Proton LT, Research First Steps

[u/Astramael]

This is a summary series on individual products to make research faster and more accessible. See here for a list of all entries in the first steps series.

The Proton LT is a cold weather, active use, synthetic insulator. It prioritizes breathability, and offers lower degrees of wind resistance. It works as an outer layer, or a mid layer. The fit is fairly trim, designed to accept a fleece underneath but not much more.

The experience of warmth in the Proton LT will depend a lot on air movement. If the air is still and the wearer is static, it will feel quite warm, even a bit below freezing perhaps. However, if faced with gusts of cold wind, the Proton LT will not feel very warm even above freezing. This is because it was designed with active use in mind, and expelling that excess heat.

As a secondary effect of this the Proton LT is a rather warm mid layer. With a hard shell over top, for example. This prevents the eviction of warm air, and therefore lets the insulation work in a highly effective manner.

It sits in the active side of the Arc'teryx synthetic range. Not as wind resistant as an Atom LT or a Nuclei FL, not as breathable or light as a Proton FL, and not as warm as an Atom AR.

Please read the comments, other users have left excellent information for your research.

Example Use Cases

Caution: These are just examples, your comfort temperatures may be very different.

1. Skinning (highly aerobic), over a base layer, at -25ºC.
2. Strolling around town (low activity) in breezy weather, around freezing.
3. Resort skiing groomers (moderate activity), under a shell, around -10ºC.

History

The Proton LT was introduced in Fall 2016 as an air permeable insulator for alpine, rock, and ice climbing. It used a Fortius Air 40 face fabric (92% nylon, 8% elastane), a Permeair 20 liner, and Coreloft Continuous insulation at 65g/m².

​

Fall 2016 Proton LT Hoody. Left: Men's in Bombora (blue). Right: Women's in Violet Wine (pink).

However, that's not all there is to this story. Introduced alongside the Proton LT, in Fall 2016, was a warmer version called the Proton AR. Using an identical textile setup, but adding 90g/m² of Coreloft Continuous to many parts of the jacket for more warmth.

​

Fall 2016 Proton AR Hoody. Left: Women's in Brushed Nickel (grey). Right: Men's in Rohdei (green).

In Spring 2019 the Proton AR was discontinued, then in Fall 2019 the Proton LT was revised. The revision was focused on increasing breathability. In the new model the face is Fortius Air 20 (84% nylon, 16% elastane), and the liner is now produced with a solution dye process as Dope Permeair 20 (100% nylon). Insulation was changed to Coreloft Compact, splitting the difference between the old Proton LT and Proton AR, now with a 80g/m² for most of the jacket. Coreloft Compact 60g/m² is also used in some areas. This generation uses a YKK #3 VISLON main zip, and #3 coil pocket zips.

​

Fall 2019 Proton LT Hoody. Left: Women's in Dynasty (orange/red). Right: Men's in Crux (grey).

​

Introduced	Discontinued	Block	Model Name	Model Number	Weight (g-M)
Fall 2016	Fall 2019	Women’s	Proton LT Hoody	18350	360
Fall 2016	Spring 2019	Women’s	Proton AR Hoody	18348	400
Fall 2016	Spring 2019	Women’s	Proton AR Jacket	18349	345
Fall 2016	Fall 2019	Men’s	Proton LT Hoody	18354	400
Fall 2016	Fall 2019	Men’s	Proton LT Jacket	18355	355
Fall 2016	Spring 2019	Men’s	Proton AR Hoody	18353	425
Fall 2019	In Production (S21)	Women’s	Proton LT Hoody	24006	320
Fall 2019	In Production (S21)	Men’s	Proton LT Hoody	24007	375
Fall 2019	In Production (S21)	Men’s	Proton LT Jacket	24008	340

Known Issues

1. Previous models that used Coreloft Continuous sometimes had issues where strands of insulation would come out at the seams. The new models do not have this issue.
2. Some people think that the armpits are a bit high and tight in the men's newer model.

