Types of Insulation
High performance products are made with synthetic and down insulation. Synthetic insulation is best for extended trips in wet conditions where you expect to get wet, stay wet for an extended time period, and where opportunities to dry the bag out are limited. Down insulation offers significantly more warmth for its weight, is more compressible, more comfortable, and more durable than synthetic insulation. Its Achilles heel, however, is moisture: down bags are not suitable for conditions where the bag may get soaking wet. Down's insulation value, the amount of loft, decreases with as it collects moisture. It's important to prevent a down bag from getting wet and, on multi-week trips, to have an opportunity to dry the bag out if it becomes soaked. For most conditions in most places, howeverm down insulation offers the greatest performance for multi-day trips where weight and packed size are top concerns.
Down insulation comes from tiny particles of a bird's plumage that lie under its feathers. Although other cold weather birds produce down, Goose down has been shown to the highest quality available. Down quality is measured by its fill power, the number of cubic inches displaced by one ounce of down. A bag with 900-fill power will be lighter, more compressible, and more durable than an identical bag and equally warm with a lower fill power, such as 700. The more volume displaced by higher fill powers requires less total down to fill the same volume.
Down can be harvested in a variety of ways. Some birds are killed for their down and meat, while others are killed solely for their down. Some birds, geese in particular, are live plucked of their breast feathers. Others, such as the eider duck, line their nests with down, making harvest a pain free process. Animal rights activists consider live-plucking to be a cruel and painful process, especially because it happens regularly. Some companies, such as Patagonia, do not use down from live-plucked geese. Instead they use down from force-fed geese that are produced for foie gras. Some of the smallest premium companies, such as Western Mountaineering, source the highest down from geese that aren't live-plucked or force fed; they collect naturally shed down from molting. Some vegans might consider avoiding down down products or purchase them from a company such as Western Mountaineering, that guarantee no harm to animals.
Down quality has not improved over time.
Fill power ratings have increased over time as fill power testing procedures have changed. Fill power ratings represent the maximum potential performance under ideal laboratory conditions, i.e. after a one ounce sample of down has gone through a five-day "spa retreat" of washing and drying and fluffing. Geese have not developed better ways to produce down. We've simply improved our ability to test down's potential performance under ideal conditions. The result is that fill ratings today are roughly 130 points higher than they were fifteen years ago even though the down is the same.
Hydrophobic coated down has the potential to radically address down insulation's only drawback- its susceptibility to loft loss from moisture. Nano technology coatings claim to prevent down from absorbing water and allow it to dry out faster once it does absorb water. Hydrophobic coatings have the potential to make a very good thing even better. But how much better and is it worth the additional 10% price increase?
Our preliminary field testing has shown that treated down is not a substitute for synthetic insulation in wet conditions. Testing by one well respected company (that prefers not to be named) has shown a negligible difference between treated an untreated down in real world experiments, such as if the bag gets soaked, perhaps during a river crossing, does it dry faster and re-loft faster? Their testing suggests that it does not. Another concern is durability. Down is extremely durable: it's able to withstand hundreds of compressions, wide swings in temperatures, and proper washing restores twenty and even thirty year old down close to its original performance. We aim to test treated and untreated down side by side in sleepings bags and jackets over the long-term (and then test the down fill power) to see if treated down reduces durability. OutdoorGearLab is also in the process of developing quantitative tests through dummy skin temp probes and thermal imaging that will compare the insulation values of treated and untreated down.
Our testers use synthetic insulation for very wet conditions or trips with a moderate to high probability of getting soaking wet. Synthetic insulation is good for expedition mountaineering trips that cross multiple climates, it's the only bag suitable for big wall rock climbing, and it's generally chosen for "if it gets wet you die" circumstances. Synthetic insulation is also great for lazy people that don't like to put the extra effort of caring for down products. Synthetic bags can go in the washing machine without specialized, expensive detergent. Unfortunately, synthetic insulation is far heavier and far bulkier than down insulation and is much less durable. Repeated compression breaks down synthetic fibers and reduces warmth over time-- a synthetic bag may retain its warmth less than half as long as a down insulated bag. Our specification tables, found in each review shows insulation types.
