Made to move with you during high-output activities, active insulation jackets—like those in Arc’teryx’s Proton collection—merge warmth and weather resistance with class-leading breathability. For spring 2019, the Proton FL (fast and light) joins the AR (“all round”) and LT ("lightweight") as the lightest and most summer-specific jacket of the bunch. We wore the women’s Proton FL while backpacking in the Grand Canyon and running, climbing, and skate skiing outside of Bend, Oregon, and were surprised by how much we were able to accomplish in the jacket without wanting to take it off. Below we break down the Arc’teryx Proton FL’s warmth, breathability, mobility and comfort, weather protection, weight, fit and sizing, and more. To see how the Proton FL stacks up, see our article on the best synthetic jackets
 

Performance

Warmth

Arc’teryx bills the Proton FL as a summer alpine and rock climbing layer, which is reflected in its lightweight build and synthetic insulation. Stuffed with a thin layer of polyester Octa Loft—known for its breathability and temperature-regulating abilities—the jacket provides that small dose of warmth during activities where a t-shirt or light baselayer just won’t cut it. I now routinely find myself reaching for the Proton FL in situations where I previously paired a baselayer (like the NW Alpine Black Spider Hoody) with a wind jacket (most often the Patagonia Houdini). Compared to this combination, the Proton FL performs much better across the board, with added breathability and freedom of movement in a lightweight package. In short, the Proton FL quickly has shifted from a jacket I didn’t know I needed to one I now can’t live without.Arc'teryx Proton FL (front zip)

Although the Proton FL is surprisingly cozy for its thin and streamlined build, it’s a far cry from the Arc’teryx Atom LT or Patagonia Nano-Air. Even the Coreloft insulation in Arc’teryx’s ultralight Atom SL—a slightly lighter active insulation piece—offers significantly more warmth. As a result, in the Grand Canyon when nighttime temperatures dropped to the low 50s Fahrenheit, I found myself swapping out my Proton FL for a lightweight down jacket. In addition, for winter activities like skiing and ice climbing, I certainly would want more insulation and weather protection (enter the Proton AR). But for active pursuits during summer adventures in the mountains or shoulder seasons closer to home, the Proton FL provides that small but vital boost in warmth.Arc'teryx Proton FL (liner)


Breathability

Breathability is the hallmark of the Arc’teryx Proton FL—it’s a piece that truly is built for activity. Both the insulation and shell fabric are air permeable, which make for a jacket that you can put on and keep on. I’ve worn the Proton FL while climbing technical pitches of rock, hauling a 40-pound backpack through the Grand Canyon, and running the trails near my home, and have found it to be impressively breathable for an insulated layer. Where I would have built up a sweat in jackets like the Atom LT (synthetic midlayer), Patagonia Houdini (wind jacket), or even the similar Outdoor Research Refuge Hybrid, the Proton FL has kept me comfortable and well-ventilated. Simply put, it has become a layer that I don’t shy away from wearing for any sort of activity, no matter how aerobic it may be.Arc'teryx Proton FL (hiking)


Mobility and Comfort

Along with breathability, the Arc'teryx Proton FL excels by offering impressive range movement for high-exertion activities. First, it’s made with 16-percent elastane for a stretchy fit that moves with your body. Second, articulated elbows and gusseted underarms increase mobility and comfort. Given these features, I’m able to wear the Proton FL while climbing demanding 5.12 pitches without it restricting movement or riding up underneath my harness. And as I touch on in the “Fit and Sizing” section below, the Proton FL allows this freedom while still being streamlined, trim-fitting, and lightweight. For comparison, the Arc’teryx Atom SL provides similar mobility, while the Outdoor Research Refuge Hybrid sacrifices a bit of stretch in the core, resulting in a bulky, less free-flowing jacket.Arc'teryx Proton FL review (logo)


Weather Protection

Weather protection is not the Proton FL’s strong suit. When the wind really picks up or the rain starts falling, this is not a jacket that will protect you from the elements. It is neither windproof nor waterproof, but the combination of Fortius Air 20 shell fabric and light insulation means it provides decent protection from light wind. That said, in high winds, we’d much rather be wearing a dedicated wind layer or even a jacket like the Outdoor Research Refuge Hybrid, which has thicker insulation in the core and highly wind-resistant fabric (borrowed from the impressive OR Whirlwind Hoody) in the arms. Further, jackets with a DWR coating on the outer shell—such as the Patagonia Nano-Air Light Hybrid—will perform better in a surprise rain shower. But if you do get caught off-guard in the Proton FL, the thin outer fabric and synthetic fill mean that it will dry out quickly.Arc'teryx Proton FL (snow)


