Make your own free website on

Sleeping Bag Guide

How Warm A Sleeping Bag Do I Need?
What Design Sleeping Bag Should I Get?
What Insulation Should I Get?
About The Shell Material Used In Sleeping Bags.
About The Internal Construction of Sleeping Bags.

How Warm A Sleeping Bag Do I Need?  There are many different sleeping bags on the market, each with a different temperature rating.   The temperature rating of a sleeping bag represents the minimum air temperature in which the bag will keep the user comfortable.  There are many factors that affect the ever so elusive issue of warmth.
     Manufacturers of sleeping bags rate the minimum temperature of a sleeping bag by a simple test. A fiberglass or a copper dummy is placed inside the bag in a controlled environment. An Ensolite pad is put under the sleeping bag to prevent Conductive Heat Loss. Then the temperature of the dummy is raised to 98.6 degrees Fahrenheit, as the room temperature is dropped to the temperature that the bag is being tested for. After an elapsed time, usually eight hours, the temperature of the dummy is re-measured. If the temperature of the dummy is higher than 98.6F, the bag is too warm. If lower, the bag is not doing the job and changes need to be made in order to increase the warmth of the bag.
     Not all sleeping bags have the same amount of insulation on the bottom portion of the bag as they do on top. The reason being that as we sleep in a bag, the fill will compress under our weight which in turn will reduce its insulating power. Understanding this point, manufacturers place a higher percent of insulation on the top portion of some sleeping bags. Now, this is a great idea if you sleep on your back, but not everyone does. Some people sleep on their side, or even on their stomach. If you are such a person, you will find differentially filled bags particularly cold, since your back will not enjoy the same amount of insulation as your front. Fiberglass dummies are always laid on their backs.
     Understanding that temperature ratings in sleeping bags are guidelines and not written in stone will put you in the driver's seat in terms of choosing a bag that you will be comfortable in. If you know yourself to sleep warm, buy a bag that is a bit cooler than you need. If you sleep cold, buy the next warmer bag.
     Every time a sleeping bag related discussion arises, the main topic of conversation becomes the subject of Loft. Loft, simply put, is the height of the bag when it is fully fluffed up and is lying at rest on a flat area. The thickness of the bag is primarily determined by the type of insulation and the amount of it in the bag. The loft of each bag is traditionally measured by thoroughly skaking the bag, laying it on the floor, and measuring the thickness or the height of the bag from the floor to the top of the bag. The loft of sleeping bags can range from an inch in light weight summer bags to as much as ten inches in expedition quality winter bags. The correlation between the loft and the temperature rating of a given bag can be discussed differently, depending on the source. The Army Quartermaster Corps says that one inch of loft at rest will give the average user 40 degrees of protection. But by no means are the Army's standards of insulation acceptable by the market. In fact most manufactures suggest that only 10 degrees of protection is given per inch of loft. This means that a sleeping bag with four inches of loft will maintain a temperature in the bag of 98.6F in 30 degrees above zero temperatures. Of course one needs to keep in mind other factors, such as the users metabolism and the overall design of the sleeping bag. During the next segment we will discuss the importance of a sleeping bag design in terms of the role design bags play in trapping body heat. There are sales clerks that put most of the emphasis on the poundage of insulation in a bag. Although this is a very important issue an even more important point is how the insulation is distributed. For example, it is common knowledge that the body loses 60% of its heat through the head and neck area. Therefore a sleeping bag with a 'hood' will do a more effective job of controlling heat loss.
     Also, different fills function differently in changing climate. The most vivid example is the lack of the insulation value of down when wet. So, it is safe to say that there are many factors that play important roles in helping a sleeping bag do what is is supposed to do.

What Design Sleeping Bag Should I Get?  There are four basic sleeping bag designs available on the market today:

     Rectangular Sleeping Bags: This is the most common bag design available on the market. People who have owned sleeping bags consider rectangular bags to be the most comfortable bags to sleep in. These bags have a minimum girth of 32 inches which is consistent throughout the bag, girth being the measurement done by encircling the bag around the chest area. But while it is true that rectangular sleeping bags are quite comfortable to sleep in, they are also the least efficient bag design on the market.
