Steven in Sierra Designs sleeping bag inside Sierra Designs tent on snowy day

How to Choose a Sleeping Bag

There are a number of important factors to consider when choosing a sleeping bag. Let’s break them down.

Insulation Type

The insulating material inside the sleeping bag is often called fill. The fill is a very important choice when considering buying a sleeping bag because it can affect the weight, cost, durability and warmth of the bag. 

We’ll break insulation into four main categories: 

HOW DOES A SLEEPING BAG KEEP YOU WARM?
Insulating materials, whether they are natural or manmade, trap air within networks of fibers. This motionless air is a very good insulator because the trapped air forms a barrier between the warm inside and the cold outside.

Diagram showing how insulation works to isolate cold and warm air

Synthetic Insulation

Synthetic insulation uses manmade fibers of various shapes and lengths to trap air. There are branded types of synthetic insulation used by numerous sleeping bag manufacturers. There are also proprietary synthetic fills developed by individual sleeping bag manufacturers. This means that you’ll find a variety of synthetic fills on the market.

There are technical differences between different brands of synthetic fill but they share some general features.

  • Water resistance – synthetic fibers do not absorb water so they retain their ability to insulate even in wet or damp conditions
  • Hypoallergenic and cruelty-free – human-made fill contains no animal products and doesn’t promote common allergies 
  • Low maintenance – synthetic materials are easy to care for and often machine-washable
  • Budget friendly cost – synthetic materials are often lower in cost than other options

Despite advances in technology, synthetic fill has drawbacks when compared to natural down. Synthetic fills are not as efficient as down, requiring more fill to achieve the same temperature rating, so a synthetic bag will be heavier than a comparable down bag. Also, synthetics do not compress as effectively as down, resulting in a larger stuffed size.

Down Insulation

Down cluster
A cluster of down feathers

‘Down’ is a term we use to describe a certain type of feather. Down performs the same role for birds as it does for us:  it keeps them warm in cold conditions by trapping air. Down is a time-tested favorite in sleeping bag insulation because it is both warm and lightweight. For its weight, there is no better insulator for outdoor gear than down.

The notable and well-known weakness of down is its vulnerability to water. Down feathers absorb water, so they stick together in wet clumps when exposed to moisture. The bottom line is that down loses most or all of its insulating ability when exposed to wet or very humid conditions.

These are some important facts about down:

  • Allergies – down feathers can harbor allergens, or even act as allergens
  • Cost – natural down is labor-intensive to gather and is therefore more expensive than synthetics
  • Water – when exposed to wet or damp conditions, down loses some or all of its insulating properties

Water-Resistant Down Insulation

In an effort to mitigate some of the challenges of down in wet or damp conditions, some manufacturers have introduced water repellent treatments. A microscopic, hydrophobic finish is applied to each down feather, which slows the absorption of water by the feathers.

It is important to understand that while most water-resistant down will stay dry and insulating longer than traditional down, this material will eventually get soaked when exposed to wet conditions for too long. Another benefit to water-resistant down is that it recovers (dries) faster when it does get wet.

The takeaways for water-resistant down:

  • Warm and safe – water-resistant down combines the lightweight, insulating abilities of down with the water resistance of synthetic insulation
  • Cost – water-resistant down is more expensive than synthetics

Hybrid Insulation

Hybrid insulation is an innovative mix of synthetic and down fill which seeks to take advantage of the strengths of both natural and synthetic materials. The performance, weight and cost of these bags tends to fall between the extremes of 100% down and 100% synthetic sleeping bags. 

Temperature Ratings

Sleeping bags are rated for certain temperatures. Some companies test their own bags. Other companies have their bags tested by certified laboratories, resulting in a EN/ISO (European Norm) rating. The accuracy of temperature ratings is a complicated discussion, but you can use them as a guide. 

THERMAL EFFICIENCY
A warm night in a sleeping bag is the result of your body generating heat which warms the air inside the bag. A bigger bag has more space inside, while a smaller bag means that your body doesn’t have to work as hard to warm the interior. 