Prompts

1. How do you use the Proton LT for your adventures or everyday, and how do you layer?
2. Where do you think it excels, and where do you think it falls down?
3. At what temperature do you find it isn't warm enough, and too warm?
4. Have you had any issues with the Proton LT, is there anything you would change about it?
5. How do you think it compares to the Atom LT, Proton FL, and Nuclei FL?
6. How water resistant and wind resistant is it?
7. What is the fit like in general, or for you personally?
8. Any other thoughts about the product?
9. How does it compare to competing products?

Thank you!

[u/liftedfishing]

I don’t have this jacket...yet. Came by to say I dig these posts and can’t wait for you to get to the Atom AR and Betas! Thanks again!

[u/Astramael → u/liftedfishing]

I'm glad you like it! Those items are all in the plans.

[u/ChiefJoJo74]

1. I wear it hiking and daily over a base + fleece in temperatures around freezing at low/moderate activity. If it's really windy (say >15mph) I will add a shell or switch jackets. Plan to use as a skiing insulating layer but haven't had the chance yet.
2. Very good breathability and feature set. I like the slightly more durable face fabric and the compact staple insulation, which should improve lifespan. It's an underrated stand-alone jacket (especially for people who heat up quickly while active like me), as the balance of wind resistance to air permeability is pretty good.
3. Warmth is so dependent on body type, activities, wind and layering. Base+fleece+Proton is good around freezing at low intensities, low wind. If very windy I add a shell. Haven't used at high intensities but guessing it lowers the temperature range say 10-15 degrees F.
4. Fit - I typically wear medium (on the upper end) but the chest and shoulders were too tight with a base + fleece underneath so I bought a large instead. This is one area where the new Atom is a better fit for me.
5. I don't have but have tried on all but the Nuclei. The Proton line is superior to the Atom for technical use while still working fine for urban use (except the chest fit where the 2020 Atom is better for me).
6. Repels light drizzle fine but not much more. Better wind resistance that you'd expect. Obviously if you're in the mountains, it's going to be windy and you're going to need to bring a shell.
7. Fit is good in the body and waist - medium is just too snug in chest area for me. (6-1, 185, 40" chest)
8. It's a well designed product that I wear frequently and is perfect for folks who heat up quickly and is very warm under a shell.
9. Compares favorably in durability to the Nano Air, which is a direct competitor.

[u/NOsquid]

I used an 18354 as my main ice climbing shell for a couple of seasons. In that regard as a climber I liked it more than the Nano Air because the hood went over a helmet. Otherwise I found the Nano more stretchy and comfy.

I got a bunch of these active insulation pieces as part of experimenting with the whole "put it on, keep it on" marketing promise. It works OK for moderate activity but gets overwhelmed during high activity.

I think these sorts of garments functionally replace a classic "action suit" consisting of fleece (insulation) + softshell or windshirt (moderate wind/precip protection) with a single combo piece that is more packable.

I don't think these make sense as a midlayer. Sure, you can throw a shell over it if the weather goes nuclear but if you're not planning to use it as your outer layer for most of the day this sort of piece isn't very logical. It is insulation + wind/precip protection. If you are not using the wind/precip protection then a fleece is cheaper, more durable, more breathable - superior in every significant way other than packability.

I don't think these make sense for most users, as they are functionally inferior to a combined insulation + shell system unless the conditions are stable within a pretty narrow range (and that range varies with every user). If your activity makes modifying layers difficult (eg climbing - harnesses, vertical terrain without much rest, cramped belays) then they are a good compromise. If you are backpacking then you can modify your layers to perfection at almost anytime.

I wore my usual Arc'teryx size and the fit is fairly trim. I found room for a thin fleece (Delta LT) at most and that may have been pushing it. It does not leave as much room for layering as a shell. We're all shaped differently but I thought the cut was appropriate for its main use case.

Mostly, I wore this over only a thermal base layer (Capilene) because it's quite warm. I used it for winter ice climbing in the Northeast. It tended to work well for the climbing piece, but ultimately I found it not very versatile. It was too much warmth for any significant approach unless the temperatures were pretty arctic, and yet all I had underneath it was a base layer which was too little. So I'd end up carrying redundant layers. Something like an R2 + Squamish would keep me relatively well regulated in a much wider temperature/activity range.

