric

The choice of shell fabric and finishing affects the weight, water repellency, comfort, cost, durability and longevity of a sleeping bag. There is no universal consensus on the best fabric. Often bags will have a tightly woven shell with a DWR (Durable Water Repellent) finish. High thread count fabrics (270+ per square inch) are designed to minimize down leakage and to improve wind resistance. Any sign of down or fiber coming through the fabric is cause for suspicion that the weave may not be tight enough.

Nylon and Polyester are the most common shell fabrics. You can find bags made with fabrics as heavy as 2.2 oz./yd. and as light as 1 ounce. Lighter fabrics may not be as durable or as down proof but with careful use may suit your extreme lightweight needs for many years. Nylons are often calendered to make them more downproof and fiberproof.

Polyester is a softer fiber than nylon, and especially when woven with microfiber yarns, has a nice hand and drape. It also absorbs less water. Polyester holds a DWR finish better than nylon, but it's not as strong for the same weight.When compared to natural fibers, nylons and polyesters are stronger, absorb less water and dry more easily.

Cotton is comfortable next to the skin and is preferred by people who are sensitive to synthetic materials. It is often woven into flannel and used for general purpose sleeping bags. Cotton shelled sleeping bags pretty much disappeared twenty five years ago. Per unit weight, cotton is not as strong as nylon and polyester, absorbs moisture easily, and is hard to dry.

PTFE (polytetraflouro-ethylene) laminates are waterproof/breathable shells made with a shell fabric laminated to an expanded PTFE membrane (originally patented as Gore-Tex™). They have proven very effective in keeping sleeping bag insulations dry. This laminate is wind-proof and makes a bag warmer in many conditions. This fabric is often used in the outer shells of more technical bags, especially those insulated with down. There are a varity of expanded PTFE laminates being offered for bag shells. A PTFE laminate can add to the cost of a bag. As technology improves, the breathability of waterproof/breathable polyurethane-coated fabrics may provide a lower cost alternative to PTFE laminates.

Inside liner fabrics are generally a light weight nylon, usually lighter in weight than the shell fabric. In cases where gear may be stored at the foot of a bag during cold weather use, a heavier fabric may be used in that location.

12)Insulations

Though synthetic fills are constantly improving, down remains the leader in warmth-for-weight, compressibility, and resilience. Down is the fluffy undercoating of ducks and geese, located just beneath their outer feathers. Down insulation breaths well, and drapes itself over your body reducing internal air spaces. It is also more comfortable over a wider range of temperatures.

Although down has a high initial cost, because its useful life is measured in decades rather than years, it offers superb value in the long term.

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.

Synthetic fills are non-allergenic, retain more of their insulative properties when wet, and have a lower initial cost than down. Combine these benefits with ever improving warmth-for-weight, and synthetics become a viable, and for some users, preferable alternative to down.

Synthetic insulations are extruded polymers, essentially plastic threads. These threads may be either continuous filament (a single long strand) or short staples, pieces 1.5 to 4 inches long. Either type may be made with hollow threads. The hollow interior reduces weight, and traps air for insulation. Some synthetics are microfibres with textured surfaces that reduce air circulation and increase warmth.

More technical bags are designed for maximum lightness and compressibility, and some loss of thermal effectiveness is expected over time. Relatively bulky, heavy and durable insulations are used for applications where weight is less critical. Most synthetic fills keep their loft when wet. For this reason, many will consider synthetics if there is any chance of getting wet. The wide price range virtually assures you of finding a bag warm enough at a price you can afford.

13) Performance when wet

All bags will yield poor performance when wet. The best advice is 1) keep them dry, and 2) evaluate the number of times your bag has gotten wet in the past, (usually zero). Bags used on paddling trips are easily protected with a dry bag. and tents generally cover bad weather. If your tent leaks, fix the tent! All bags should be air dried during the daytime hours. Also, most shell fabrics are treated with a DWR. Keeping your bag's DWR maintained will improve its performance. It is not usually difficult to keep water problems to a minimum.

