Duck, duck, goose, synthetic… Rachel has been spinning around in circles trying to figure out what is up, so to speak, with down. If you’ve never put much thought into the stuffing inside your winter jacket, read on.

I’ll admit it, I came into this review thinking any sort of ‘synthetic’ insulation was just a cheaper, more plastic-y, worse version of down. But when I realised the folks at Arc’teryx were filling their top of the line jackets with synthetic filling, I began to realise there must be a reason.

I’ve been a down snob for some time and have worn various brands of down jackets – Kathmandu, Macpac, Patagonia, Uniqlo — on a range of adventures, from multi-day camping trips to hikes in Nepal, and inside ski shells in the Australian backcountry. However, I’ve also reached for them just as frequently before walking the dog or on my city commute.

Despite this much down in my life, I have never owned a synthetic-filled outdoor jacket.

So when I embarked on this review – to compare the women’s Arc’teryx Cerium Hoody (down filling) to the Atom Hoody (synthetic filling) – I thought the Cerium was in for a sure win. But while testing the jackets over the winter season, I’ve found myself appreciating the seemingly slight but important differences between the two jackets.

And I’m here to break it down for you – pun very much intended.

What’s the big deal about down jackets?

It’s worth mentioning that ‘down jacket’ has become the catch-all word for any kind of, invariably black, puffer jacket these days (which sometimes includes synthetic filling) so for the sake of consistency, I’m going to stick with ‘down’ in this article.

Although it’s been unseasonably warm here in Sydney, you can’t leave the house without spotting a down jacket on the streets. They’re widely adored by all adventurers, frappuccino dog walkers, and puffer-vest-wearing-investment-bankers, and for good reason – down is warm, lightweight, and compact. In fact, the Cerium Hoody compacts neatly into its 1 litre sized bag, attached to the inside pocket of the jacket – it’s the size of a Nalgene!

Want more down, lower down? Best Down Booties In Australia 2023


Down Versus Synthetic Filling —Arc'teryx Cerium vs Arc’teryx Atom Jackets, Rachel Holland, two jackets, gear, compact jacket

Wearing the Atom, doing the very natural ‘how great is this!’ product dangle of the Cerium. It’s way smaller than expected!

Duck, duck, goose – which one is better?

‘Down insulation’ is defined as goose or duck plumage (the soft stuff underneath the feathers – not the feathers themselves). While both goose and duck down provide excellent warmth, goose down is considered superior due to its larger and stronger down clusters, which make them more resistant to compression. If you leave duck down jackets compressed they can lose 100 fill power points or more.

Fill power is the warmth-to-weight ratio, it’s a measure of a filling’s efficiency and is commonly measured by a number, usually ranging between 350 – 900. The larger the fill power number, the warmer, and generally more expensive, the down. It’s a common mistake to think a higher number will correlate with a thicker, bulkier jacket, when in actual fact it just reflects better quality insulation.

The Arc’teryx Cerium jacket contains goose down and has a fill power of 850, taking it to the top of the market, but not without the hefty price tag of $580.

Water off a duck and/or gooses back then?

Like any high-performance gear, down doesn’t come without its weaknesses. The phrase water off a duck’s back doesn’t apply to down (remember, down is made from the stuff underneath the waterproof feathers), meaning that when down gets wet, it loses insulation.

This can be a problem for outdoor sports, especially multi-day adventures when the forecast can’t guarantee it’ll be 100% dry (read: skiing in Australia on a ‘snow day’). And even the premium quality of the Arc’teryx Cerium jacket won’t keep you dry. Although it’s worth noting that the Cerium subs out down for synthetic insulation in areas of the jacket that are prone to moisture (e.g. the cuffs). Some brands will treat the down itself to make it more water resistant, but this can have performance and environmental drawbacks.

Read more: How To Wash Your Down Jacket


Down Versus Synthetic Filling —Arc'teryx Cerium vs Arc’teryx Atom Jackets, Rachel Holland, two jackets, gear, compact jacket, Cerium, cuffs

Deliberate design right down to the cuffs – expected for the price bracket but good to know

So, what are the alternatives to down?

Enter… synthetic insulation.

Synthetic insulation was designed to directly address the aquaphobia of down insulation and outdoor companies have a number of patented synthetic insulation technologies. Some examples include PrimaLoft, Thinsulate and The North Face’s ThermoBall. Arc’teryx’ contribution to the mix is its patented Coreloft technology, made from 100% recycled polyester.


Toasty warm in the synthetic Atom Jacket

Atom vs Cerium

While both jackets have an eco-friendly Durable Water Repellent (DWR) finish to help water shed off, the Atom’s Coreloft insulation means better water resistance and permeability (a fancy word for breathability) than its Cerium cousin. Down jackets like the Cerium have pretty poor breathability, so they’re only good for low intensity activities.

The Atom also contains stretch side panels for easy range of movement, and a durable outer lining.

All this for an impressive 310g, only 20g more than the Cerium’s 290g.

The Atom is also $180 cheaper than the Cerium Jacket.

Down Versus Synthetic Filling —Arc'teryx Cerium vs Arc’teryx Atom Jackets, Rachel Holland, two jackets, gear, compact jacket, Atom

It looks pretty slick too


While the weight of the jackets is comparable, the Cerium jacket can be stuffed into its own internal pouch at roughly half the size of the Atom, making it more convenient when shoving it into a backpack.

I found both jackets exceptionally warm. I was fine wearing a t-shirt underneath both at 10-15 degrees. Unsurprisingly, the Cerium was toastier and while I wasn’t able to test it in freezing temperatures (thanks to a ruptured ACL meaning no southern hemi ski season for me), I’m confident its 850 fill power would keep me toasty in sub-zero temperatures.


Although the Cerium contains goose plumage (so it’s not considered a vegan product), its down complies with the Responsible Down Standard, ensuring animal welfare is respected and ‘provides certification at each step of the supply chain from hatchling to final product.’

On the other hand, the Atom jacket is 100% vegan and is made from recycled materials.

Whatever jacket you pick, you can be comfortable that sustainability is a core tenet of the Arc’teryx design process and that both jackets are made to last.


Down Versus Synthetic Filling —Arc'teryx Cerium vs Arc’teryx Atom Jackets, Rachel Holland, two jackets, gear,

I think I’m officially down with synthetic

So, what am I reaching for and when?

I’ll be reaching for my Cerium Hoody (down):

  • As a midlayer while skiing in cold climates like Japan or Canada
  • To stuff into my pack for overnight hikes as an extra layer around the campsite
  • As a casual winter jacket on cold, dry days
  • To check the surf on frosty winter mornings

I’ll be reaching for my Atom Hoody (synthetic):

  • To wear on hikes, bushwalks and ski touring (thanks to its permeability)
  • As a midlayer while skiing in Australia or on warmer days overseas
  • As a casual jacket when the rain clouds are unpredictable


Down Versus Synthetic Filling —Arc'teryx Cerium vs Arc’teryx Atom Jackets, Rachel Holland, two jackets, gear, compact jacket, Atom

Our reviewer was given this product for testing and was allowed to keep it afterwards – they could say whatever the heck they wanted in the review. Check out our Editorial Standards for more info on our approach to gear reviews.

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Photos by @mattwisemanmedia