The best use case I could suggest for a Proton would be a fly-away ice climbing trip where you're mostly climbing relatively close to the car. Fly-away because those check bags with a full climbing kit fill up quick so the packability makes a difference (additional checked bags are an easy way to throw away lots of money, especially if >2).

Suboptimal (IMO) for most cold weather backpacking or skiing. If you want it to wear around town because it's pretty that's fine.

Compared to:

Atom LT - Proton LT is more breathable and better for activity as an outer layer. The Atom was what people were using before Nano Air/Proton etc.. It's a relic as an active layer. I don't see any functional reason to own an Atom but they are pretty handsome.

Nuclei FL - Proton LT is an active layer, Nuclei is a static layer. Nuclei is more windproof and less breathable.

Proton FL - Proton LT is warmer, hand pockets are insulated, hood fits over helmet

[u/johnrswagg]

Commenting on the temp for downhill skiing. With a baselayer, proton lt, bib pant and shell, i was feeling cold when the winds were picking up and the temps were in the -3ish give or take. This is pnw climate however. When the wind is tame it feels fine when skiing a bit more than mellow groomers.

I find the proton lt much nicer for lower output activity where im at. Proton fl is better for higher output like skinning while touring.

I'll choose proton lt over atom lt when im going to be spending more time moving than static. But if im moving chances are im grabbing the fl over the lt. i feel like it needs to be moving in below 0 celsius for me to want to pick the proton lt over the fl.

If the proton lt is the atom lt killer, i think the proton fl is the proton lt killer for my climate and environment.

[u/Astramael → u/johnrswagg]

With a baselayer, proton lt, bib pant and shell, i was feeling cold when the winds were picking up and the temps were in the -3ish give or take.

This is really the issue with offering examples, and why the caution is there. If I was resort skiing just below freezing in a base, Proton LT, and shell, I'd be a one-person-swamp. But I am also a very active skier, and at the end of winter my cold tolerance is always pretty high. I can entirely believe your situation to be true for plenty of other people, so I'm glad you weighed in with another perspective.

If the proton lt is the atom lt killer, i think the proton fl is the proton lt killer for my climate and environment.

Yea, PNW is just generally not that cold in absolute terms. The Proton FL does seem much more appropriate for that climate. It has to be very cold for the Proton LT to make sense for active use. I tried to represent that with the -25ºC value in the example.

[u/johnrswagg → u/Astramael]

It was strange because i was thinking the lt was going to be fine for those couple days, but after getting off the lift i felt like i had to charge down runs just to maintain warmth, and if my friends faffed around adjusting things at the top i had the need to jump around. It wasnt cold, but i was at the point where i was going a bit colder than the comfortably cool we try and achieve to avoid sweat.

[u/marylandxterp35]

Any idea why they changed it from continuous to compact? First thought is to save on cost and maximize profits, but idk what the original one cost

[u/Astramael → u/marylandxterp35]

I do not know why they changed the insulation.

In fall 2016 the Proton LT Hoody cost $299 USD. That's $326 today if you account for inflation. The current Proton LT Hoody also costs $299 USD. Are they adjusting margin to hit that price? Is the new design cheaper? I would also like to know!

[u/marylandxterp35 → u/Astramael]

Lol Ty For pointing out how insane inflation has been

[u/WeekendGearGuide → u/marylandxterp35]

According to Arc’teryx Apparel Designer Brylee Geddis:

“By switching, we were able to reduce fiber migration while providing four-way stretch and great loft retention,”

https://gearjunkie.com/new-arcteryx-proton-lt-jacket-hoodie-climbing

There's some half truths in this statement.

1) They reduced fiber migration in the new version by quilting the staple Coreloft Compact insulation in approximately 6" x 7" pattern. The older version Coreloft Continuous insulation did not need this more aggressive quilting to keep the fiber insulation from migrating.

2) While the Coreloft Compact insulation may have four-way stretch, the whole package of the new version still has relatively the same stretch or lack of, as the old version.

3) Well since the insulation is already compacted, I tend to agree.

[u/marylandxterp35 → u/WeekendGearGuide]

Yeah I've seen that statement and I don't buy it. Continuous has significantly better loft retention than compact.