Unlike insulated jackets, sleeping bags are rarely exposed to abrasive conditions that might tear a fabric. Thus, backpacking bags use the lightest possible down proof fabrics. Some ripstop fabrics, used on the exterior, weigh as little as 0.8oz per square yard. All of our top rated bags use very similar, if not the same fabrics. In general, lower priced bags use heavier fabrics. We've found that waterproof breathable fabrics are unnecessary in three-season in three-season conditions. The exception here is at the a bag's foot box. Nemo uses a waterproof breathable fabric on the bottom of its bags and this helps to prevent the insulation from absorbing water. Depending on its use, a properly cared for down bag may last ten to twenty years.
Types of Construction
Three-season down bags use three major types of baffles (compartments for down). Sewn-through baffles have a seam that pierces through both fabrics- this is the lightest, coldest, and cheapest way to build a down bag. See the Western Mountaineering Highlite and Mountain Hardwear MTN Speed 32. Vertical baffles run from the bag's head to toe and usually have several mesh walls to prevent the down from migrating. Vertical baffles make it easy to construct comfortable hoods and toe boxes and are found on the Marmot Plasma. Horizontal baffles create the highest warmth to weight ratio are used in what we believe to be the best three-season and winter bags. Continuous horizontal baffles offer the greatest versatility for three season conditions because they allow you to shift down from the top of the bag to the bottom of the bag, for warm weather, and from the bottom to the top of the bag, for cold weather. Our highest rated backpacking bags have continuous horizontal baffles.
Synthetic ones are constructed in similar ways. Some are sewn-through, others alternate the location of seams to increase warmth, and others (such as the Mountain Hardwear Ultralamina) laminate the insulation to the shell fabric, which saves weight and increases warmth.
Types of Sleeping Bags
The classic design encloses the user's top, bottom, and foot area; fastens with a zipper; and has an adjustable hood. This type of bag is wonderfully comfortable, but, from a weight and thermal efficiency standpoint, is not the best option. Why not? There are three reasons: 1) Any type of insulation provides little to no warmth when it's compressed, so the bottom of a bag adds more weight than warmth. 2) Zippers increase user comfort by providing an easy way to enter and exit, and allow the user to vent the bag on one side. Unfortunately, zippers are usually the first part to break and can add significant weight (the baffles need to be sewn shut, a zipper needs to be sewn on, and a draft tube is usually added to prevent cold air from entering through the zipper). 3) Attached hoods are attached. You can't take them off when you don't need them (adds weight) and, if you have hooded clothing to insulate your head they're unnecessary in most three-season conditions. Our testers have unanimously found detached hoods to be more comfortable than attached hoods. This is because they allow you to turn your head and the hood at the same time- gone are the days of "getting lost in your bag" and facing the back of the hood (which sends exhaled moisture vapor directly into the down- reduces loft and adds weight). We've found that very few attached hoods are highly comfortable. Most are uncomfortable when cinched fully and some are too tall (head goes up too far and obscures your nose).
Applications for Traditional Bags
The best application for the traditional style is cold weather use. When weight matters, our testers generally use quilts down to-- depending on our clothing system-- around 10 degrees F, and traditional style bags for anything colder. Traditional bags are also best for when comfort is top priority. Although quilts are more comfortable in hot weather (use them as a blanket) they involve slightly more effort to setup than a traditional bag, are best for people who don't thrash about at night, and aren't as good for spooning with another person in cold weather (because you need to stay on your pad to be fully inside the bag).