     Dead air keeps you warm, moving air keeps you cold. In rectangular design sleeping bags, since the loft of the bag does not follow the contour of one's body, there is a lot of empty space inside the bag. This area still needs to be warmed by your body heat, and this is the heat that is going to waste. In other words, valuable body heat is being used to heat useless areas. Especially the area around your feet, which are hard to keep warm to begin with.
     Here are a few helpful things to look for in your sleeping bag, especially if it is a rectangular design, to help you keep warmer on a cool night.
     First, make sure that your sleeping bag incorporates an insulated baffle or tube along side and behind the zipper through out the bag. This insulating baffle will prevent heat loss through the zipper, which is helpful since rectangular sleeping bags also have the longest zippers. Baffles are often sectional to prevent the shifting of the insulation towards the bottom.
     If you are indeed looking for a warm bag with a 20F degree rating or lower, make sure that your sleeping bag is not sewn through. Sewn through bags utilize no insulation along the stitched areas. Consequently these areas will become cold spots in your bag, especially on the top portion, since the bottom of your sleeping bag will be on a pad. Take the top half of your sleeping bag, hold the inner liner and the outside shell firmly and pull apart. The shell and inner liner should separate quite easily except in sewn-through bags. With your index finger feel along side the stitch marks, you will feel no insulation there.
     Another helpful design feature in a rectangular bag that will increase its warmth is a drawstring along the neck portion of the bag. A simple and useful, but often neglected feature, the drawstring will allow you to close the top portion of your sleeping bag snugly around your neck. This will prevent heat loss through the top opening and will trap your heat inside the bag where it is needed most. Also on cold nights when the bag is just not doing the job, slide into the bottom of the bag and tighten the string over your head. This will do two things: It will completely trap your body heat inside the bag and prevent respiration heat loss, not unlike a scarf around your mouth and nose. You will notice the difference right away. This is why it is important to buy a sleeping bag this is longer than you need!
     Modified Rectangular/Tapered Sleeping Bags: If you like the comfort of the rectangular design sleeping bag, but are looking for a bag that is a bit more efficient and has the potential to be lighter and more compact than a rectangular bag, this design may be for you.
     In modified rectangular sleeping bags, manufacturers have tried to reduce the empty space inside the bag by tapering the sleeping bag from the hip area down, so that by the time your feet reach the bottom of the bag, it more or less wraps around them. The tapered bag design does not confine your feet quite as dramatically as a mummy style sleeping bag, but it goes a long way toward reducing the amount of empty space inside the bag and giving you more warmth per weight.
     Modified rectangular bags are a good choice for those who are looking for a two to three season sleeping bag that will allow them to backpack. This reasonably efficient design, in conjunction with an efficient insulation, could easily fall under the lightweight category. If you are looking for a cold weather sleeping bag, still demand the draw string around the neck area for extra closure, and a substantial baffle backing the zipper. Without these extras, the tapered design will not amount to much.
     Mummy Sleeping Bags:  The final word in efficiency is this design sleeping bag, but this does not mean that the compromises one has to make for achieving this efficiency, will be to everyone's liking. In fact, people who have used mummy style sleeping bags have, and will continue to have, a love/hate relationship with the idea. Those who like the design will boast of the fact that the bag goes to great lengths to reduce heat loss through the channels discussed previously. The secret to the function of a mummy shaped bag is this: The user's body plays the role of a heater, and the less the amount of space to be heated the warmer the bag will be. In following this rule of thumb, mummy bags are designed to follow one's contour as closely as possible, reducing the amount of empty space that translates into a colder night's sleep. And that's not all, by now we know that one loses as much as 60% of their body heat through their head area, Mummy bags all, without exception, incorporate an insulated hood extension into their design. This three dimensional 'hat', in conjunction with a drawstring attachment for the sole purpose of regulating the fit over the head area, will eliminate heat loss through one's head. The importance of the hood section of a mummy style sleeping bag warrants few words about the design of this all important addition.
     Remember reading about the loft in a sleeping bag and what it represents? Well, the loft of a mummy bag's hood can easily be flattened, since fitting a hood over the head by pulling on a draw cord, will create wrinkles, which in turn will flatten the loft of the hood in certain areas. Manufacturers will combat this problem by pre-forming the hood section like a hat. This will ensure a better fit to begin with and so, fewer wrinkles. This feature is especially important in sleeping bags designed for lower temperatures. So remember, the flatter hood to begin with, the less efficient the design, and the more wrinkled it will become once fitted.
