How Do Sleeping Bags Work and is Down or Synthetic Fill Better?

Sleeping bags are an essential piece of camping equipment, but how do they actually work? Well, the physics are fairly simple. Sleeping bag performance is directly dependent on the sleeping bag fill material, the outer layer, and the overall shape.

It’s best to understand exactly how sleeping bags works to make the most of this essential camping equipment so you can fully enjoy your time outdoors.

So, let’s get into it!

Article Contents:

How Does a Sleeping Bag Work?
What is the Difference Between Down and Synthetic Fill?
How are Sleeping Bags Rated?
Standard vs. Mummy Shape Sleeping Bags
Summer vs. Winter Sleeping Bags

backcountry camping tent sleeping bag pad the north face

How Does a Sleeping Bag Work?

The physics of how a sleeping bag works is fairly simple. It all boils down to air.

Air is a fantastic insulator. Air molecules have a very low density and have a low heat transfer capacity. That means a layer of air doesn’t transfer heat from one side to another side easily.

In modern homes, double-panned glass is used in windows because it’s designed with a layer of air in between the glass panes. The trapped air reduces the rate at which heat transfers between the outside and inside of a home, keeping it cool in the summer and warm in the winter.

A sleeping bag functions the same way:

Sleeping bags work by utilizing trapped air to insulate your body from the outside temperature. The fill material inside of a sleeping bag traps air to create a layer of insulation.

The overall performance of a sleeping bag primarily depends on the type of fill material, as well as the quantity and quality of the fill material.

What is the Difference Between Down and Synthetic Fill?

Sleeping bags are typically filled with either natural goose down feathers or a synthetic polymer manufactured to mimic down feathers.

Down feathers are a natural animal-based product. Down is the underlying layer found in birds but commonly harvested from geese or ducks. When fluffed up, down becomes very lofty which means it contains a high portion of air per volume.

Synthetic fill materials such as PrimaLoft®, Thinsulate™, and Polartec® are derived from synthetic polymers (plastics) such as polyester that is specifically manufactured. The manufacturing process produces a low density material with a high proportion of empty space that can trap air.

Is Down or Synthetic Better?

Since the introduction of synthetic materials, people tend to ask which fill material is better for sleeping bags, synthetic or down?

Well, the overall performance and selection comes down to a few key factors which include the following:

Insulation capacity
Performance when wet
Weight
Cost
Durability
Safety
Animal-derived, vs. animal-free

1. Insulation Capacity

When it comes to insulation performance, the key factor is how well each material traps air which then warms up from your body heat.

Both synthetic and down filled sleeping bags can achieve the same insulation and temperature rating but at varying weight, size, and cost metrics.

Insulation fundamentally boils down to the material microstructure. Down feathers and synthetic fibers try to achieve similar structures that are conducive to air retention.

The microstructure of down feathers is made from barbed filaments that branch into one another creating a 3-dimensional plume. Because of this physical characteristic, down is able to give lots of volume with very little weight.

However, due to the variability in quality, down is rated in “fill power” which is a measure of volume to weight.

Fill power is a measure of how many cubic inches one ounce of down can fill when fully lofted.
Two down-filled sleeping bags rated for the same temperature can be filled with different amounts of material – 12 ounces of 900 fill power down can be equivalent to 20 ounces of 550 fill power down.
Higher fill power is better since it can loft more for less weight and compress down better.

Synthetic filaments and fibers are manufactured to mimic the natural microstructure of down feathers. They offer similar lightweight insulation capacity, loftiness, and compressibility.

So, generally both synthetic and down filled sleeping bags offer great insulation capacity but will differ dramatically as far as weight, packed size, and cost.

2. Performance When Wet

A sleeping bag performance killer is moisture. When wet, some materials can loose all of their insulation capacity. The material becomes wetted and can no longer effectively trap air.

This can be extremely important when camping in wet conditions, or during high altitude climbing where snow and ice can melt and dampen a sleeping bag.

A wet sleeping bag runs you the risk of loosing a lot of heat trying to keep your body warm, which may develop into hypothermia.

Well, down and synthetic fill material function very differently when wet.

Down feathers loose all of there insulation capacity when wet. The down micro-structure collapses and it can no longer trap air. A down filled sleeping bag must fully dry before it can trap air to keep you warm.

Ducks are geese naturally retain the waterproofness of their down because they excrete a waxy substance from a gland and spread it to their feathers.

Synthetic fill materials, since they are primarily made from plastics, are naturally hydrophobic. So, a synthetic filled sleeping bag can retain some of its insulating properties when wet, and it can dry out much faster than down.

If you’re worried about wet environments and need to stay warm, a synthetic sleeping bag is the clear choice.

3. Weight and Compressibility

Packed weight and compressibility can certain influence which type of sleeping bag you choose.

Weight and compressibility aren’t very important for car camping and back yard adventures. However, for backcountry camping, mountaineering, or travel backpacking, a compact and lightweight sleeping bag is highly desirable.

Down lofts better than synthetic which results in a better warmth to weight ratio for similar performance requirements.

Down filled sleeping bags are up to 35% lighter than similar performing synthetic filled sleeping bags depending on the fill type.
Down filled sleeping bags pack down 50 to 60% smaller than similar performing synthetic filled sleeping filled sleeping bags.

4. Cost

For the average consumer who is just getting into camping, price is probably the most important factor when determining which type of sleeping bag to purchase.

The cost difference between down and synthetic fill can be fairly significant. Here is the range in cost for similar performing sleeping bags at the same size and shape.

Down filled sleeping bags cost between $200 to $500
Synthetic filled sleeping bags cost between $100 to $200

So, if you’re on a limited budget, a synthetic filled sleeping bag is the clear choice.