Clothing and Traditional Fit
There's more innovation and variety with down bags, because down is the highest performance insulation, than with synthetic bags. Choosing the right type of fixed girth bag will come down to efficiency versus comfort. The most efficient bags will give your body just enough room to move a little, but not enough to create drafty dead air space. The most efficient sleep system involves clothing. Our testers choose their clothing and traditional design with the intention of just being warm enough on the coldest night of a trip. And on the coldest night(s) our testers wear all of our clothes, except waterproof materials in the bag. Mammut has a good report on the history of bag warmth and testing procedures, see it here.
When it comes to traditional bag comfort, wiggle room is important. The more space to sprawl about, twist and turn, and toss around-- the better!! When choosing the most comfortable bag our testers aim to be warm enough on the coldest night with minimal clothing. Unfortunately, wider bags are less efficient at keeping you warm. Your body heats up the air around you, and when you move (in a wide bag) you may open a pocket of dead space (cold, unheated air) and need to start over by heating up a new area. This is the case with larger mummy bags and all rectangular and semi-rectangular bags. Therefore, from a weight and thermal efficient standpoint, the single most important part of selection is fit. In three-season conditions the solution is a quilt style.
Quilts are the highest performance type for multi-day trips in three-season conditions. They don't have zippers, hoods, or insulation on the bottom, which makes them warmer for their weight and more versatile than traditional style bags. Quilts are comfortable in a wider range of temperatures: they can serve as a blanket on warm summer nights, be cinched down in colder conditions, and are usually easier to wear around camp than a traditional mummy bag. Quilts dry faster than those with half length zippers or no zipper and, most importantly, quilts have adjustable girths that allow them to be tailored to the clothing you're wearing or to various body types.
Adjustable girths are the solution to sleeping bags' greatest drawback: fit.
A bag that's too large will leave cold, dead air spaces- the bag is less thermally efficient. A bag that's too tight won't be able to loft properly. Variable girths accommodates changes in clothing (in colder conditions we might wear a lightweight down or synthetic jacket). Traditional styles are plagued by a fixed circumference that is incapable of adapting to the varying conditions typically found by backpackers. The most thermally efficient traditional bag (a trim fitting bag) will be too slim to accommodate additional clothing in colder conditions, too slim for a broad-shouldered male friend, and too large for a petite female friend. Quilts better fit a wider range of conditions and body types and are the best type of bag for backpacking.
Bottomless Traditional Bags
In an effort to save weight some traditional bags omit a zipper and replace insulation on the bottom of the bag with a thin single layer of fabric. There are many variations on this: some bags, like the Therm-a-Rest Navis, attach to a sleeping pad and others, like the Rab Infinity SL, lie on top of a pad without an attachment mechanism. Further, the Feathered Friends Vireo is a hoodless and zipperless bag with more fill in the foot area than chest area- it's designed to be worn with a hooded jacket in colder conditions or used in alone warmer weather. These types of bags offer excellent performance for specific conditions, but their fixed girths and lack of zipper severely limit their versatility.
Hood-less Sleeping Bags
The Hood-less style offers three significant advantages over their hooded counterparts: (1) They're much more comfortable. Only a handful of the seventy products we've tested have hoods that are truly comfortable when fully cinched. Separating the hood from the bag lets you turn over without getting lost inside the hood and prevents you from blowing moisture vapor into the bag (which adds weight and reduces loft). (2) Hood-less bags are lighter. Most people do the majority of their backpacking in above freezing conditions when attached hoods are rarely needed. Why carry it if you don't need it? Instead, bring a hat, wear hooded clothing, or get a down hood for below freezing conditions. (3) Hood-less bags cost less to manufacture. Why pay the additional cost of a complex hood if you don't need one? 90% of the time our testers do not bring a separate hood.