Weight and Packability

The Arc’teryx Proton FL clocks in on my scale at 9.6 ounces for a women’s small (0.1 ounces lighter than the advertised weight), which is very competitive for an active insulation piece. In comparison, the Patagonia Nano-Air Light Hybrid Hoody is 10.2 ounces, the Outdoor Research Refuge Hybrid is 9.5 ounces (10.2 ounces on my scale for the women’s small), and the Atom SL is a superlight 8.3 ounces. Interestingly, my combination of baselayer and wind jacket that I mentioned in the “Warmth” section weighs in at 14.3 ounces, making a strong case for any of these innovative active insulation jackets. Further, the Proton FL is relatively packable but unfortunately does not stuff into its own pocket. However, both the packed size and lack of stuff sack are fairly standard when held up to the competition (only the Refuge Hybrid packs into its pocket). A wind jacket or even a softshell/wind jacket hybrid like the Outdoor Research Whirlwind will stuff noticeably smaller, but you’ll give up the insulating properties of the layers listed above.Arc'teryx Proton FL (packing)


Features: Hood, Hem, Cuffs, and Pockets

The Proton FL is designed with a fairly trimmed-down feature set to save weight and bulk. That said, the jacket still incorporates a hood, adjustable hem, well-built elastic cuffs, and four generously sized zip pockets. The hood is uninsulated and fitted with a simple one-pull adjustment on the back, meaning it can be worn under a helmet for next-to-skin wind protection. I usually wear jackets with hoods that fit over my helmet, but this can restrict movement and vision. It’s a bit of a toss-up: the hood of the FL requires you to remove your helmet to take it on and off, but the under-helmet feel is much more comfortable. Plus, only the top of the head is uninsulated, so you still get the full warmth of the jacket along the neck. All told, I like the unique design, but think my hood will stay off (when I’m wearing a helmet) for all but the windiest of conditions.Arc'teryx Proton FL (cuff)

Aside from the hood, the Proton FL’s quick, one-pull hem adjustments on each hip allow you to tighten the jacket so it doesn’t budge underneath a climbing harness or backpack hipbelt. Unlike the hem, the cuffs are not adjustable, instead featuring elastic that’s stretchy enough for easy on and off but snug enough to stay in place. I find that these cuffs keep wind from entering at the wrist and the sleeves from falling over the hands, and they’re so low maintenance that I can adjust the sleeves while perched on small foot holds mid-way through a rock climb. Finally, each of the four pockets have smooth-moving zippers, and while the handwarmer pockets are lined with insulation on both sides, the chest pockets have a stretch-woven outer fabric for added durability. In my opinion, two chest pockets are overkill, but I appreciate the handwarmer pockets that are fairly (although not fully) accessible over my harness.Arc'teryx Proton FL (pocket)


Build Quality and Durability

Arc’teryx products are known for their high-quality design and construction that stand the test of time, and the new Proton FL lives up to the billing. The jacket has a sleek fit and finish, a noticeable attention to detail, and features premium fabrics and zippers. The 20-denier face fabric is thin, but the addition of 16-percent elastane allows the jacket to stretch and give rather than abrade when it comes into contact with sharp objects. After a month of wearing the Proton FL almost every day, there are no holes or tears. However, I have noticed that my turquoise-colored FL (the “Dark Illucinate”) has gathered a fair amount of dirt along the cuffs, hem, and front zipper (especially where my harness rides), and the fabric seems to stain easily. Thankfully, the jacket is easy to launder—I simply wash and dry it with the rest of my clothing.Arc'teryx Proton FL (front)


Fit and Sizing

Made with a trim fit for athletic performance, the Proton FL lies close to the body and falls hip-length. The women’s jacket in particular has a great shape that follows my torso’s contours, avoiding that “boxy” feel more characteristic of the Outdoor Research Refuge Hybrid. I am a standard size small in most clothing and the small Proton FL fits me perfectly, layering naturally over both a thick baselayer or a tank top. Plus, while the FL is meant to be worn as an outer layer, it is trim enough to layer underneath a shell or even a heavier midlayerArc'teryx Proton FL (hem)


