fill types - down vs. synthetic

Mobile sleeping systems are fairly simple mechanically. The user is the warmth generator, and the bedding is there to retain as much heat as possible. The insulation in the sleeping bag creates tiny air pockets that slow your body heat from escaping.  In its most basic form, the more loft you have surrounding you, the more of your own heat you retain, and the warmer you stay throughout the night.

There are also differences in bag type or design that can play a huge roll in warmth as well. Mummy bags contour your body, and leave less dead air space, so they are more thermally efficient. They also do a great job of locking out drafts. While rectangular bags and quilts might not be as efficient, they have the added benefit of a less restrictive shape. You can read about other factors that lead to warmth and value in our blog post about Price | Temperature Range | Weight | Fill Type.

Down and Synthetic Fill are the most common fill types that are seen in mobile sleeping systems today. There is an overview of each as well as some advantages and disadvantages. Down is a bit more complex and controversial, so there is more information on how down is classified and valued. 

Synthetic Fill

  1. Synthetic fibers come in a few different forms, mainly nylon and rayon. Various raw chemicals are mixed together and spun to the specifications needed. There are a few of different brands of synthetic, but the essential properties are nearly the same.
  2. Synthetic filling is ideal for people who often encounter wet and rainy conditions. It’s also great for those who may not camp as often and need an inexpensive solution for mobile bedding. Applications include: car camping, recreational vehicles, festivals, budget campers, and families with children. 
    1. ADVANTAGES: Synthetic is almost always less expensive than down. It is more water resistant, so it will retain it's loft when wet. This is great for campers who often face damp conditions. The material is also very durable and low maintenance.
    2. DISADVANTAGES: Synthetic is significantly more bulky and heavy than down. This is not an issue if you typically camp from a car and have extra space. It becomes an issue when backpacking because you are adding extra weight and removing space from your pack. Lastly, there are few options to meet user specifications. 


  1. Down is a byproduct of the meat industry. It is the natural fibers that typically come from a goose or duck. It is stripped from the deceased animal, cleaned, and sorted by type and size. There are three main types of fiber: down clusters, fibers, and feathers. The clusters are most valued because of their natural ability to expand, compress and effectively trap air and keep warmth. Down is measured and classified by a few different standards.
    1. Fill power
      1. Fill power is a measurement of volume. 60 grams of down are measured out, and set aside at a very specific temperature and humidity. It is then placed in a graduated cylinder with a weight on top and let to rest. The resulting measurement is fill power.
        1. The higher the fill power number, the more volume the down takes up. Basically, the more it will retain heat with less weight. Fill powers typically range from 550 to 950.
        2. NOTE: The higher fill power means you can use less down (in weight) to achieve the same amount of loft or warmth. For example a bag with 20 ounces of 850 down could be just as warm as a bag with 26 ounces of 650 down. We discuss the merit of weight in our post about Ultra-Light vs. Lightweight (COMING SOON).
    2. Percent Down
      1. Percent down is the ratio of how much down clusters there are compared to other fibers and feathers. Percent down is a less common measurement, but can be a quick indicator of quality. The higher the percentage, the more clusters are present, and the better the down is as an insulator. Percent down typically ranges from 50% to 95%.
    3. Species
      1. Goose Down
        1. Goose down is commonly referred to as the leading standard in quality down. As the down comes from a larger bird, it has the largest clusters and can reach elite fill powers. Goose down can reach up to 1100 fill power. It is typically more expensive than duck down.
      2. Duck Down
        1. Duck down has much the same properties as goose, but it typically has smaller clusters. Duck down can reach fill powers up to 850 fill power. It is typically less expensive than goose down.
      3. Duck vs. Goose
        1. When the fill power is equal there is no difference in warmth between duck and goose down.
        2. Duck down is typically less expensive than goose down. The price is based on animal growth rates and industry demand. Goose down can achieve higher fill powers than duck, so in ultra-elite sleeping bags goose down of fill power 900 to 1100 is used.
        3. The industry typically leans toward a preference of goose down as top quality, however there is little scientific evidence to support such claims. There are also arguments that favor goose down because the cluster size is larger, and therefore tend to break down slower over time. Again, there is little scientific evidence to support this claim.
    4. Water resistance
      1. Water resistant down is treated with chemicals that allow the down to become more hydrophobic. The typical measurements often include placing the down in small jars with water and shaking them up and measuring how long it takes for the down to sink into the water. The water resistant down excels in this scenario, but these measurements are not entirely indicative of sleeping bag fill performance. However, there is subjective evidence to support the claim that water resistant down is better at retaining loft in damp real world conditions. Ultimately, there is an inherent weakness in down towards water. Water resistant down is better in damp conditions, but the reality is that all down will fail with enough water saturation.
  2. Down filling is ideal for people who typically camp often as it has the widest range of applications. Down's ability to compress, expand, and retain warmth with a relatively light weight is ideal for mobility. Applications include: car and motorcycle camping, through hiking, backpacking, and adventure traveling. 
    1. ADVANTAGES: Down offers the best warmth to weight ratio available. It is extremely compressible, and for the weight it provides an unbeatable amount of warmth. When backpacking, down provides the most comfort available because it is lightweight on your back during the day, but warm at night. It also has a big range of different specifications to customize. Down has a range of options that can fit into a wide range of budgets, temperatures, and weight requirements. 
    2. DISADVANTAGES: Down is significantly more expensive than synthetic fill. It is naturally vulnerable to moisture. It also requires more maintenance. Down must be stored loosely in a dry location, or it will lose it's natural ability to regain loft. Lastly, even with the best down there still could be a small amount natural odor. 

As with any purchasing decision, it is a very personal choice. At NoZipp, we use only the highest quality down filling at ultra competitive prices. We think our sleeping bag is the bee's knees. It comes with a great warranty and we'll be sure to deliver you an great product. We realize that our bag isn't for everyone, so if you'd like to compare between other brands take a look at our sleeping bag comparison matrix. It compares over 250 different mummy bags for your convenience. 

Thanks for reading!