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How to Choose a Sleeping Bag: Fill

A key component to the comfort and warmth of your sleeping bag is the type of insulation it contains, commonly referred to as the “fill." Here we explore the three primary types of insulation used in sleeping bags: synthetic, down, and treated down.

How to Choose a Sleeping Bag: Fill


Synthetic fill is comprised of human-made fibers designed to mimic the structure of down. The strands are spun to create a framework that traps and maintains body heat. The ever-changing field of technology has allowed designers to create synthetic fills with these amazing properties:

  • Water resistance that promotes increased thermal efficiency even in wet or damp conditions

  • Human-made fibers contain no animal products for a cruelty-free and hypoallergenic product

  • Low maintenance nature allows for easy care and machine washing

  • Budget friendly cost

Many manufacturers have created their own proprietary synthetic insulations that are trademarked, so you may see a large variety of brand-named synthetic fills on the market.

Despite amazing advances in production, synthetic fill has a primary drawback in comparison to its down counterpart:

  • Lower warmth to weight ratio results in a heavier sleeping bag

  • Does not compress as compactly as down


Down is a time-tested favorite in sleeping bag insulation because it is both warm and lightweight. The loft created by the feathers traps body heat in a manner that is incredibly efficient.

Some notes on fill power: the power rating is displayed as a number that correlates to loft, or warmth-to-weight ratio. Specifically, the number refers to the volume occupied by one ounce of down.  

  • It would take more 500 fill down to achieve the same temperature rating as 900 fill down

  • Example: a 20 degree sleeping bag with 500 fill will be heavier than a 20 degree sleeping bag with 900 fill down, even though they provide the same warmth

Down does require some additional care, due to its more delicate nature. Some things to consider with a down bag:

  • In wet or damp conditions, down loses some of its insulating properties

  • Can harbor allergens, or even act as an allergen itself

  • Higher price tag than synthetic bags

Treated Down

In an effort to mitigate some of the challenges with down in wet or damp conditions, some manufacturers have introduced water repellent treatments to down. A microscopic, hydrophobic finish is applied to the down, which allows it to shed water and retain warmth, even when damp.

  • “Best of both worlds” combines the lightweight compressibility of down with the water resistance of synthetic insulation

  • Dries faster than down when wet

  • Most expensive of the three fills

General Guidelines

As with all gear, there are many considerations at hand when selecting a sleeping bag. It is always recommended to consider the intended use of your sleeping bag when selecting a fill. What conditions will it be subject to? Is weight or space a key factor? How will you care for the bag? Answering these questions can help guide you to your best bag.