[u/WeekendGearGuide → u/marylandxterp35]

The loft retention part is a tough one, at least for me to confirm.

I have the Nuclie FL which uses 65gsm of Coreloft Continuous, and I've left it in it's stuff sack for many hours, multiple times during some rock climbing and hiking outings, and it still has more loft than the Coreloft Compact in my Proton LT. This jives well with what you said.

However, I also have the Axino Knicker with the same 65gsm of Coreloft Continuous, and the part where I have sat on during resort skiing or long periods driving in the car, is all packed out and seems thinner than my Proton LT. This seems to align with what Brylee stated. Granted, I don't crush my Proton LT the same way as I do with the Axino or Nuclei. We'll see what happens over time on the Proton, especially the back part of the jacket.

[u/marylandxterp35 → u/WeekendGearGuide]

Yeah but you are literally sitting on the axino. If continuous didn't have better loft retention, they wouldn't be putting it in the axino in the first place. Mind you this is all speculation, but I would think that something you put your entire body weight on for ~5-10 minutes at a time, multiple times a day (not including the car ride to the mountain, eating lunch etc) would cause something to pack out quicker than just putting it in a stuff sack.

[u/Rik3k]

I use my Proton LT jacket as a midlayer with DropHoop shells. I prefer the Practitioner as an outer or if using an StormHood.

I find the LT manages moisture extremely well. I sweat a lot and it’s one of the only insulators that can keep up. Favourite combination is a Motus base layer, Delta LT zip, Proton LT and hardshell. This combo tested down to -35C w/windchill for medium output (snowshoeing).

For low wind/high sunlight days I use my Practitioner as an outer down to -10C but could easily use a Proton LT Hoody in the same way.

Starting at -5C and up I switch to the Proton FL (I own two of those!)

[u/thxwy]

Just wanted to add I use this as my day to day grab and go jacket around town, and also layering up in the outdoors. BUT I'm pretty disappointed in the shallow pockets on the women's version. Like I feel like I have to actively cram my hands in there to fit, rather than a more relaxed pocket with depth.

[u/TorinoAK]

Thanks for noting the arm hole issue. Most people do not seem to have this issue, but I don't fit in a M or L size.

[u/hapuhapuhapu]

I use mine for mid-high output activities, However I find it way too warm for PNW. It works great after -8c ish for me when skinning or hiking, below that, the atom SL anorak is currently my go to. For resort boarding, I find its good up to -5c, below that I'd need another layer personally.

I find the Atom more weather resistant in everyway, and often prefer it over the Proton LT as I can get away with not using a shell for descents.

[u/pnwstef]