14) Mated bags

If bags are of similar design have zippers the same size and roughly the same length, they can often be zipped together. Mated bags allow a warmer sleeper to share heat with a colder sleeper, and in addition, each partner can select a warmer or colder bag for their side. If you are mating barrel or rectangular bags, you can carry bags of different warmths, putting the thicker one on top of both of you in colder weather and vise-versa. However this system is less efficient than separate bags, since there are more gaps and voids and the movements of two people pushing out warm air.

15) Size and volume of bag-length and width

Sleeping bags with lower internal volume are more efficient since there is less air inside the bag to heat up. First the air inside the bag then the dead air within the insulation must warm up before the bag will begin to insulate. Lower volume bags heat up sooner and insulate sooner. However, comfort in regards to claustrophobia is also a factor, so it is individual preference how much room you may need.

 

C) How to keep warm in a sleeping bag (draft 8 May 98)

 

C) How to keep warm in a sleeping bag (H. Heyburn and A. Lee)

INTRODUCTION

Even during summer nights cold weather can be expected. In mountainous areas, cold is even more common. Knowing how to properly use a sleeping bag can make the difference between a cold miserable night or a comfortable restful sleep till morning.

Keeping warm in a sleeping bag is divided into three main parts; human components, environmental components, and your sleep system components.

Remember, your sleeping bag does not produce heat. It is only an insulator that slows heat being lost from your body. In cold conditions it is important to start the night warm. You will sleep comfortably if you maximize your body’s heat production and minimize heat loss.

1) Human components

a) Role of metabolism and body mass

The human components that directly affect how warm you will be is based on your metabolism. Taking a short walk or light exercise is a way to increase metabolism by getting your muscles to generate heat. But do not work to the point that you will begin to sweat, this could cause the inside of your bag to become damp. Your muscles are responsible for generating heat, and don't expect fat to be a good insulator. Therefore being in good shape is important for a warm nights rest.

b) Diet (protein, carbohydrates, sugars, hot beverage)

It is important to have enough to eat and drink before going to sleep. Poor food and water intake during the heavy exertion associated with backpacking can disrupt your body's metabolism and prevent it from heating your body. Your body will burn fats throughout the night and generate heat. A hot beverage with sugar just before retiring will warm you up so that your bag heats quicker. Sweet food will boost your metabolism and warm your sleeping bag's insulation.

c) Age/Physical condition

Your mental state along with your physical condition or level of fatigue will contribute to how well you will sleep. Both your age and physical condition will affect your metabolism. In general older people or people in "poor" health will have a lower metabolic rate and therefore , they will have a harder time warming their bag. In this case they should buy a warmer rated bag.

2) System components/Auxiliary products

Your sleep system includes more than just your sleeping bag. Other components that affect your warmth are a sleeping pad, ground cloth, bivy sack or tent. All of these items will increase your chances of having a warm night’s sleep by reducing the amount of heat lost through convection and conduction.

a) Ground and ground pad, conductive surfaces

Using a ground pad made with an insulative foam will help prevent conductive heat loss between you and the ground. Polymers are a major building block of foam and polymer is a good conductor of heat. It's been found that open cell foam is a better insulator than closed cell foam because open cell contains less polymer. For winter use, an insulative pad with an R value greater than 2 is recommended.

b) Cots, hammocks and convective areas

The use of a cot or hammock presents a unique situation. Many people believe that a foam pad is just meant to provide a soft surface to sleep on, and therefore they elect not to use one when using a cot or hammock. The result is that there is very little insulation under the sleeper and air currents moving under the user will rob heat.

c) Tent

The use of a tent or bivy sack helps to create another layer of dead air space and helps to reduce convective heat loss. Lab tests show up to a 10° F benefit by using one of these.

d) Clothing

Since the majority of heat is lost from your head, wearing a hat when you turn in, will help increase the chances of having a nice warm relaxing night. The amount of dry clothing layers you have on will also help keep you warm. The more clothing layers, the more insulation, leading to a warmer sleep solution. But do not allow the additional clothing to compress the insulation of the sleeping bag. The theory that sleeping naked is warmer, may be fun, but it is untrue.

