Written by Amelia Steele, Shackleton 

Down is the first choice for Shackleton designers when it comes to insulation due to its unrivalled air trapping performance and packability.

Insulation layers usually use two types of insulation materials: down which comes from ducks or geese, and synthetic man-made fibres in the form of polyester.

Both down and synthetic insulation function as insulating materials by trapping air within their structures, and keeping it stable. 

Shackleton designers have engineered our mid and outer-layers to use primarily RDS certified European goose down and recycled polyester insulation where performance benefits require. In layers such as a Faraday Ultra lightweight Down Gilet a combination of both down and synthetic insulation is used to maximise each individual elements' unique properties and enhance the technical capability of the jacket.

Read on to learn about the benefits of each type of insulation. 

What is Down?

People often refer to down as feathers, however down is in fact, the fluffy clusters which develop underneath the outer feathers of ducks and geese, which keep them warm and buoyant. 

The fibres that make up down are incredibly fine and light. This material is the most efficient means to trap air and hold it in position. This trapped air creates an incredibly efficient thermal barrier. Down's performance is unrivalled by any other insulation natural or synthetic. Individual down clusters or “plumules” (superfine featherlike plumage) expand to fill an area much greater than their size, which is called loft.

Down’s volume is in fact made almost entirely of trapped air, which means apparel made with down is incredibly lightweight and packable. Higher quality down fluffs back up to create more loft after compression, providing more heat-trapping air space within the down fill.

Down Cluster Ratio

The ratio of down to feather found in a piece of clothing constitutes the down cluster ratio. The higher the ratio, the more down the item contains. The more down the item contains, the better the warmth to weight ratio due to the down's lofting capability. For Shackleton down jackets, we use a down cluster ratio of 90:10, which means the minimum down content is 90%.

Fill Power Explained

Fill power is a number that indicates the relative quality of the down. The number derives from a lab test that measures how many cubic inches of loft one ounce of that down fill produces. Higher fill power numbers indicate greater loft and insulating efficiency. For the Rothera, Ross and Ronne jackets we use a fill power of 850, but for our even more lightweight performance gilet, the Faraday we use 750 fill power.

The Down Fill-Power Test

The test is typically done by filling a large cylinder with one ounce of down. A lightly weighted disk is placed on top which slowly sinks to compress the down. After exactly one minute, the position of the disk is recorded and the volume of down compressed at that point is calculated.

A result of 450 cubic inches of down equates to 450-fill-power down. More resilient down compresses less and will loft higher. It also yields a higher test result—900 fill power is about as good as down gets.

Responsible Down Standard

The Responsible Down Standard (RDS) is an independent, voluntary global standard which certifies down against animal welfare requirements and tracks it from the source to the final product. The standard aims to ensure that down and feathers come from animals that have not been subjected to any unnecessary harm. All Shackleton virgin down is certified to the Responsible Down Standard, and you can track where the down in your jacket came from by scanning the QR code on the swing tag.

What is synthetic insulation?

Synthetic insulation is usually created from polyester to form either sheets of wadding or lofted structures inspired by down. In this process different sized filaments are intertwined with each other to form a 3D web-like structure which traps air to create a thermal barrier. This structure is held in place by binder fibres which are heat activated (melted) so that they bond where the fibres cross, which holds them in place.

Synthetic insulation's key benefits include that it is fast drying, and remains effective in damp conditions. It is also durable and lightweight. Synthetic sheet insulation is designed to respond to how it is being used, either insulating or breathing to regulate body temperature across a wide range of conditions.

Down Vs Synthetic

Synthetic fills have no equivalent test to down fill power, and each brand of synthetic insulation has unique characteristics. So, it’s not possible to use down fill power to compare insulation efficiency to any synthetic fill, and there’s no way to compare the efficiency of two different synthetic fills. 

Although both down and synthetic insulation share many of the same properties, there are certain situations where one trumps the other.  Down offers unrivalled warmth for weight and is highly compressible and packable, whereas synthetic insulation copes best with moisture, remains warm when wet and is highly resilient and fast drying making it ideal for damp conditions. 

It's for this reason that the Faraday Gilet uses hydrophobic recycled polyester inside the flat panels across the shoulders and upper back, areas more prone to water ingress, to avoid cold spots. The rest of the gilet's baffles are filled with 750-fill RDS certified goose down, paired with a 100% recycled ripstop shell fabric and DWR water repellent finish, the Faraday performs both as an outer layer over a T-shirt, fleece or sweater or as an excellent insulator under a parka when conditions demand.

Which Shackleton Down Jacket is right for you?


Hercules Ultralight Down Jacket Men

800 Fill power

100% recycled nylon ripstop fabric shell

Micro fleece chin guard

Protective hood design

Optimal baffle construction

Hem adjuster




Rothera Hooded Down Jacket

RDS-certified 850 fill goose down jacket

Resilient, Japanese-made 100% recycled nylon ripstop shell fabric

Performance fit

Windproof and breathable

Packable outerwear that also performs as a versatile mid layer

Sculpted hood construction to snugly protect the head

Storm cuffs and thumb holes


Ronne Down Jacket

850-fill 90/10 RDS-certified European goose down

100% recycled nylon mini ripstop fabric shell

Lightweight and packable

Performance fit with distinctive baffle design

Articulated arms for range of movement

Adaptive Layering System Compatible

Mirco fleece lined collar

Low-tensile webbing on cuffs


Ross Down Gilet

850-fill 90/10 RDS-certified European goose down

100% recycled nylon mini ripstop fabric shell

Lightweight and packable

Performance fit with distinctive baffle design

Windproof and breathable


Escape Expedition Down Jacket

RDS-certified 800 fill goose down jacket

Strategically placed extensive baffles

Garment dyed with Bluesign certified PFC free Bionic Finish ECO DWR

Detachable hood



Haakon Tactical Parka - Navy Edition

850-fill 90/10 RDS-certified European goose down

Fully windproof and breathable

100% recycled, bluesign® certified 3-layer fabric

Waterproof to 10,000mm hydrostatic head (H/H)

Snorkel hood with reinforced visor and detachable faux fur ruff


Endurance Parka

Lightweight with OEKO-TEX® certified shell fabric

Waterproof to 10,000mm hydrostatic head (H/H)

Packable into own stuff sack

RDS-certified 850 fill goose down

Mouldable snorkel hood with reinforced visor and detachable faux fur ruff

Made in Italy



Read how to maximise the lifespan of this incredibly long-lasting and efficient insulator, in this journal written by Head of Product Performance, Gord Betenia. 


Call us on +44 (0) 208 138 0212 or email info@theshackleton.com to talk to a member of our team, who will happily assist you.


Drop into our London store (214 Piccadilly, London, W1J 9HL) to explore the range. All information can be found HERE.