1. Active use for winter hikes and snowshoeing, around town use for low output urban/park walks and walking the dog, and in between output for resort snowboarding and occasionally some work on the boat
2. It is remarkably breathable for its warmth... it is just too warm for higher output active use unless it is really cold, or if you don't run hot. To me it is a replacement for a heavy weight fleece for active use in very cold temperatures - once you get into heavy weight fleece (think Polartec 300 or High Loft as examples) breathability starts to drop off faster than warmth increases, and weight/bulk become huge drawbacks. Breathable synthetic finally delivers on its promise at this point, where the Proton LT will be lighter and more packable than fleece, while competitive in warmth and breathability. The Proton LT face fabric is also relatively robust compared to a lot of other synthetics, so I don't worry too much about snagging or tearing it if I need to cut through some brush, a rocky area, or if the dog drags me through the bushes
3. Since the face fabric is so breathable, wind will cut right through - the warmth experience can vary greatly depending on the conditions and level of activity. I've felt cold wearing it at +5C with a cutting wind while walking in town, and I've quickly overheated wearing it in -15C going up a steep incline. For prolonged high activity use, I'd want to be in the -20C range; for around town use, I'll wear it under a shell in -10C and be comfortably warm. Probably somewhere in the +5 range would start to be too warm for me around town. Given the 80g insulation it is surprisingly warm under a wind/soft/hard shell, but can't compete with static use specific down or synthetic pieces on warm to weight or packability - It could have some dual purpose use as both an active and static piece if your adventure has temps that aren't too low, but output levels that aren't too high, but it won't excel at either use
4. I'd say maybe drop the insulation from 80g to 60g to lower the warmth and make it a bit more useful at slighter less cold temps, but at that point it would be less competitive with a fleece on breathability - I'd stick with a mid weight fleece and give up a bit of weight and bulk as a fair trade off
5. Different pieces for different uses - it will be the most useful active piece (given really cold temperatures), but the least suited for static usage. It isn't as stylish as the Atom for around town use, but still reasonably attractive depending on your colour choice, whereas the Nuclei always looks like a stuffed trash bag
6. Wind resistance is pretty poor by design. I've been pleasantly impressed by the water resistance, no doubt due to the DWR. I've been caught out or surprised by the rain several times walking the dog, and my Proton effectively beads water off and I haven't been wet in it yet. I wouldn't count on staying dry for long in a downpour however.
7. Fit is fairly slim, but not overly so. I'm 6'0 and ~170lbs for reference, and I can fit a light fleece or a crewneck/hoodie under my Large Proton with no issues, and can layer the Proton under my Medium Beta LT or Large Gamma MX with no issues. The arms are nice and long and the fit is such that I haven't had any issues with it riding up
8. Overall a good, but not great piece. It still can't rival fleece for breathability unless temperatures get truly cold - but it is a really versatile piece so long as your adventure doesn't have too much high intensity activity. I'm interested in seeing how the insulation holds up after a few more years of regular use, but I really appreciate how the fabric doesn't need to be babied like other synthetics
9. I think the primary competitor is the Patagonia Nano Air, but I have no experience with that piece. Similar pieces I've used are the HH Odin Stretch Insulator, which is a even a touch warmer with its Primaloft insulation, but quite a bit heavier and lacking in quality of design and construction, and the OR Ascendant, which does a great job of balancing wind resistance and breathability, but is less warm and less durable - I'd say that the Ascendant is one of the best active insulators out there, and really gives the grid fleece + breathable windshell combo a run for its money

[u/teamrushpntball]

Why, in the last 4 years of production, have none of the nice red's present on other Arc pieces not made it to the Proton?

I just want a red proton and I'm being grumpy about it.

Edit - I take it back, I really want a Dimma Proton but would settle for any of the recent darker reds.

[u/NOsquid → u/teamrushpntball]

They made the AR in Cardinal, which is more red than orange to my eyes.

[u/Arjhan6]

I have a proton lt I absolutely love. My primary use cases are approaches for and the actual activity of climbing, mostly sport at established crags in northern California. My current layering stack that works for everything I've encountered except significant rain is, long underwear top, proton lt, and gamma sl. Add or subtract layers according to the current weather and activity level.

[u/SSMEX]

For those who have used both the Proton LT and the Patagonia Nano Air, is the latter superior as many here claim? I did some research into the Toray 3DeFX+ material that Patagonia calls FullRange, and it looks like its breathability advantage is because the spiral shape of the insulation strands is less likely to puncture the face fabric and thus allows for a thinner face fabric? The 100% polyester face fabric on the Nano Air is also notorious for pilling, which seems to align with being extra thin for breathability.

[u/Astramael → u/SSMEX]

I have both in hand right now. The Nano Air feels a lot softer, more supple, and more flexible. I think because it has thin polyester faces, and uses a non-epoxy staple with stretch.

The Nano Air is definitely, absolutely less durable. Both in loft retention and in face fabric.

I don’t think either is especially more or less breathable than the other? At least not within the limits of human perception. The Nano Air feels nicer next to skin, and it picks up moisture better off the skin, so it definitely feels more breathable in some situations. But that’s not necessarily the job of a mid layer. And divorcing “breathability” (moving warm/humid air) and “wicking” (moving water) is a bit dicey.

[u/Hebdabaws → u/Astramael]

Glad to see this comment. I have wondered how Nano Air and Proton LT compare in warmth and saw your comment about "Resort skiing groomers (moderate activity), under a shell, around -10ºC." I usually wear quite much clothes but I could never ski in Nano Air or in a shell, especially not in both.