     Mummy bags also incorporate an insulated baffle along the zipper in order to reduce heat loss through the zipper itself. And in cold weather, mummy bags usually have an insulated collar placed along the chest area. This addition will plug up the upper sleeping bag opening so that no trapped body heat can escape through that area.
     All the constructive thoughts that have gone into designing today's mummy style sleeping bags have resulted in incredibly thermally efficient bags that give the user great weight and bulk to temperature ratio. For this reason, almost all cold weather campers, and most backpackers, choose mummy style sleeping bags for their backcountry adventures.
     Modified Mummy Sleeping Bags:  In the outdoor equipment world, compromises can be made to ensure certain amount of comfort without cutting corners to endanger the user. In modified mummy sleeping bag design, although most of the design features of the mummy style have been preserved, the girth of the bag has been enlarged in all areas so that the fit of the sleeping bag is not very snug. Going by what we learned about mummy bags use and their efficient design to keep us warm, it is also fair to say that the wider cut in the modified design bags take away from the design's efficiency. So, find the happy medium between comfort and function.
     In order to compensate for the lack of comparative efficiency in modified mummy bags, manufacturers need to increase the amount of insulation used to achieve the desired temperature rating. This, along with the extra materials used in a larger girth sleeping bag, will mean a heavier and bulkier sleeping bag, and for a backpacker this is not good news. So, when comparing a mummy design to a modified mummy design, try both on for size. Get in them and see how they feel, since no two sleeping bags are made the same, you may be surprised how comfortable some mummy style sleeping bags feel.

What Insulation Should I Get?  The question of insulation preference always starts with whether a person wants down or synthetic in his or her sleeping bag. There are two different kinds of insulation available, natural or man-made fibers. As far as the natural fibers, goose and duck down are the only two insulation that are used, whereas there are more than seven different synthetic fibers available for use in sleeping bags.
    Down Insulation:  The down used for insulation comes from foul raised for food. In other words, down is a meat byproduct. Since the United States consumer does not show much appetite for goose and duck, most of the down in this country is imported from European countries, Poland to name a major source. It is also important to consider that it takes ten to twelve large geese to produce one sleeping bag, therefore, down products are expensive. If you require a high loft down product like a 650 fill power sleeping bag, the process to separate down from impurities, such as flight feathers, must be repeated until purity is reached, this also adds to the cost. Limited availability of down along with the cleaning and separating process, are enough to make down an expensive insulation. It is important to emphasize that cleaner down will be warmer per ounce, will give you better loft, will pack smaller, and will have fewer feathers that break down the insulation over time.
     Down, however is temperamental, its biggest problem is that it does not take to water very well. If you've watched a goose or duck, you've noticed that it spends a great deal of time preening itself. This preening serves to distribute a gland-produced oil through the feathers. Without this constantly maintained layer of oil, a goose would sink. Therein lies the problem, the minute a plume of down is plucked from the underside of a goose, it has received it's final oiling. Therefore the effort must be made to preserve the oil the goose has so carefully applied to its down.
     Down, once in your sleeping bag has been cut off from its normal maintenance schedule. It will retain its insulative capability only if it is not subject to abuse. To abuse a down product is to get it wet without any means of drying it. When wet, down will become soggy and limp. Dried properly, it will regain it's full loft. But the user will be faced with a temporary loss of insulation, in the backcountry, this situation may be life-threatening. For this reason down is usually recommended for cold and dry climates where snow does not have the moisture capacity rain has. The other way of abusing down is to wash it wrong, and washing it wrong means washing the natural oils away, in which case down will loose its insulating properties.
     Wash your down product in an industrial quality washing machine that washes clothes by tumbling action as opposed to agitating cycles. Us a non-detergent solution, like Ivory flakes, which are soap, not detergent. Use cold water and very little flakes, maybe a tablespoon. Hand dry, but not on a clothes line. Hanging a sleeping bag on a line will likely damage the internal box construction of the product. Hang your bag from a nail or a tree. Be sure that you are hanging the sleeping bag from the webbing usually sewn to the end section of the bag.  One final note about down, it is a natural fiber and is allergenic to some people.