Wearable Sleeping Bags
From a weight saving perspective, jackets are thermally inefficient. Sleeping bags offer much more warmth for their weight. Although many can be draped over your body around camp, they don't perform as well a dedicated jacket. Thus, some companies offer wearable ones that eliminate the need to bring a jacket. We tested the Jacks R Better Sierra Sniveller, a wearable quilt, and the Feathered Friends Rock Wren, a wearable traditional bag. Wearable bags offer budget and weight conscious backpackers a viable option for saving weight, but they are not as warm or as comfortable as either a dedicated jacket or dedicated sleeping bag.
Sleeping Bag Anatomy
Down bags have baffles, or channels, that hold down in place. There are three types: continuous baffles (allow you to move down from the top to bottom of bag), vertical baffles (allow you to move down from the head to the foot), and side block baffles (which keep the down in place for maximum heat retention). We believe that continuous baffles are the most versatile and side-block the warmest.
A hood can be drawn around your head in cold weather. Hoods are important in cold weather to insulate your exposed body parts. If you were naked outdoors, most of your heat would be lost through your chest and core, but once you are tucked in a sleeping bag, your exposed head is a primary spot for heat loss. Since hoods vary in comfort we encourage you to get inside the bag you're considering and fully tighten the hood and neck baffle. Can you still breathe? Can you turn over or change position without suffocating? Does the draw cord feel constricting or uncomfortable? Try out different bags because hood designs vary considerably. The Mountain Hardwear Phantom and Marmot Plasma have our favorite hoods.
A neck baffle (a.k.a. draft collar) is an insulated tube that prevents heat loss from around your shoulders and neck. While rectangular or ultralight bags may not have draft collars, they remain an essential component of winter bags. Neck baffles should be sufficiently voluminous to fill the void between your neck and shoulders, but not so gigantic that you fill like you're wearing a tire around your neck. Good neck baffles have elasticized drawcords that stretch when you move. The best neck baffles (see the Mountain Hardwear Phantom 15) are asymmetrical, to conform to your neck, and have differential pull cords (one round and one flat) so that you can feel what part of the bag you're tightening in the dark.
Since your feet take up more room than your shins do, many bags are designed with flared or trapezoidal foot boxes. Some mountaineering bags have extra space to accommodate boot liners or water bottles.
Thoroughly examine the zipper, the zipper's teeth should be large enough to run smoothly without catching on the fabric. Most bags have a reinforced strip along the inside edge to prevent snags. Some bags, such as the Montbell U.L. Super Spiral Down Dugger, can be zipped to another bag for a larger and perhaps more interesting sleeping experience. Look for reinforced strips alongside the edge of the zipper- these prevent it from snagging, and eventually tearing, the interior fabric. Also, make sure the fabric and seams have good integrity. Tug at the seams to make sure they don't open up. The bag's overall construction, features, and finish will determine its quality.
Almost all bags have an insulated tube or flap that runs parallel to the zipper in order to reduce heat loss. Cold weather bags may have two heavily filled draft tubes. The tube(s) should be sewn only to the inner liner material, not all the way through, since that creates holes and air leaks. Draft tubes are often sewn to the top zipper so that they hang down when you sleep. Make sure that the tube is sufficiently large to cover the entire zipper area. Those that are too small can interfere with the zipper, so examine the zipper action before you buy.
Some bags have stash pockets. These are generally more for fashion than function, but perhaps you might use one for a watch alarm. We think internal pockets are better than external pockets because you can access them without opening up your bag and an alarm will be more likely to wake you up.
Several of the zero degree down bags we've tested have waterproof-breathable shell materials. These resist frost, spindrift, and snow, and keep the bag much drier on multi-day trips. We only recommend a waterproof shell for camping on snow and ice.
Some companies offer a lifetime warranty on material and workmanship. Others don't. Most will repair damaged bags for a reasonable fee.
Stuff and storage sacks
Stuff sacks pack your bag down for your time in the mountains or on the water. Storage sacks let the insulation loft when the bag is not in use. Of the 30 plus products we've reviewed none of them include the ideal stuff sack. Get yourself a small and light compression sack that's durable enough to withstand several hundred compressions.
