Sleeping Bag Technology 101

How do modern sleeping bags work? Understanding the basics will help you choose a great sleeping bag and get the most out of it.

When you’re shopping online or browsing through a store, how do you decide which sleeping bag to buy? With so many choices and designs, it’s difficult to decide. The following will help you understandidentify the technology built into bags that will keep the most heat in, the weight down, and still fit in your pack – and your budget.


Because air is a poor conductor of heat, sleeping bag insulation is designed to trap as much still air between you and the outside world as possible. The trapped air forms an insulating barrier, keeping your body’s warm air from escaping. Sleeping bag manufacturers use many different natural or synthetic fibers create this dead air space.

There are many new synthetic fibers available that are an attractive alternative to natural fibers. There are always new competitors coming into this area, with names like Primaloft, Climashield, etc. The advantages of synthetic fibers is their lower cost and performance when wet. They are cheaper than down insulation, and will retain heat a little better than down does when the bag is soaking wet.

Down feathers are the finest feathers on the underbelly of geese and ducks. Down has several advantages over synthetics. First it is much more compressible than synthetic insulation, which means it will fit better in your pack. Second, a good down bag is much lighter than a synthetic bag of equal temperature rating (as much as a full pound less!) There are three downsides to down. First, its more expensive than synthetic. Second, if the down is soaked (for example, in a river crossing) it will not hold hold as well as synthetic fill until the down has dried out enough to fluff back up. Third, down requires a little more care than a synthetic bag.

So what’s in my pack? Down, all the way. I am currently packing a Marmot Plasma 15, which is filled with 900 fill down (Down Fill power is a measure of the quality of down. Fill ratings range from 300 on the less impressive end to 900 on the highest quality end.) I chose this bag because it is extremely compressible and very light weight, which meets my needs as an ultralight backpacker. The Marmot sleeping bag is available from Amazon with this affiliate link if you’d like one shipped to your house: Marmot Helium 15 Sleeping Bag – Cobalt Blue/Dark Azure Regular/Left Zip

Bag Shape

Bags come in many different shapes. The more efficient shapes are mummy bags, which are tapered to match your body contours and include a hood that encloses your head, and the least efficient at the rectangular bags often used for car camping.

Rectangular bags are usually the cheapest, and are filled with a lower quality insulation. They work well for car camping and the warm season. With a large opening, the bag does not retain heat well in the cold. The rectangular shape gives you lots of leg room and space to move while you sleep. They’re too big and bulky for backpacking however, and weight considerably more due to their size and cheaper fill materials.

Mummy bags are typically more expensive, but are more efficient in retaining heat. If you haven’t slept in a mummy bag, know that it will take you a few nights to get used to the form-fitting shape which will restrict your movement somewhat. With a little practice, you get used to the idea that turning over required you to rotate the bag with you rather than just rolling over in the bag.

Bag Construction

There are several construction features to look for in choosing a bag.

Stitching methods affect the bags warmth. Cheaper sleeping bags are through-stitched – meaning that the thread passes from one side of the bag completely through the insulation and out the other side. This is not as effective because heat can pass through the stitching holes to the outside of the bag. Sewn-through bags have cold spots near the seams, so they are better for warm-weather uses. Better designs use quilting, where the stitching does not pass all the way through the insulation.

Baffles are thin mesh material that is used to hold the down in place throughout the bag. There are often organized into channels, which are walls that may direct heat flow around the bag horizontally or vertically. Some baffles and chambers allow you to adjust the position of the down so you can adjust the location of the insulation to address cold spots. Some baffles are welded instead of stitched. Welding can create a more permanent, more water-tight baffle.

Layers or shingles are a technique used on synthetic insulation. In a layered sleeping bag, the insulation consistis of thin sheets of insulation, the outer layers are often stitched to the sleeping bag’s shell. In a shingled sleeping bag, the insulation is cut in strips and sewn into the bag in layers, like the shingling on a house roof.

Differential cut is a technique based on the fact that the inside of the sleeping bag lining has a smaller circumference than the outer outer lining (the outer lining must cover a larger area as it wraps around the sleeper.) Most mummy bags are differentially cut, allowing the manufacturer to add additional fill in the outside sections of the baffles.

A collar may be added to a bag for additional heat control. A collar is a tubular ring of insulation sown in above the shoulder. A collar functions much like a scarf, better trapping heat in the body of the bag as the head may move around, temporarily opening the top of the bag as it does so, letting heat escape.

Vents are zippered mesh openings, typically in the footbox. In warm weather, the zipper can be opened to allow air to circulate better through the bag. A vent will add some weight to the bag, but can extend a colder-rated bag’s usefulness into warmer months.

External Factors

Finally, there are a few important external factors to consider.

Most manufacturers’ advertised temperature ratings assume the use of a good ground pad. The weight of your body will compress the fill beneath you, which greatly limits the insulating power and puts you in direct contact with the ground. A good mattress will insulate you from the ground, as well as provide some cushion for comfort.

Sleeping in a tent can add 5 or more degrees of warmth to your sleeping bag. While large tents do not add as much warmth as smaller tents, they still do protect you somewhat from circulating wind.

Heat Loss Occurs in 3 Different Ways

In choosing or troubleshooting your sleep system, keep in mind the ways in which heat can be lost:

Convection happens when heat leaves the body by traveling through the air. In a sleeping bag, this occurs in places where the bag is open, or in places where there are large gaps between the body and the bag.
Radiation is the process of a body’s radiant heat leaving through the air – passing through the material of the bag.
Conduction is the transfer of heat through direct contact, particularly through the ground. If you’re cold underneath, upgrade ground pads.

Mike Harmer

I created this website back in 1996, and have slowly added content to it over the years. Some resources have been contributed by viewers and other people who love the outdoors.

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