3) Human/Environment Interaction

a) Campsite selection/protection from the elements

The environmental components of keeping warm in a sleeping bag are critical to prevent a good evening from turning into a nightmare. Avoid camping in low areas where cold air collects. Moving air causes convective heat loss so camp sites should be sheltered from the wind. Be prepared for high elevations to cool quicker when the sun sets or goes behind clouds.

b) Bag conditioning in the field

It is best to set camp early enough for you to prepare your sleeping site. It is suggested you open up your sleeping bag as soon as you can. Giving your bag as much time as possible to fully loft will maximize its insulative value. Pull your bag out of its sack and shake it and fluff it to get air mixed into the insulation. Do this several times before retiring.

4) Manipulating bag features, adjusting insulation, controlling heat loss

Some bags offer the possibility of adjusting and distributing insulation. Some models of down bags for instance are constructed with continuous baffles. By distributing the insulation from top to bottom you can better control the insulative value of your bag and tailor your needs to the given climate. In warmer weather move the insulation to the bottom of the bag and when you find the weather cooler than you expected, move more of the insulation on top of you. Use your cord lock to snug the hood about your face, or distribute the inside hood material to create a hood ruff. By learning to minipulate the features of your bag you can better control heat loss.

5) Optimize performance

a) Revitalize in home dryer before trip

It is not a bad idea to put your bag into a dryer set to low heat before leaving on a trip. The tumbling will reloft the bag and at the same time, the heat will drive out any moisture that may have accumulated during storage. This works for both down and synthetic sleeping bags.

b) Keeping your bag clean

By keeping your bag clean its loft will be maintained. Using a ground cloth under your bag will protect the shell from abrasion, dirt and ground moisture. You may also use lightweight underwear and socks to keep body oils and sweat out of your bag. Think of your underwear as sheets on a bed. That's why you don't often launder your mattress.

6) Bags deteriorate over time

Be aware of a sleeping bags life expectancy. Shell fabrics and insulations deteriorate over time at different rates. Exposure to UV radiation may speed up some deterioration. Comparing current loft to the loft when the bag was new will provide an indication of how much the insulation has deteriorated. Washing a sleeping bag may improve loft. Down bags often have the longest life spans.

 

D) How to choose a sleeping bag (P. Hickner and S. Klien)

INTRODUCTION

The best advise is to learn all you can. Designs and materials change, and you may find that your knowledge is out of date.

Try these sources of information:

Equipment reviews from the library or internet services. Remember that reviewers are not infallible, and may have biases. Manufacturers catalogs are very helpful. Read the text and specifications carefully. Speak with guides and other mountain professionals. People with specialized knowledge are usually happy to share it.

Outdoor store sales people can be a wealth of knowledge. Choose your store carefully after you have read some catalogs. Visit two or three stores for a more varied opinion. Ask about brands that the store does not carry. It is easier to learn negatives from someone who does not sell the product.

You will have to consider price, temperature range, weight, size, water resistance, durability, and compressibility as well as, zipper placement, compatibility with other bags, and even the feel of the fabric.

1) Define intended use

Carefully determine your needs. But be realistic about how you will use your bag. You probably will never find a bag that meets all your needs and wants, so first decide where you will not compromise and where you can.

a) Time of year, range of conditions, environment. Single/Multi-season range of use

Carefully consider the temperature range to be covered. If the range is wider than any one bag can cover, you need to decide which end of the range is most important to you. Buy the bag for the weather you will MOST often encounter. You may be able to add extra clothing, use a bag liner, a vest, or an over bag, to extend the range of your bag. That will save you expense, spare you carrying extra weight, and keep you from sweating on warm nights. Do not use temperature ratings to cross reference between companies. There are currently NO accepted standards between manufacturers for rating a bag in degrees. Part of the problem is that everyone has their own metabolic rate.

