ArcTeryx Proton FL – The Goldilocks Alpine Hybrid Jacket

The Arcteryx Proton FL is equally at home on the skin track as it is on the descent

ArcTeryx Proton FL – The Goldilocks Alpine Hybrid Jacket


Jace Jackson

The modern trend of “hybrid” softshell jackets can be confusing and tough to navigate, and often seems like a marketing tactic to get you to buy a jacket that you don’t really need. I’ve always steered away from hybrid softshells, as they seem to try and do too much and come up short in many different ways – Jackets of all trades, masters of none.

The Arcteryx Proton FL is equally at home on the skin track as it is on the descent

The Arcteryx Proton FL is equally at home on the skin track as it is on the descent

That is, until I encountered the Arcteryx Proton FL ($259). In typical Arc’teryx fashion, they answered the call and knocked it out of the park, creating what I believe is the perfect “put on and leave on” jacket for 90% of outdoor pursuits in a place like Colorado.

I hesitate to give it a label – terms like “hybrid softshell” and “insulated midlayer” put the Proton FL in a box and don’t do it justice.  It has the breathability of a micro-grid fleece, the warmth of an Atom LT, and the weather protection of a light softshell – all in a package that seems to defy the laws of physics with its light weight.

Initial Impressions: 

To be honest, I was underwhelmed when I first received the Proton FL. I had read so much hype about this piece, and it came strongly recommended from several reputable folks in the outdoor community, that my expectations were set pretty high.

When I pulled it out of the box, it seemed to be no different than other lightweight synthetically insulated jackets— If not a little thinner, lighter, and more fragile feeling. I was also a bit put off by how unusually wrinkly the face fabric was, even after several hours outside the box.

In hindsight, this seems to be a common initial impression of this piece. However, the consensus is that you will fall in love with this jacket after its first use… That was certainly the case with me.

First use with the ArcTeryx Proton FL. The wrinkles smoothed out after a couple uses. For contrast, Taylor is wearing her Atom LT (right)

First use with the ArcTeryx Proton FL. The wrinkles smoothed out after a couple uses. For contrast, Taylor is wearing her Atom LT (right)

Overall, the fit and finish of this piece is on par with the level of quality and attention to detail I’ve come to expect from Arc’teryx. This jacket is what they consider a “trim” fit, so those between sizes might consider sizing up.

For me, I ordered the same size I normally wear in Arc’teryx and I was pleasantly surprised by the more tailored fit. My wife sized up for her Proton FL to allow for more room in the chest and for layering underneath.

Arc’teryx has become known for having ample room in the shoulders and plenty of length in the sleeves, and this jacket is no exception (in fact, it has even more room in the shoulders than the jackets in the Atom lineup).

ArcTeryx Proton FL – The Goldilocks Alpine Hybrid Jacket 1Proton FL Hoody Men’s


The main purpose for this is for climbers, who typically have bigger shoulders and spend a lot of time reaching overhead; However, it is a welcomed fit for this ex-rugby player as well.

The liner is a highly breathable low-pile fleece that is insanely comfortable, even against bare skin. Despite having a fleece-like texture (which is typically a nightmare for layering), it still layers well with something like a long-sleeved Merino base layer underneath.  

This jacket is equally at home on a windy peak while working up a sweat in the middle of winter, as it is walking the dog… I have used it for both, and everything in between. It’s comfy like your favorite cotton hoody when chilling around the house, but it is more than capable of keeping you protected on a windy, exposed ridgeline when things get sporty.

Put on and leave on -- perfect for low angle pow laps!

Put on and leave on — perfect for low angle pow laps!

I have long sought after a jacket that I could put on and leave on when splitboarding, and the Proton FL fits the bill perfectly. It strikes an incredible balance between breathability on the way up (especially in temps below 32° F), and weather/wind protection on the way down. Don’t tell my mom, but it’s usually the only jacket I bring with me on fast-and-light tours anymore. 

When you do need to throw on a hardshell, this makes a fantastic midlayer. The uninsulated hood makes it great for layering under all types of shells – even those with a Drop Hood like the popular Beta AR.

It will do everything that your fleece, softshell, and lightweight puffy do – only better.

Weather Protection:
As with other hybrid jackets and softshells, the Proton FL is not waterproof. However, it does have a robust DWR treatment that has yet to show signs of wearing off after 6 months of hard wear and washing after nearly every use (Note: it’s best to wash this piece with something like Nikwax Tech Wash to preserve the breathability and DWR of the face fabric).

