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
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
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
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
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
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
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
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
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
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
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.