     Synthetic Insulation:  Virtually all synthetic insulation try to duplicate the positive properties of down:  High thermal efficiency and rebound.  Manufacturers have examined the deficiencies of down to gain an advantage by including features in their product that down cannot claim.  For instance, down, as a naturally occurring material, will vary in consistency, the same way trees, animal fur, and vegetable do.  Down is subject to periods of difficulty in production; the duck and goose crop is not always the same.  For manufactures there can be additional problems because down is mostly imported, occasionally making it difficult to get the necessary amount in time to satisfy production needs.  Down is expensive, and when wet loses most of its insulating value.  Synthetic insulation have done an excellent job of surpassing down in certain categories.  They are readily available, their insulating value when wet far surpass down, and synthetic insulation offer a significant cost savings.
     Over the past fifteen years there have been major advancements in the field of synthetic insulation, and most of them thanks to the US Military and NASA.   The goal in recent years has been to produce a synthetic insulation which is as efficient as down, is light and compact, will remain insulative when wet, and is cheaper than down.  The primary insulations are Hollofil II, Quallofil, Polarguard, Polarguard HV (High Void), Polarguard 3D, Micro Loft, Lite Loft, PL2, and Prima Loft.
     Hollofil is a trademark of Dupont.  It is synthetic fiber about 2 1/2 inches in length.  As its name implies it features four hollow compartments throughout its length.  With four compartments to trap air, Holofil II typically provides good warmth, but this fiber is heavy and does not pack very well.
     Quallofil is also a registered trademark of Dupont.  Just like Hollofil, this fiber incorporates the hollow core design as its primary feature.  The difference is that Quallofil features seven hollow compartments that run the length of each fiber.  These hollow tunnels are engineered to capture and retain body heat even more effectively than Hollofil.  This configuration provides great insulating characteristics, softness, compactness, good loft per ounce, and is able to maintain 85% of its insulating value when wet.  Furthermore, Quallofil dries three times faster than down.
     Polarguard batting is made from continuous filament polyester through a threaded roll processing system patented and owned by Hoechst Celanese corporation. The batting is then lightly bounded with resin and heat cured to ensure dimensional stability. thus, thousands of filaments, bonded together, are spread to produce an exceptionally lofty web which traps large quantities of air. Very few people will question Polarguard's durability. It retains only 1% moisture, dries quickly, is rather inexpensive, and is extremely light weight, Polarguard has also been a popular choice among manufacturers. The biggest drawback to this synthetic fiber is the fact that its compactness leaves something to be desired.
     Polarguard HV:  This polyester insulation offers all the advantages of regular Polarguard but it is 25% lighter. This was achieved by engineering the polyester fiber with a triangular void. This will trap a greater quantity of air in comparison to the regular Polarguard fiber, thereby providing a more effective barrier against escaping body heat.
The result is a great warmth-to-weight ratio. And since, much like the regular Polarguard, HV is a stable fiber, requiring no additional internal structures to prevent shifting. This will help an HV bag be lighter weight.
     Polarguard 3D is the softest Polarguard fiber ever. It offers durability, warmth as well as down-like softness. Polarguard 3D is made from continuous filament fiber which keeps it from pulling apart, clumping or matting. Each fiber is made from a high-void cross section that prevents fiber collapse, allowing it to retain loft after years of use. To achieve Polarguard 3D's softness, the filament diameter is decreased by 40% as compared to HV. This results in a finer, more silky fiber.
     Micro Loft:  Micro denier fibers are insulating fibers with a denier under 1.0, remembering that your hair is 50 denier in size.  As have the other micro denier insulating manufacturers, Dupont paid attention to the finest features of down while keeping in mind its deficiencies. Dupont decided the best way to equal down's insulative performance was to produce a fiber blend approximating down's mix of quill and fine filaments.
     A typical down cluster of one central quill and many smaller braches. The quill provides the loft, the ability to compressed down to spring back, and structurally makes the room for the fine branches to fluff out. The interconnection of finer fibers is what makes down insulation warm by controlling both radiant and convective heat loss. Nature conveniently mixes quill and fine fibers in a ratio of about 80% fine to 25% quill.
     Dupont Micro Loft is mechanically constructed under a patented process to be a blend of micro denier and larger synthetic fibers that simulate down cluster. And Dupont states that, "Micro Loft offers down-like comfort and warmth-to-weight with some obvious advantages, such as resistance to moisture and relief for those with down allergies." Micro Loft offers great softness, it retains its insulating value even when damp and is durable, moisture resistant and can be machine washed and dried.