The DWR treatment in action

The DWR treatment in action

This jacket sheds precipitation extremely well, but even when things get crazy and the jacket wets out, the synthetic Octaloft insulation will retain its loft and continue to keep you warm. I am also surprised how quickly the Proton FL dries out. Even when the DWR is not enough to fend off the deluge, you’ll be dry again in a matter of minutes.

Despite the mind-blowing breathability, the Fortius Air 20 face fabric is surprisingly wind-resistant (comparable to that of Arc’teryx’s superlight softshell, the Gamma SL). 

Not waterproof, but mighty water-resistant

Not waterproof, but mighty water-resistant

Every time I don the Proton FL, I am continually shocked by how breathable it is. I run super hot, yet I rarely find myself taking it off, even during high output activities in cool weather (like splitboarding or even trail running).

At a first glance, the Proton FL looks a lot like other jackets in this category (i.e. the Atom LT), but they could not be more different inside, which is why the breathability of the Proton FL an entirely different league. It all comes down to how the insulation is constructed…

In the simplest terms, Coreloft is constructed basically like a bunch of strings woven and packed together; On the other hand, The Proton FL’s Octaloft insulation is composed of millions of individual pieces (think microscopic cotton balls), which allows body heat to pass through nearly unrestricted. The breathability of this jacket is comparable to a good microgrid fleece like a Patagonia R1 or a Melanzana Hoodie – maybe even a little better than the Melly.  

The liner is as cozy as it is breathable.

The liner is as cozy as it is breathable.


Arc’teryx’s proprietary Octaloft insulation used in the Proton FL provides more than just great breathability – it is proven to be more resistant to packing out than the popular Coreloft insulation used in the Atom lineup.

Jackets like this are often worn in the car, underneath a heavy pack, or shoved inside the pack when not in use – none of which are good for the life of your insulation. However, Octaloft is claimed to have a much longer loft life. Anecdotally, I have found this to be true… I’ve worn the Proton FL hard for the last 6 months and still looks brand new, whereas my Atom LT started to lose its loft after the same period of time. 

The Fortius Air 20 fabric is wonderfully durable for its weight. It is much more abrasion resistant than other jackets in this category. At first I babied this jacket, but I’ve grown comfortable wearing this as my only layer while splitboarding. I’ve had brushes with trees, frozen snow, and rock, and the jacket is no worse for it. 

ArcTeryx Proton FL – The Goldilocks Alpine Hybrid Jacket 2Arc’teryx Proton FL Insulated Hoodie – Men’s | REI Co-op


Room for Improvement: 

I typically avoid light colors because of their propensity to show sweat when I’m really working hard. The “Proteus” color of this jacket is no exception. For a piece that is advertised for high output activities in the cold, it would be nice to see some more sweat-friendly colors (which it looks like Arc’teryx has updated in the 2021 lineup).

On the bright side, this jacket dries so quickly that you’re back to looking normal in a matter of minutes, rather than looking like a wrestler during a weight cut while you have Après beers with your buddies.

Other than that, any changes to this piece would be messing with perfection.


The Proton FL has an almost cult following among die-hard fans of the Dead Bird – and for a good reason. The Atom LT is most people’s introduction to Arc’teryx, and the Proton FL is where you are likely to become a die-hard too.

Over the years, I’ve acquired nearly every piece in the Atom lineup (SL, LT, AR, and Vest), and I believe the Proton FL is the MVP of the Arc’teryx insulation lineup. I find myself reaching for the Proton FL most often, and my Atom LT and Patagonia R1 are seeing much less use as a result.

Arc’teryx uses the word “microclimate” when describing the Proton FL, and I think that hits the nail on the head – Never too hot, rarely too cold, just put it on and forget it’s there while you enjoy the comfort of your personal microclimate. A true Goldilocks alpine jacket, the Proton FL is “just right.” 

Jace Jackson

Jace is a third-generation Coloradan, raised in the foothills of Evergreen. He fell in love with the outdoors at a young age, summiting his first 14er at 8 years old. He studied business at the University of Denver, where he was an All-American Rugby player.

He lives to play in the mountains with his wife, Taylor, and their super-mutt Samson. You can find them (usually together) playing on Colorado’s high peaks… Backpacking, mountaineering, snowboarding (split and solid), trail running, and mountain biking — these are a few of their favorite things.

He has an affinity for high-quality gear, but he’s not afraid to use it and is notorious for testing his equipment to its fullest potential.

A lover of Type 2 fun, Jace is always up for a good sufferfest as long as there is a cold beer or two after.

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