5. Durability

Most campers take care of their gear, both during and after the camping season. However, with extended use, camping equipment does degrade over time.

And because sleeping bags are used somewhat constantly when camping, after a while the fill material can get damaged, breakdown, and eventually loose its insulating performance.

Synthetic fill material is less durable than down and slowly breaks down after repeated compressing and decompressing.
Down feathers are resilient and can endure many cycles of compression and decompression without degraded loft or performance.

Personally, I’ve had the same down-filled sleeping bag since 2011 and its performance is still the same – that’s 11 years of repeated use so far!

How you clean (wash and dry) a sleeping bag also depends on the fill material. Oils and dirt can seep into the fill material causing a decrease in performance.

Both down and synthetic filled materials can be washed with very little, non-harsh soap on a gentle cycle.
Never dry cleaning a down-filled sleeping bag, use fabric softener, or bleach
Tumble dry sleeping bags on low heat

Pro-tip: During the off season, store your sleeping bags uncompressed. This extends the life of the fill material inside which is especially true for down filled sleeping bags.

6. Safety

Often, sleeping bags are used in close proximity to open fires. Under the right conditions, excess heat, or a rouge spark can land on a sleeping bag causing it to melt or ignite.

So, which is more flammable – synthetic or a down filled sleeping bag?

Synthetic fibers and fill materials such as polyester are ignition resistant and not considered flammable [Reference]. However, they can melt which can lead to severe burns.
Down feathers are inherently quite flammable. Down can easily catch fire if not treated with a fire-resistance chemical. Campers need to be extra careful using down filled sleeping bags around open flames and campfires.

However, most high-quality sleeping bags are manufactured with a flame-resistant synthetic outer shells that limits ignition.

7. Animal-Derived vs. Animal-Free

Another reason people may choose a synthetic filled sleeping bag is that it’s derived from petrochemicals instead of animal-derived materials.

A majority of down feathers produced every year is a by-product of the goose and duck meat industry [Reference]. Live plucking of feathers, or force feeding practices are also common before the birds are processed.

For individuals who are strictly vegan or want a cruelty-free product, synthetic fill sleeping bags are the clear option.

However, it’s becoming more common for companies to ethically source down feathers. Certifying bodies and standards have been developed to evaluate bird living conditions, slaughterhouses, and processing facilities.

The strictest standard is the Global Traceable Down Standard (GTDS) which ensures no feathers can be derived from live-plucked or force-fed birds and the animals have access to the five basic freedoms. Patagonia is the only company to ethically source down feathers according to the GTDS.

Tent sleeping bad pad therm-a-rest coleman

How Are Sleeping Bags Rated?

When you start looking at different sleeping bags, you’ll notice a temperature range each bag is rated for. But, what does that mean, and how is that tested?

Well, there are typically two temperatures listed for a sleeping bag – comfort temperature, and limit temperature. Both ratings are based on laboratory testing and a subjective temperature value a sleeper would still feel comfortable:

Comfort temperature rating is the ambient temperature a cold sleeper would still feel comfortable.
Limit temperature rating is the ambient temperature a warm sleeper would still feel comfortable.

Personally, I’m a hot sleeper and my wife is a cold sleeper. That means I can tolerate a lower ambient temperature because I tend to overheat at night.

So, if the ambient temperature dips down to 10^oC overnight, my wife and I would have to use different rated sleeping bags. I would be able to use a sleeping bag with a limit temperature rating of 10^oC whereas my wife would have to use something that 5^oC warmer.

Rectangle vs. Mummy Shape Sleeping Bags

There are two popular shape of sleeping bags. The standard shape is a rectangle, and the alternative is a mummy shape which follows the contour of the body. But, which shape is better for camping?

Well, both shapes have pros and cons. And, depending your sleep habits, pack weight and size restrictions, and performance criteria, one is probably better than the other.

Rectangle Sleeping Bags

A rectangular shape is simple (and cheaper) to manufacture, with plenty of leg room for stretching.
Rectangle sleeping bags can feel less claustrophobic, allowing you to roll or shift around easily.
For the same temperature rating, a rectangular sleeping bag weighs more since there is inherently more material around your legs.
Rectangular shapes can allow warm air inside to escape more easily.

For car camping, traveling, or cottages, a rectangle sleeping bag is good enough!

Mummy Sleeping Bags

Mummy sleeping bags are shaped to closely hug the body, and taper down to the feet.
The offer better heat retention since there’s less empty space for air to heat up.
Mummy sleeping bags pack smaller and weigh less because they don’t have any excess / unnecessary material.

For backpacking, cycle touring, or mountaineering, a mummy sleeping bag is recommended.

tent moisture condensation inside sleeping bag pad mattress backcountry

Summer and Winter Camping Sleeping Bags

A majority of people camp during the summer months when the average ambient temperature is 10^oC or higher.

A summer sleeping bag with less insulation will allow for more circulation, keeping you warm but not overheated.

Outdoor temperatures in late spring and early fall may drop down to 0^oC. A sleeping bag with more insulation is recommended. Alternatively, using a sleeping bag liner with a warm weather sleeping bag, or wearing additional insulating layers at night will help keep you warm.

For true winter camping, at temperatures well below freezing, you need to use a cold weather rated sleeping bag. The lower the temperature rating, the warmer you’ll be.

Also, sleeping bags with more fill or a higher fill power will keep you comfortable in frigid conditions.

But, if you’ll only experience a few degrees below zero, then you don’t need to get a -20^oC rated sleeping bag which will be bulkier, heavier, and more costly.

Pro-tip: A sleeping bag should be paired with the proper rated sleeping pad. Check out my completed guide to camping sleeping pads.

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