The first thing is to determine what kind of conditions you expect to use the sleeping bag. What is the lowest temperature you expect? What about the average? A 45 degree sleeping bag would be appropriate for summer camping, while a 0 degree bag would be better suited for winter adventures.

After you have a general temperature range in mind, think about how you personally sleep. Do you get cold easily or does your internal furnace cause you to throw off the covers at night? This knowledge can help you fine-tune your choice. For instance, if you settle on 30 degrees as an expected low temperature for your adventures but you tend to sleep cold, you might select a 20 degree bag for comfort.

A sleeping bag liner is a lightweight way to increase the warmth of a sleeping bag. Liners are available in different sizes and shapes to suit every bag.

Shape

Sleeping bags are available in three general shapes:  mummy, rectangular, and semi-rectangular. The shape of a sleeping bag influences its thermal efficiency.

Diagram of three sleeping bag shapes: mummy, semi-rectangular, and rectangular

Mummy-shaped bags are usually the lightest and warmest and are preferred for backpacking. As popular as they are, some people find the slim, tapering shape confining. 

Rectangular-shaped bags are comfortable; they provide plenty of room to move around. As a result of the extra material used in their construction, they tend be heavier and not as efficient (~warm) as mummy bags. These bags are popular for car and base camping, when weight is not an issue and comfort is desired.

Semi-rectangular sleeping bags are a compromise between the other two shapes. They are roomier but less warm than a true mummy bag, while being lighter and warmer than a true rectangular bag. These are useful for those seeking more space in cooler camping conditions.

If you and your partner don’t want to be apart during your camping trips, there are sleeping bags made for two. These double-wide sleeping bags supply enough room for two sleepers.

Weight

How much a sleeping bag weighs reflects some of the things we’ve already talked about, such as shape and insulation type. A rectangular bag will weigh more than a comparable mummy bag and synthetic insulation weighs more than down. 

The temperature rating is another important factor because a bag intended for use at 0 degrees needs more insulation and will therefore weigh more than a comparable 30 degree bag. 

Other factors influence weight, such as the face fabric (the material covering the outside of the bag) and even the zippers.

Gender- and Age-Specific Designs

Children

We explained that excess space inside your sleeping bag reduces your ability to keep warm. You might imagine that this is a serious concern for children, who can be much smaller than the adults for whom most sleeping bags are designed. To address this, some manufactures produce sleeping bags that are appropriately sized for children.

Women

Women also tend to have smaller frames than men, but there are also important physiological contrasts (circulatory patterns, fat distribution, basal metabolic rate, etc.). To address these differences, manufacturers design women’s bags with specific female-friendly features:

  • shorter lengths for better thermal efficiency
  • narrower shoulders and wider hips to accommodate female body shape
  • increased insulation in the footbox and upper body areas, where women often lose more heat compared to men

Specialty Features

Some bags feature special features to enhance their function. For example:

  • Draft tubes and draft collars – these insulation-filled flaps hold warm air inside your bag and prevent cold air from entering. These are most important on cold-weather bags.
  • Sleeping pad loops/straps – these systems differ between manufacturers, but the goal is the same:  prevent you from rolling, sliding, or slipping off of your sleeping pad
  • Stretch panels – incorporating stretch fabrics allow people more freedom to move while maintaining a bag’s thermal efficiency
  • Zipperless construction – this innovation cuts down on weight and snags

Alternatives to Sleeping Bags

A backpacking quilt or blanket is a packable, lightweight insulated layer that you can attach to your sleeping pad in the campground or the backcountry.

These layers are similar to sleeping bags in terms of insulation and temperature ratings. However, their single-layer ‘backless’ construction means that they generally weigh less than sleeping bags.

Because quilts and blankets do not completely enclose the sleeper, they offer greater freedom to move than sleeping bags. This construction also means that their thermal efficiency is lower and their use in extreme temperatures is limited.

The Takeaway

Our goal was to give you the information that you need to find the perfect sleeping bag for your adventures, whether you’re planning to sleep in a snow cave in the Brooks Range, in the shadow of the Tetons, or with the kids in the backyard.

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Questions? Talk to a gear specialist.