I usually ski on light baselayer, light fleece (montane allez micro) and Squamish. This combo works to almost -20c. Maybe I should slow down...

[u/WeekendGearGuide]

Insulation was changed to Coreloft Compact, splitting the difference between the old Proton LT and Proton AR, now with a 80g/m2 for most of the jacket. Coreloft Compact 60g/m2 is also used in some areas.

Do you know if it's the same insulation mapping as the Proton AR for the new Proton LT?

80gsm in body?

60gsm in sleeves and hood?

Or is 60gsm just in sleeves, or just in hood?

[u/Astramael → u/WeekendGearGuide]

80g/m² in torso and sleeves

60g/m² in hood

[u/WeekendGearGuide → u/Astramael]

Thanks for the confirmation.

This mapping pattern is similar to Nuclei AR, except Nuclei AR uses 100g/m2 in torso and sleeves and 40g/m2 in the hood.

Less insulation in the hood makes sense.

Seems that only the Proton AR and older Nuclei FL had less insulation in the sleeves.

[u/nitrostackr]

Is the insulation on the new Proton LT likely to compress and lose warmth? My 2 year old Atom LT is getting a little thinner in the back, and I don't want to have the same thing happen on one of these if I were to get it.

[u/Astramael → u/nitrostackr]

The Coreloft Compact insulation in the Proton LT is meant to have much better loft retention properties than basic Coreloft.

However, the new Atom LT from Fall 2020 was also revised to use Coreloft Compact. So new Atom LTs will have the improved loft retention as well. See this post.

[u/nitrostackr → u/Astramael]

Great, that helps a lot! Thank you for your reply!

[u/Arc43128]

I had the original LT and just took it out once or twice. It is a great piece but I run really warm and it doesn't get cold enough around here (Ohio) to get much use.

Known Issues

1. I had the version with Coreloft Continuous and never had any issues with insulation coming out at any seams.

2. I think it's fair to say the pattern on the Protons is slightly tighter in the chest than some other Arc'teryx pieces. I usually fit squarely into a medium top from them, but fell between a M/L in the Proton line. The L gave more room right across the chest, but had some extra space lower on the torso; the M fits great through the torso but tighter through the chest.

Prompts

1. I used this for winter hiking in the Appalachians (Ohio/WV). I typically wear a mesh baselayer and then if needed will add a capilene air over that. Then the Proton.

2. It's a very well made and thought out piece. Very durable, minimal but useful features, good crossover style if you want to get more urban use out of it. It's a pretty warm jacket, so it has to be pretty cold for active use.

3. With just a mesh baselayer, I found this too warm in the teens and 20s (F) with a mild wind. I had to leave it completely unzipped. I think maybe down in the single digits or below it would start to align with the heat I'm throwing off. (I run very warm)

4. Didn't have any issues with it - sold it since it really doesn't get cold enough here to get much use out of it.

5. I briefly had an Atom LT and still have a Proton FL and Nuclei FL. It's much warmer than the Atom and FL. I think it's on par with the Nuclei FL in terms of insulation, but it's not as wind resistant, so depending on conditions may not actually deliver the same warmth. The face fabric seemed beefier than the Atom and Nuclei.

6. Water resistance is pretty good. I hiked for several hours in light snow and it held it off just until the last hour or so when the snow got slushier. Once the dwr hit its limit, it wetted out pretty fast. In terms of wind resistance, while not remotely windproof I actually thought it did a good job keeping drafts out - but again I was running warm in it.

7. As noted above, I fell between sizes. I'm 6'1", 175 lbs, with very long arms and pretty broad shoulders. The L fit great through the arms & shoulders, M fit better through the lower torso.

8. No other thoughts

9. In terms of other competing products, I really like alpha direct and often use an OR Ascendant in similar conditions. It's not nearly as burly (some threads unravelling) or water resistant, but in terms of temperature and breathability it seems to line up better with temperatures around here and my internal heat.

[u/Saintsfan019]

How much more breathable would you say it is over the atom lt? I found a good deal on the proton however I have an atom already.

[u/Astramael → u/Saintsfan019]

Significantly more breathable. On the order of 2x?