Backpackers in the mountains can encounter variable conditions even in the summer, and you will need to be prepared for all possibilities. You could buy a Two-season, or Three-season bag to cover a wider temperature range. If you're only a fair weather summer camper, you may get a single season summer bag with a higher temperature rating. On the other hand, you may be the extreme winter mountaineer. Then you would best be served with a single season winter bag with a low temperature rating. These bags are only useful for cold conditions so don't be tempted to get a winter bag for all your neeeds.

b) Type of camping

Car camping - Weight and volume aren't really an issue. The bag of choice could be a rectangular or semi-rectangular bag . If you want more comfort, you could choose a bag with a cotton/polyester liner.

Backpacking - Lightweight and low volume would be important. A mummy style bag would be most practical.

Bicycle camping - Lightweight and low volume would be important. A mummy style bag would be most practical.

Kayak or canoe camping - Low volume for storage, usually inside a dry bag.

c) How much use/durability

Determine how much you will use your bag. You may need to consider a more durable bag or you may evaluate your skill level for caring for a bag. Durability can add cost and weight, so evaluate your needs and skills carefully.

2) Determine bag requirements

a) Size and Shape

Mummy bags are the most efficient. Many mummy bags are available in wide girths so that you do not have to be squeezed in a mummy bag. People who car camp or use a sleeping bag to double as a quilt should consider a rectangular bag. Semi-rectangular bags hold appeal for those who feel constrained in a mummy bag and want some kicking around room. They also make good zip together combinations, either with another semi-rectangular bag or a ground sheet combination.

b) Fit

For most people, the smallest size that is comfortable is the right size. A smaller bag will warm up faster and save weight. You should try on a few bags to get a good sense of size. If you have a bag already, measure it and compare it to prospective bags. Be aware that there is some inconsistency in published measurements. Measure for yourself if you have the opportunity. Remember that bags with equal shoulder and foot measurements can have different contours in between. Decide if you need to leave room for wearing extra clothing. Consider if you want to leave room at the bottom of the bag for clothing, boot liners, and to keep water bottles warm.

c) Weight, compressibility

Narrow cut bags are lighter and more compressible than wider cut bags. Wider bags use up more

materials in their construction and so are often more expensive than other bags at the same temperature rating. Many hikers make weight and compressibility the primary consideration in choosing a bag, but you should recognize that a comfortable sleep is worth hauling a few extra ounces up the trail.

d) Price, budget

Dream of the perfect bag if you want, but limit your search to what you can afford. We all started with equipment that was barely adequate and we had a great time anyway. But be aware of what money will buy, if you need it you'll have to pay for it. You won't get something for nothing. When considering cost consider also longevity and quality of the product. Reliability also costs more. Cheaper products may have to be replaced more often and won't magically keep you warm. Learn the principles first then know your compromises when you make your decision. In most cases, there will be a bag in your price range that can fit your needs.

3) Be aware of acceptable levels of variation and construction

a) Materials

There are many different shell fabrics available for sleeping bags. Understand the trade offs for different features that may make a bag "better" but only for a specific use. As water resistance increases, breathability often decreases. "Bearding" may result when a shell fabric doesn't have a high thread count or tight weave to contain the insulation and the fibers migrate through the surface of the fabric. Make sure the fabric has enough integrity to hold up along the sewn seams. Pull at the side seams lightly to see if the fabric is separating and allowing seam slippage. Make sure the zipper opens and closes easily. Understand the variations in down fill power and how it is derived and used. Learn as much as possible about the materials in your bag so that you can better understand what to expect.

b) Workmanship and Quality

Stitching should be close and tight. Loose stitching will snag and shorten the life of a bag. Ask if the internal seams are stabilized to prevent unraveling. Learn to look at the overall construction, features and finish of the bag to determine its quality.

c) Warranty

Look for a lifetime warranty on materials and workmanship for an indication that the manufacturer believes in the quality of its products. But recognise that "Lifetime" will usually apply to the product and not necessarily your good health.