Other Versions of the Proton

The Proton FL is the newest member of the Proton family, joining the AR (“all round”) and LT ("lightweight") in Arc’teryx’s lineup of active insulation. The AR ($349; 15 ounces) only comes in a men’s hooded jacket but is the warmest of the bunch, with two weights (65 and 90g/m2) of Coreloft Continuous insulation patterned throughout. For $40 more than the FL, the hooded Proton LT is about 3 ounces heavier (12.7 ounces for the women’s jacket) and also features warmer and bulkier Coreloft Continuous 65. Further, both the AR and LT use 40-denier shell fabric (rather than the 20D of the FL), which offers a bump in durability and weather resistance. All three are made for high-output activities, but the warmer and more weather-protective versions do sacrifice some breathability. In the end, we recommend the AR for wintertime use, the LT for shoulder-season activities, and the FL for summer in the mountains or mild spring and fall days.Arc'teryx Proton FL (dinner)


What We Like

  • The Proton FL excels both in breathability and mobility, making it a fantastic jacket for high-output activities like climbing, hiking, running, and cross-country skiing.
  • Surprising warmth given the jacket’s low weight, packability, and streamlined build.
  • Stretchy face fabric provides a nice boost in durability and is less likely to tear. 
     

What We Don’t

  • With thin, air-permeable fabric and no DWR finish, the FL is less wind- and water-resistant than much of the synthetic jacket competition.
  • We like the FL so much for climbing that we wish it came with the option of stuffing into its own pocket.
  • The face fabric on our turquoise model seems to gather dirt and stains rather easily.

Arc'teryx Proton FL (hiking 2)


Comparison Table

Jacket Price Weight Insulation Fabric Packable
Arc'teryx Proton FL Hoody $259 9.7 oz. Octa Loft  20-denier No
Outdoor Research Refuge Hybrid $159 9.5 oz. Vertical X (60g) 75-denier Yes
Patagonia Nano-Air Light Hybrid $249 10.2 oz. FullRange (40g) 20-denier No
Arc'teryx Proton LT Hoody $299 12.7 oz. Coreloft Continuous (65g) 40-denier No
Arc'teryx Atom SL Hoody $229 8.3 oz. Coreloft Compact (40g) 20-denier No


The Competition

We love the Proton FL for its great fit and finish, impressive breathability, and lightweight warmth, but there’s no shortage of competition for this new active insulator from Arc’teryx. Outdoor Research’s Refuge Hybrid is another intriguing option that comes in at a similar weight (9.5 ounces for the OR vs. 9.7 for the Arc’teryx). However, while the FL has a consistent layer of insulation throughout, the Refuge Hybrid stuffs all its insulation (60 g/m2) into the torso and uses thicker (75-denier) and more wind-resistant fabric in the uninsulated arms and hood. This OR definitely keep you warmer on cold days, but we took the Refuge Hybrid on a winter run and quickly overheated. And in terms of fit, the OR is much boxier and its bulky core results in a bit less freedom of movement. But for only $159 (a $100 price drop from the Arc’teryx), the OR makes a great budget choice and is a solid layer for days on the skin track.Arc'teryx Proton FL (lacing)

Patagonia threw their hat into the active insulation ring with the Nano-Air Light Hybrid Hoody, which retails for a comparable $249 ($10 less than the Proton FL). The Nano-Air combines lightweight 40-gram FullRange insulation with a waffle-knit fabric reminiscent of their popular R1 baselayer. Similar to the Refuge Hybrid, the 10.2-ounce Patagonia features insulation exclusively on the front of the core, arms, and hood, with the breathable and stretchy knit fabric across the entire back. That said, we found the jacket to be overkill for high-exertion activities in temperatures above freezing. Further, while the front panel’s DWR coating does a great job of repelling water, the stretch knit fabric soaks through in a hurry. In the end, we like the Nano-Air Light Hybrid for high-output winter adventures, but the Proton FL is a better breather in mild temperatures.

Finally, some of the biggest competition comes from within Arc’teryx’s own lineup. As we mentioned above, the heavier and more expensive Proton LT and AR offer more warmth and weather protection than the FL, making them better-suited for shoulder-season and winter activities, respectively. In addition, the extremely light (8.3-ounce) Atom SL is an interesting alternative for summer adventuring. Similar to the Patagonia and Outdoor Research above, it features insulation (40 g/m2) only in the core, with thin fabric over the arms and a stretchy fleece along the sides. Overall, it’s a warmer jacket than the Proton FL and offers more water and wind resistance (with the exception of the side panels). That said, breathability is less of a strong suit and you’re likely to overheat in the SL, making the Proton FL a better choice for high-output activities.

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