     Thinsulate Lite Loft by 3M makes use of two fiber sizes, like natural insulation, in order to maintain a good structure and good insulation. Thermal bonding fibers maintain the structure of Thinsulate Lite Loft insulation without the use of resin. Thus, the end product is more weight efficient and hypo-allergenic. The fine fibers in Thinsulate Lite Loft are all less than 15 microns in diameter. Finer fibers will more efficiently block radiant heat given off by the body.
     The fiber structure of Thinsulate Lite Loft is formed to achieve the most loft with the minimum amount of fibers. This will amount to a light weight insulation and a lighter weight sleeping bag.
     The Thinsulate Lite Loft insulation, like the other synthetic insulation discussed above, are machine washable even though 3M claims that their insulation holds up to the abuse of washing better than most, many have disputed this claim.
     Primaloft:  As the current incarnation of synthetic fiber fills matured in consumer and market acceptance, technical users began seeking insulation that were more similar to down's strong points, but improved on some of the weaker points of those fibers presently available. The US Army and Albany International led the change, and the result was Primaloft synthetic down. Primaloft is manufactured to duplicate down's combination of extremely fine fibers supported by the larger diameter quills found in the typical down cluster. Primaloft uses minute .5 denier polyester fibers to minimize convective and radian heat transfer in combination with a small percentage of larger diameter fibers which give the insulation high loft and springy recovery from being compressed.
     Primaloft is water repellent. The microscopic air pockets created by Primaloft's .5 denier fibers resist passage of water. All larger diameter synthetic fibers have air pockets many times larger than Primaloft and offer little (if any) resistance to water.
     Primaloft PL2 is the newest generation of microfiber insulation from Albany International, engineered to satisfy the experienced backpacker with a sleeping bag that delivers long lasting, high loft performance never before available in this price range. The technological advantages of Primaloft .5 denier were addressed in the previous segment. Sleeping bas with Primaloft PL2 .9 denier insulation incorporates most of the same qualities as Primaloft except to lack some of the loft, compactness and moisture resistance of Primaloft. Sleeping bags using this fiber for insulation are perfect for those who want all the advantages of microfiber insulation without the high cost.

About The Shell Material Used In Sleeping Bags.   A sleeping bag has an inner shell and an outer shell, with insulating materials in-between.  The outer shell plays the important role of protecting the bag from wear and tear, moisture, and it also needs to prevent the insulation from coming out through it's weave. Most manufacturers use nylon or one of its derivatives as their choice for this purpose. In some cases, such as sleeping bags are concerned, rip stop nylon happens to be the most common choice.  It tends to meet all the shell requirements very well. Rip stop is a 70 denier woven fabric characterized by extra heavy yarn introduced at regular intervals in the weave of the cloth. Since sleeping bags are subject to less wear and tear than garments, or backpacks, lighter weight rip stop cloths are used in manufacturing sleeping bags in order to aid them in packing in smaller areas. But if compactness is not as big an issue, or if manufactures want to reduce cost, taffeta can be used as a fair substitute.
     Taffeta is a tightly woven nylon cloth characterized by a uniform appearance. Although it is true that rip stop cloth will stop a tear from continuing to 'run', taffeta will not tear as easily so, in fact, it is a stronger material.
     Supplex fabric, which is also a low denier nylon weave, has also been used in constructing sleeping bags. Supplex produces a silky and cottony hand. It is water resistant and breathes very well. Finally, some of you may remember sleeping bags use cotton as their exterior shell. Since cotton is heavier and bulkier than nylon and takes a long time to dry, not many manufacturers incorporate it in their bags.
     One final note, sleeping bags require breathability. This will allow condensation to rise away from the interior of the sleeping bag, which in turn will keep the inside drier. Breathability will also prevent the insulating materials form getting damp. To test whether or not the shell of your sleeping bag is breathable hold the outer shell in both hands and blow into it. You should be able to blow easily. But what if an outer shell of a sleeping bag also needs to be waterproof in order to protect the insulating materials, like down, which will fail to function if wet? In such circumstances a shell can be backed or coated with water proof/breathable material, such as Gore-Tex, Ultrex, etc.  Application of such material will dramatically increase water repellency of shells, protecting the sleeping bag from outside moisture. Unfortunately, it is also true that waterproof/breathable materials never breathe as well as uncoated shells. So in choosing the shell material, one has to weigh the pros and cons of such cloths. Waterproof/breathable materials will also add considerably to the price of sleeping bags.
