How Down Insulation Works

Down is a remarkable insulator.

Not only do these tiny feathers have an incredible warmth to weight ratio, but they also pack super small and loft back to life again with a few quick shakes.

The range of products made with down are as diverse as duvets to down hats. With so many products advertising the incredible insulating properties of down it can be difficult to ascertain the quality of down products. Here are a few specs consider when purchasing down products.

The Low Down on Down

Down is comprised of the small feathers underneath the protective outer feathers or sometimes on the belly of birds. Although all birds have down, the type of down most often used in commercial outdoor products is goose and duck down.


  • High Warmth-to-Weight Ratio
  • High Compressibility


  • Will not insulate when wet
  • Expensive

Although down has its drawbacks, down’s ultralight and ultra packable properties make it a perennially popular insulator for backpackers.

Goose Down or Duck Down?

The down industry is inextricably linked to the duck and goose meat industry. Feathers harvested for down are simply a byproduct of birds harvested for their meat.

Ducks are a more popular food source in Asia, where ninety percent of feathers used globally are produced. Due to the demand for duck, sixty percent of the global down market is comprised of duck down. The higher demand for duck results in lower prices for duck down. Fewer geese harvested for meat means less goose down and higher prices.

But are there differences in the quality of duck and goose down?

Yes and No.

Down is made up of small feathers called clusters. These clusters insulate by trapping air within filaments that make up these clusters. The larger the cluster, the more air trapped within the down. The more air trapped within these clusters, the warmer the garment. Therefore, larger clusters form higher quality down. 

The largest and highest quality clusters come only from geese, which have more large cluster down than ducks due to their larger size.

But barring the highest quality down, goose and duck down remain largely indistinguishable in size and in quality.

Fill Power

Down quality is measured by fill power. Fill power is the amount of room one ounce of down will occupy in a cylindrical glass container when allowed to loft to its full capacity. If it takes up 650 cubic centimeters of space, it is 650 fill power down. 750, 850 fill and so on.

Besides the technical definition, fill power can also be thought of as the measured loft of down. High quality down has a higher loft than low quality down because of the size of the down clusters.

Higher fill power down has larger down clusters than lower fill power down clusters.  The larger clusters trap air more efficiently with their big fibrous filaments. Higher Fill Power =’s  more warmth with less weight. 

Duck down can be found in down up to 700 fill power, whereas large, fibrous Goose down clusters are used in higher fill power downs of 750 fill and higher.

Fill Weight

Although fill power is a primary measurement of down products, another important specification to consider is fill weight. Fill weight is simply how much down is in the product. Fill weight is usually measured in ounces.

Sure, aspiring ultralighters always check the weight of their items, but how much of that weight is actual down insulation? And how much is fabric, zipper, and bells and whistles weight?

When choosing down products, it is helpful to use a part to whole ratio of overall weight to fill weight to ensure the most down for the buck. Down sleeping bags should have a higher fill weight to fabric weight. Down jackets can have a lower fill weight to overall weight ratio. This is because less construction in needed to produce a sleeping bag then to create a shaped garment.

Hydrophobic or Untreated

Down outdoor products have made vast improvements since Eddie Bauer (yes, that Eddie Bauer) patented the first quilted goose down jacket in 1936. One of the significant drawbacks of down is the loss of insulating properties when wet. When down gets wet, the filaments in the cluster collapse. That means all that lofty goodness is now a sopping wet mass that will do anything but insulate.

Hydrophobic down, however, is the same down clusters we know and love, but treated with a Durable Water Repellent or a (or similar water repellent) finish. Many a backpacker or aspiring gearhead may recognize the letters DWR. It is a fabric finish that is water repellent (not water proof). 

While this new innovation will not allow for day long downpour jaunts, the use of hydrophobic down is extremely effective in reducing down cluster collapse from other forms of moisture.

Down can lose loft due to moisture in the air during high humidity, as well as from sweat that is drawn out from the skin by moisture wicking outdoor apparel.

But perhaps the most impressive aspect of hydrophobic down is increased drying time. One proprietary hydrophobic down brand claims a 33 percent faster drying time than untreated down.

And the best part about hydrophobic down? It won’t add even an ounce to your pack weight.

