Climber’s Air-Permeable Insulation – Arc’teryx Proton LT

Arc'teryx Proton LT

Air permeability is the name of the game these days in insulation. With all this innovation, it’s tough to know which jacket to buy for each activity. What if you’re going out on super-aerobic backcountry tours? Maybe you’re an ice climber? For the latter folks, for whom reach and flexibility are paramount, let me introduce you to the Arc’teryx Proton LT hoody.


In designing the Proton LT, Arc’teryx’s goal was to move beyond the softshell and into a garment that can provide warmth and protection across a wider range of weather and activity levels. The Proton LT is the result of combining recent innovations in face fabric and insulation into a hoody that keeps you warm but simultaneously lets sweat vapor out. That seems to be the buzzwordy stuff going on these days from just about everyone. Let’s talk about how the Proton LT stacks up to other offerings, including Arc’teryx’s own Atom series.

Design and Construction

The Proton LT looks and feels like a traditional synthetic layer, for the most part. There’s some stretch to the Fortius Air 40 face fabric – likewise with the Permeair 20 liner. (Arc’teryx always has the best names for their fabrics, don’t they?) Insulation duties are served by Arc’teryx’s own, continuous filament Coreloft in 65g weight.

A climbing helmet compatible hood sits on top. You get the standard, stretchy, napoleon pocket on the chest and two hand-warmer pockets at the hips, high enough to avoid interference with a hip belt. A single drawstring cinches the hood around your head and another, with dual adjusters, keeps drafts from blowing up your waist.

Slipping the jacket on, the first thing I noticed was the elbows. That sounds strange, doesn’t it? Normally, a sleeve is just a tapering tube that slides around your arm when you move. In the Proton LT, it feels like there’s a pouch in the elbow of the jacket that cradles your arm, fitting it perfectly. When you move, there’s no interference, This is especially important in a jacket designed for rock and ice climbing.

I also appreciated the cuffs on the sleeves. They’re an elastane material that seals the elements out of your wrists. The elastane creeps up the sleeve in a small wedge, giving the lower sleeve much more versatility. You can slide them up your arms to cool off.


So does the Proton LT do what it’s supposed to do? In a word, yes. In another word, no. Sorry. Let me explain.

Arc’teryx always makes choices based on the purpose of a garment. In the case of the Proton, they expected the wearer to be performing strenuous activities, but not necessarily sustained aerobic activity. They also expected abrasion. This brings us back to my original comment about backcountry touring versus ice climbing.


My perception, backed up by Arc’teryx’s marketing copy, is that the Proton LT is less breathable than some of its competitors (and other Arc’teryx jackets), on purpose. Fortius Air 40 is windproof. It’s also a lot more durable than some of the face fabrics that other air permeable insulated jackets use. I feel much more comfortable rubbing the Proton LT against a rock or snagging it on a branch than I would, say, the BD First Light or the Patagonia Nano Air.  (We should note that Fortius Air 40, itself, is marketed as being more breathable than the Tyono 20 used in the Atom series.  The Atom LT’s Power Stretch underarm panels make a big difference.)

On the flip side, I don’t take the Proton LT out on 30° days for a tour. While I would use it for backcountry touring, the weather would have to be cold and precipitation would have to be, at the very least, in the forecast, if not already falling from the sky.  Why?  Because the Proton LT sacrifices some of that breathability for protection. Within Arc’teryx’s own lineup, I would usually select an Atom series jacket over the Proton for touring (in fact, I did just that on Tuesday).

However, cold rock and ice are where the Proton LT really shines. The Fortius fabric is exceptionally durable without being heavy. Reaching for holds or to swing a tool is a dream because of where Arc’teryx placed seams. I’ve never once felt like my movements were restricted by the Proton.



Arc’teryx generally runs a bit smaller than some American brands with some variance across the line. A large Proton LT is just about too small on me. It’s also a bit smaller than the Atom series. At 5’10”, 230, I’d be hesitant to size up because I expect the sleeves would be too long. However, I could use a bit more room in the hips. The waist, chest, shoulders, and arms are perfect – not too baggy, but not skin-tight. Let’s face it – it would be more cost effective and performance enhancing to lose weight anyway.


Arc’teryx also makes a Proton AR (AR meaning All-Round), which adds 90g Coreloft to the torso and an extra napoleon pocket to the chest. Pick the AR over the LT if you tend to run colder or will be using the jacket in colder conditions. You only add 0.9oz (25g) for the extra warmth.  MSRP is $349 – $50 more than the LT.


Final Thoughts

The Arc’teryx Proton LT is the cold-weather climber’s jacket. If you already have air-permeable insulation and want something that adds durability and weather-resistance to your jacket quiver, grab one. If you’re looking to add your first air permeable piece to your closet, however, the Proton LT may be too specialized to be your only jacket. In that case, it may make sense to look elsewhere and come back to the Proton LT when you need to fill the niche it was designed for. MSRP $299.


See also –

Patagonia R1 TechFace Hoody Fleece – Better than R1?

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