     In discussing the inner shells of sleeping bags, the issue of comfort seems to surface first, and rightfully so. The inner shell of a sleeping bag lies against your body. Since most issues applying to the outer shell of a sleeping bag also apply to the inner shell, rip stop nylon, taffeta, and Supplex are also used in constructing the inner shell. The only materials used exclusively in the construction of inner shells, but not outer shells, is Tricot fabric.   This fabric is a blend of cotton, polyester, and nylon. Tricot is very soft, comfortable to sleep in, and quite durable. And since it only uses a small percentage of cotton in its construction, it absorbs very little moisture. Tricot is indeed quite breathable, the only problem with the Tricot fabric is that it doesn't pack as well as rip stop nylon or lighter weight taffeta and Supplex cloth.
     Cotton or flannel lining is also used for the inner shell material of sleeping bags. While cotton is quite comfortable to have next to the skin, it is bulky, heavy, and it takes a very long time to dry. The ability of cotton to stay wet and hold moisture next to your skin will compromise your safety in colder conditions.
     Insulating materials lies between the inner and the outer shell, and there are two kinds of cuts: Differential cuts and contour cuts. In the contour cut, the inner and the outer shell are the same size. A sleeping bag using this style of manufacturing, will slump around you filling empty spaces. The problem is that if you press the inner liner, it will freely displace the insulating material until there is no insulation left at the pressure point, creating cold spots.
     In a differential cut bag, the inner shell is cut considerably smaller than the outer shell. This eliminates the possibility of displacing the insulation in your sleeping bag.

About The Internal Construction of Sleeping Bags:   It is very important that the insulating materials in a sleeping bag is held in place properly so that it does not shift and bunch in one area, leaving other areas of the sleeping bag with less insulation, thereby creating cold spots. There are two general ways that this problem can be controlled. Sewn through quilting construction and double quilt sandwich construction.  Before getting into stabilization methods of insulation, one point need to be made. Most synthetic insulation are available to manufacturers in what are called 'batts'. Batts are cured and pre-stabilized rolls of insulation. The stabilization of these fibers is done either thermally, such as in Lite Loft by spraying resin.
     Sewn through quilting has always been the choice for manufacturing less expensive sleeping bags. This method allows manufacturers to put together a bag completely by machines and greatly speeds up the process of manufacturing. Not unlike making a sandwich, in this process the insulating material is placed between the inner shell and the outer shell, and then put under a sewing machine. However, some manufacturers will use horizontal quilts where as some will use vertical quilts. It is important to mention that in horizontal quilts the insulation will displace within a smaller area, but the vertical will allow the insulation to travel the entire length of the bag. But no matter how a sewn through bag is made, it will suffer equally by allowing cold spots around the seams, since there cannot be any insulation placed in those areas.
     In a double quilt sandwich construction, separate synthetic batts are quilted to the inner and outer shell of the sleeping bag, then a middle layer of batting is stabilized between them by sewing it to the edge of the bag. This method ensures that there is no seam which goes directly from inside to the outside of the bag; thus the cold spots of the sewn though pattern are eliminated.
     Another way of keeping insulation in place, specially down, is a series of baffles. Baffles are walls of fabric sewn at intervals between the inner and the outer shells of the bag to prevent the material from shifting. There are many different baffling systems used in the sleeping bag industry today:
     Standard box baffles are sewn between the shells so they stand vertically within the bag. This will allow the fullest loft of materials, except that the boxes must be filled with the exact amount of insulation, or the box will shift from side to side, creating cold spots.
     Slant Box baffle construction is very similar to box construction except the material is sewn in at an angle to the inner and outer shell. This method is more difficult to achieve, requires a bit more material, but it more successfully eliminates shifting and the creation of cold spots.
     The V-tube baffling uses more material than the other two, is more expensive to make, is heavier, but it is the best system for eliminating downward shifting of insulation. But V-tube baffling does not allow as full a loft as the other types.
     To further control the uniformity of insulation some bags feature trapezoidal baffling. Common slant and box baffling systems distort in use, increasing the volume, thus minimizing the pressure that maintains insulating uniformity. Trapezoidal baffles oppose one another to eliminate this distortion and keep chamber volume constant.