Ethical Down

In the early late ‘oos and early ’10s, ugly realities surrounding the treatment of birds that source down surfaced. Reports of force feedings and live pluckings inspired several large scale manufacturers to put methods in place for tracing their supply lines and eliminating these disturbing practices.

There are several ways manufacturers trace their down. The North Face helped to develop the Responsible Down Standard , while Patagonia aided in the development of the Global Traceable Down Certification Standard.

Whatever way you chose to trace your down, make sure it’s sustainably harvested.


Hannibal enjoying the sewn through baffles and 750 Fill in the Rab Microlight. Image Courtesy of Michael Pollak.

Hannibal enjoying the sewn through baffling and 750 fill of the Rab Microlight Jacket.  Image Courtesy of Michael Pollak

While source, quality, and quantity of down are all important specifications to consider in choosing down items, there is another essential specification to consider that does not integrate down insulation at all.

Construction of sleeping bags and down jackets is vitally important to the way that down insulates. Construction determines where the down insulation is placed in the item and prevents or creates cold spots.

  • Sewn Through construction is the most basic down construction technique. Shell Fabric and liner fabric are simply sewn together to create pockets or baffles that keep down in place. It is the lightest weight down construction technique because it requires no additional fabric to create these down pockets or baffles. However, sewn through technique can create cold spots where the shell and liner are sewn together. Sewn through technique is used in most down jackets as well as many warmer weather down sleeping bags.
  • Baffled Construction prevents cold spots created by sewn through construction by adding an extra layer of vertical fabric between the shell fabric and liner fabric. In essence, baffled construction creates a series of continuous fabric boxes filled with down. Although this down construction technique prevents cold spots sewn through technique can create, the added fabric needed to create the baffle boxes adds overall weight to the item.
  • Continuous Baffles use baffled construction technique to create continuous tubes of fabric throughout the item. Unlike traditional baffled construction, these tubes allow down to be shifted throughout the continuous baffles. Down can be stuffed and compressed underneath the bag in cooler weather, while down can be shifted to the top and sides of the bag in cooler weather.
  • Other Baffling Techniques include variation on baffled construction such as slanted box baffles, trapezoidal box baffles, and even V-Shaped baffles. There are horizontal and vertical continuous baffles. There are even stretch baffles made with elastic.

Construction Technique is most important for choosing sleeping bags and heavier weight mountaineering down jackets. But the bells and whistles available on down pullovers and jackets are also integral to determining their level of warmth. While construction techniques focus on where down insulates, worn down layers focus on trapping heat within the insulating layer.

  • Hem Drawcords run along the bottom of jackets and hoodies. They prevent heat loss from the hem of the jacket. As with most added construction techniques, the price of added heat loss is added weight.
  • Hoods come in many shapes and versatilities. From shaped hoods that fit the head to elasticized hood that create a seal to drawcord hoods that can be adjusted according to temperature, hoods can add warmth but also add weight.
  • Elasticized Cuffs create a seal preventing heat loss from garment sleeves.
  • Handwarmer Pockets function as much more than handy spots to stash your Snickers. They can function as a toasty home for cold hands when at rest or during stagnant activities such as belaying.

The Low Down on Down

Down is a remarkable insulator.

Innovations in the outdoor industry continue to allow down to insulate to its full potential and even improve its already remarkable qualities.

By understanding the way down quality is measured, how down products are created, and recent improvements in down technologies we can better understand how to utilize down products to their fullest extent in the outdoors.

Lead Image Courtesy of JP…

Affiliate Disclosure

This website contains affiliate links, which means The Trek may receive a percentage of any product or service you purchase using the links in the articles or advertisements. The buyer pays the same price as they would otherwise, and your purchase helps to support The Trek's ongoing goal to serve you quality backpacking advice and information. Thanks for your support!

To learn more, please visit the About This Site page.

Comments 3

  • Bob Rogers : Feb 11th

    Nice article. Now to hell with it all, where are the penguin down jackets?! 😉

  • Max : Mar 20th

    Is it better for a down jacket to fit tighter rather than loosely?

    • Caet Cash : Mar 22nd

      It shouldn’t be huge, but remember you’ll be wearing it in the cold, so make sure to leave enough room for layers underneath!


What Do You Think?