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All about DOWN
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All about DOWN

There’s a lot of confusion on the topic of down insulation. Here we attempt to offer some clarification.

An average outdoor store will have a significant number of down products: sleeping bags, jackets, and vests, even down booties for cold-weather hut trips. But –  you can’t see any of it: the down itself is packed away out of sight inside the (hopefully downproof) fabric of the product.

Mysterious numbers are embroidered on that fabric, and terms like ‘hydrophobic’ and ‘RDS’ abound. So – what do all those confusing terms mean? Well, we’ve broken those words and phrases down for you in this blog post. And what about all those numbers? We'll explain those in this blog post, too - and as far as where the numbers come from in the first place, as always, Sea to Summit goes out of its way to provide you, the consumer, with accurate specifications and independent verification when possible:

  • A detailed report from the International Down & Feather Laboratory (IDFL) is supplied with every Sea to Summit bag certifying the fill power and composition of the down. You can see exactly what you’re paying for inside the colorful shell.
  • All Sea to Summit sleeping bags use down that is RDS-compliant (Responsible Down Sourcing). See the paragraph under ‘Responsible Down Standard’.


Definition & Quality

Down is the natural insulation of waterfowl, comprised of clusters and down feathers. Thousands of tiny fibers radiate from the core of the cluster; this traps air and thus heat.

No other material comes close to the insulation value, longevity and “breathability” of quality (mature) down.

Mature Down

Mature down comes from older birds – the greater cost of raising birds over a longer lifespan means that mature down is more expensive. It has several advantages: the larger, denser clusters trap more air and are more resilient to compression than smaller and more fragile immature clusters. Mature clusters have barbs on their filaments which catch (or ‘cling’) to other clusters to make a more even insulating mass. Immature down is much more likely to ‘clump’ and leaves gaps in the down chamber which do not insulate. The maturity of the down is more important than the species (goose or duck) it comes from. The image below shows down clusters on the left and a down feather on the right.


(Down Clusters on the Left and Down Feather on the right)


Down Cluster to Feather Ratio

The ratio of clusters to down feathers is expressed as a percentage (i.e., 80/20 or 90/10). Broadly speaking, the higher the percentage of clusters, the greater the loft.

Fill Power

The more air trapped by the down, the better the insulation. The loft is expressed as Fill Power. Essentially, a one-ounce sample of down is compressed and then allowed to expand in a special cylinder. The total volume it expands to (measured in cubic inches) is described as its fill power (1 oz of 750 Fill-Power down would, therefore, fill at least 750 cubic inches). However, a higher fill power number does not necessarily mean a warmer sleeping bag or jacket – you can achieve the same warmth with a lower fill-power down, but it would require a higher fill-weight to do so.

ULTRA-Dry 750+ Loft logo _GREY1

Fill Weight

This simply means the weight of down inside the product.
A hangtag attached to each Sea to Summit sleeping bag details fill weight (so you know how much down is in the sleeping bag). The hangtag also describes fabrics and specifies dimensions  Hang Tag(to take the guesswork out of ‘will it fit?’)

Water Repellency

A number of hydrophobic/water repellent treatments are available for down. Sea to Summit sleeping bags use Ultra-Dry Down, an extremely effective, durable, PFC-free, treatment. It retains 60% more loft, absorbs 30% less moisture & dries 60% faster than untreated down.


ULTRA-DRY Down logo


Responsible Down Sourcing

All Sea to Summit down sleeping bags are filled with RDS-certified down. The Responsible Down Standard (RDS) ensures humane treatment of ducks and geese used in down production. Audits are conducted at every step of the chain from sourcing to finished item to guarantee animal welfare and correct labeling of products.


RDS logo

IDFL Certificate

As mentioned above, it’s impossible for the end-user to see the quality of the down inside a sleeping bag or jacket, because very few people possess X-Ray vision. For those of us without the ability to see through fabrics, a detailed analysis of the composition and fill power of the down used in each Sea to Summit sleeping bag is provided by the International Down and Feather Laboratory. Sea to Summit pioneered the concept of providing an analysis of down quality to the consumer.


Hopefully, the above will take the mystery out of some of those terms surrounding down products (and make clear the effort which Sea to Summit goes to provide clarity to the end user).

Down vs. Synthetic - which is better for specific uses?
Both types of insulation have advantages and disadvantages


Quality down has a very long lifespan. Correctly maintained, a sleeping bag filled with a high-lofting, mature down will last 20+ years.

High fill-power down has the highest warmth-to-weight ratio of any insulation. Less insulation is therefore necessary to achieve a given thermal performance - which means a down sleeping bag or garment can be very light and extremely compact when compressed.

Down is a natural, recyclable, renewable material. It provides a pleasant sleeping environment.

Down is relatively expensive. Taken in context of its lifespan, it costs less than synthetics; but has a higher initial cost.

Wet down will not insulate, and takes a long while to dry. Hydrophobic treatment (such as Ultra-Dry Down™) have reduced this factor, but not removed it entirely.

A very small percentage of end-users are allergic to down (a greater number are actually allergic to dust mites, also present in synthetics).


Synthetic insulation is largely unaffected by moisture. In very wet conditions, an end-user can wear damp clothes in a synthetic bag and dry them (to an extent) overnight.

Synthetic insulation is a lower-cost material than down, and does not need to be filled into complex chambers to achieve a uniform insulating layer. Synthetic sleeping bags are therefore less expensive than down.

A synthetic sleeping bag is much easier to wash than a down bag - without correct care, the down may be subject to shifting between chambers during the wash cycle.

Synthetic insulation will not last nearly as long as down. This is particularly true for generic synthetic insulation (found in lower-cost products) which may have a pseudo 'brand name'.

Synthetic sleeping bags are heavier and much bulkier when packed than down bags - this can be a key point when backpacking.

The sleeping environment in a synthetic insulation sleeping bag may be clammy under warmer conditions. 

If you have questions relating to down, feel free to contact us via the blog.

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