Tips and Tricks
1. Wrap the bottom of your bag in your hardshell or rainshell to prevent it from getting wet from condensation absorbed from the tent wall. How: close the jacket's front zipper, tuck the hood and arms inside, and slide the jacket over the foot of the bag. This prevents condensation (on the inner wall of the tent) from getting your bag wet. It's more important with down bags than synthetic, but don't do it if your jacket is wet. Keep the jacket's arms out and open the pit zips slightly if you find that the jacket isn't breathing enough.
2. For cold-weather bags get the long size. This gives you extra space for clothing, boot liners, batteries, water, and electronics.
3. Dress well. Wearing excess layers in a trim-fitting bag will compress the insulation, thereby preventing it from lofting to its fullest and keeping you warm. On the contrary, adding layers will take up dead air space if the bag is too roomy
4. Keep your bag dry. Line your stuff sack with an extra thick trash bag, stuff your bag in, twist the garbage bag and tuck it into the stuff sack, and compress further. For multi-day trips we also recommend lining your backpack with an extra thick trash bag and packing everything inside that. This will keep the contents of your pack dry even during large river crossings where the bottom of your backpack gets submerged.
EN Testing: why we don't rely on it
The European Norm 13537 is a standardized test that measures warmth. The test uses a copper mannequin, fitted with 20 sensors, that wears long underwear and socks, and lies on top of a thin sleeping pad and an elevated 12mm thick wood platform. The mannequin lies inside of a sleeping bag in a temperature-controlled room while its sensors and mathematical models attempt to estimate the products warmth. Results from EN testing provide us with three numbers:
Comfort Limit: based on a standard woman having a comfortable nights sleep
Lower Limit: the lowest temperature a standard man can sleep comfortably
Extreme Rating: a survival rating for a standard woman
(Note that women generally sleep colder than men. All women's specific sleeping pads and some bags are slightly warmer than their standard counterparts.)
The problem with standardized tests of all types is that they only measure how well a person or product does on the test, and the test may have little to no correlation to how well that person or a product performs in real world conditions. Thus, EN testing only measures how well one performs on the EN test. Is EN testing similar to real world conditions? Yes and no. Most people sleep on their backs, the position the mannequin is in, but not everyone has the same body type as the mannequin, and the EN sleeping pad is no where near as warm as those used by most people today. For example, thin closed cell pads like the one used in EN testing, generally have an R-value around 1. Modern lightweight inflatable pads like the Therm-a-Rest NeoAir XLite (R-value 3.2) are 300%+ warmer. The warmer a pad is the less insulation you need beneath you, and the less insulation the lighter the sleeping bag. Therefore, ones with little or no insulation on the bottom have poor EN scores. Yet most of us want the lightest ones possible
Many of OutdoorGearLab's highest rated contenders are not EN tested. This is because: 1) they're made by small companies that can't afford the expensive test (Marmot, for example, ships bags to a lab in Norway for testing) 2) some companies may not believe EN testing provides their customers with valuable information, and 3) a design may not be compatible with the test. For example, quilts and all non-traditional bags might not have hoods or insulation on the bottom. Furthermore, factors like and individual's body type, clothing, sleeping pad warmth, type of shelter, and food and drink can all affect one's sleeping experience. And, above all, how it fits your body has more influence on warmth than any other bag-specific attribute. We do not believe EN testing is a useful method to compare warmth across a wide range of bag types. Instead, we rely on our testers' assessment of warmth. Most of our testers have spent so much time playing and sleeping outdoors (years) that they know exactly how their body performs in various conditions and how those conditions will influence how warm they are at night. For example, after v hours of w type of exercise with x to eat and drink, y clothing, and z weather, our testers can assess the warmth and compare it to previous experiences with other bags after just a few nights. We believe this method is better at comparing a variety of types than both EN testing and putting college kids in the meat freezer (a method several outdoor companies have used).