E) How to care for a sleeping bag

E) How to care for a sleeping bag (F. Dieter and E. Weidler and P. Hickner)

1) While in use

Always read and follow the manufacturer's care instructions. Different fabrics, insulations or bags may require different handling techniques. Sleeping bags are made to be stuffed hundreds, even thousands, of times without damage. Improper stuffing, however, may shorten the life of your bag. Use a larger stuff sack if space is not your primary consideration. Recognize that frequent use of compression sacks will take a toll on the loft of the bag. Also , a larger but softer stuff sack is easier to pack around than a solid hard cylinder. Always make sure you are stuffing the bag evenly. Push the sleeping bag firmly into the bottom of the stuff sack from the outset so that the bottom of the stuff sack is stuffed as tightly as the top.

Avoid leaving a stuffed polyester bag in a hot car. Leave it unstuffed in the trunk (the trunk of a closed car parked in the sun is much cooler than the passenger compartment).

2) While in storage (air bag after use)

Store your bag in a cool, dry place that has a fairly constant temperature (between 65 and 80 degrees). Never store your bag compressed for long periods of time. Bags should be hung or stored in a large, breathable storage sack or pillow case. They should never be stored for long periods in anything watertight as condensation can occur inside the bag and result in mildew. You may wish to tumble dry your bag in the dryer on low heat for 10 minutes before storing, even if it feels dry.

Do not expose your bag to sunlight for long periods as UV radiation can degrade the nylon.

3) Cleaning your sleeping bag

Do not expect all bags to be cleaned the same way. Different synthetic fills may require special handling as well as different down products may have prefered methods for cleaning. Again, refer to the manufacturer's recommendations. In most cases washing is the only acceptable method for cleaning a sleeping bag. Dry cleaning will damage both down and synthetic bags. Body oils, hair oils, lotions, and cosmetics are all acidic and will cause down or synthetic insulation to decompose and lose it’s loft. A bag should be cleaned when it has begun to lose loft or becomes discolored due to oil accumulation.

Soap should be your first choice for stain removal. You can use cleaning solvents to spot-treat tough stains, sap, or tar. You may want to experiment on a very small area to be certain that it doesn’t damage the fabric. Try to move the down or insulation away from the spot you are cleaning and use an absorbent cloth on the reverse side. PTFE membrane laminates do not require different treatments than other fabrics.

It is best to wash your bag yourself by following the manufacturers detailed cleaning instructions. Otherwise, take you bag to a professional laundry experienced with down or synthetic insulated products. If no care instructions exist, hand wash or use a front loading machine without an agitator. Tumble dry on low heat checking frequently for overheating. A light soap is fine for synthetic bags, but a special down soap, available at your local outdoor shop, is best for down bags. Do not expect these down soaps to remove all stains since they are mild and not the most powerful stain removers. Make sure your bag is completely dry before storing.

4) Drying in field

During use in the field it is important to air dry you bag during the day. This will dry out and remove perspiration that may have accumulated during the night. The bag should be turned inside out and exposed to a moderate amount of sunlight and plenty of moving air. Be sure to carefully tie or otherwise secure your bag so that it does not blow away.

5) General tips

Your bag's inital application of the DWR finish will gradually wear off, it is a good idea to periodiocally reapply this finish. Inspect the seams under stress for any separation or distortion in the weave.

 

Edited by John Francis Maggio - Western Mountain Sports; Contributors: Roger Parry - Dupont Dacron, Dr. Liz McCullough - Kansas State University, Greg Foweraker - Mountain Equipment Co-op, Henry Heyburn - L.L. Bean, Amanda Lee - BHA, Peter Hickner - Feathered Friends, Sheila Klien - Cascade Designs, Fred Dieter - Sierra Designs, Eric Weidler - Sierra Designs.

Please send your comments and suggestions, Attention John Francis, 1025 S. 5th St, San Jose CA. 95112. Or fax to 408 287 8946.

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