The midlayer I'm taking to -25: fleece, active insulation, and the swing between the two

This is the middle chapter of a kit problem I have been chewing on all summer. I have a week booked in Kittila, in northern Finland, above the Arctic Circle, this winter. It is the same active week behind the baselayers piece and the down jacket piece that sit alongside this one: a 35km snowmobile tour, a 10km husky-sledding run, the local snow village, fell hikes if the weather allows, and northern-lights watching at night when the temperature drops to around minus twenty-five. Polar night, so no midday sun to lean on, and long static hours in the cold.

Baselayer, then midlayer, then the down and shell over the top. This piece is about the middle of that stack, the insulating layer, and it turns out to be the hardest of the three to call. The baselayer job I could reason my way through. The down job is about one number, the amount of down, and I worked that out. The midlayer is the awkward one, because for a trip like this it is being asked to do two opposite jobs at once, and the piece that is brilliant at one of them is poor at the other.

So this is not a "best fleece" list. If you want that, the full fleece guide and the fleece hub rank the field properly and I am not going to repeat their buying guide here. This is narrower and more personal: the specific midlayer decision for an active week at minus twenty-five, the two pieces I already own and use, and the one I am weighing up actually buying for the trip. It sits under our wider cold-weather gear hub, where the rest of the kit for an Arctic week lives.

The Finland problem: the swing between hot and dead still

Here is the whole problem in one sentence. On this trip I am either working hard or standing completely still, and almost never anything in between.

Working hard is the snowmobile day, wrestling a heavy machine over 35km of trail. It is hanging on to a husky sled for 10km. It is breaking uphill through deep snow on a fell if the weather lets us out. In all of that my body is a furnace, throwing off heat and sweat, and the last thing I want wrapped around me is a thick, warm layer sealing that heat and moisture in. Overheat on the move in the cold and you soak your baselayer, and a wet baselayer at minus twenty-five is how a good day turns bad the moment you stop.

Then there is dead still. Standing on a frozen lake for an hour waiting for the aurora, making almost no heat of my own. Sitting on a sled seat or a snowmobile as a passenger. Wandering the snow village in the evening. In those moments my body has stopped feeding the system and the clothing is doing all of the work, and a thin, breezy layer that felt perfect on the climb suddenly has nothing to give.

That is the swing. A plush, high-loft fleece is superb for the static side and cooks you on the move. A thin, breathable active layer is superb on the move and leaves you shivering the moment you stop. Most trips let you sit somewhere in the middle and forgive a midlayer that is only roughly right. This one does not. So the real question is not "which is the warmest midlayer", it is which side of the swing I optimise for, and whether one single layer can honestly cover both.

What a midlayer actually does at -25

Before I get to specific pieces, the principle, because it is the thing that makes the choice make sense. Nothing on this list keeps you warm on its own at minus twenty-five. The warmth comes from the system: the baselayer moving sweat off your skin, the midlayer trapping still air, the down and shell over the top holding it all in and keeping the weather out. Warmth is a stack, not a single hero garment, and the baselayers piece makes the same point from the bottom of the stack.

The midlayer's slice of that job is loft. It traps a thick band of still air against everything the baselayer is doing, and still air is what actually insulates you. A fleece does that with a lofted pile of fibres. Active insulation does it with hollow synthetic fibres that hold air the same way a very light duvet does. Either way, the midlayer is the engine room of the stack: it is where most of your standing-still warmth comes from, and it is also the layer most likely to cook you if you get it wrong on the move.

Which is exactly why the swing matters here more than on any British hill day. Pick a midlayer that only knows how to be warm, and you will sweat through it snowmobiling. Pick one that only knows how to breathe, and you will freeze under the aurora. The good midlayer for this trip is the one that manages heat in both directions: dumps it when you make too much, holds it when you make none.

My static-warmth pick: the ThruDark Phalanx (owned)

Let me start with what I already own and have actually worn, because that is where I can speak first-hand rather than off a spec sheet.

The ThruDark Phalanx is my warm fleece, and it is a properly warm one. The body is Polartec High Loft, which Polartec rates as the highest warmth-to-weight fabric in its fleece range, with Power Grid panels under the arms to shift heat when you are working. I bought mine at full retail, no discount, no relationship with the brand, and it is a lovely thing to wear. For standing still in cold air it is superb: throw it on when you stop and it holds a deep, soft band of warmth that a thin layer simply cannot.

It is also, and I mean this as praise of what it is rather than a complaint, a layer you cook in on the move. That lofted pile is a heat trap by design, and on anything aerobic it does its job too well. On a hard climb I am peeling it off inside twenty minutes. ThruDark are honest enough to rate it 2 out of 10 for wind resistance on their own product page, so it also needs a shell over the top the moment there is any breeze, which under a down jacket on a frozen lake is exactly the setup anyway.

So for the Finland swing, the Phalanx sits firmly on the static side. It is the warm layer I would put on when I have stopped and want to stay stopped, and it would be a strong pick for the aurora vigils and the sled seat. What it is not is the layer I want on while I am wrestling a snowmobile. If I take it, it is the standing-still specialist, and something lighter does the moving.

One thing worth saying plainly: we earn nothing on the Phalanx. ThruDark is not an affiliate partner of ours, so the link above is just a link. I rate the piece and I own it, and that is the only reason it is here.

My indoor layer: the Arc'teryx Delta (owned)

The other piece I already live in is the Arc'teryx Delta Hoody, a light technical grid fleece, and it is the fleece I reach for more than any other in normal life. Mine is the older Polartec Classic 100 micro-grid velour version, superlight and soft in a way that keeps it in constant rotation for cool-weather walking, travel and everyday wear.

I am taking it to Finland, but I want to be straight about the role, because it is the same thing I said in its own review: this is not cold-weather protection. A grid fleece is thin by design, it has little standalone warmth in properly cold air, and it offers next to nothing against wind. Ask it to be your insulation stood still at minus twenty-five and it will let you down, not because it is a bad fleece but because that is not its job.

Its job on this trip is indoors at the villa. It is the soft, light, comfortable layer you actually want to relax in when the heating is on and you are out of the weather, and it is very good at that. Think of the Delta as the layer you unwind in, the Phalanx as the layer you stop outside in, and you have the two ends of what a fleece can be. I own both for exactly that reason. And, same as the Phalanx, we have no Arc'teryx affiliate, so its link earns us nothing either.

The active option I'm weighing: the Salomon Thernova

Here is the piece I do not yet own and am actively thinking about buying for the trip: the Salomon Thernova Fleece Hoodie.

The name says fleece, but it is not a fleece in the Phalanx sense of a lofted pile. It is built around PrimaLoft, the synthetic active insulation, under a soft polyester face. PrimaLoft uses fine hollow fibres to trap air the way down does, which gives it a warmth-for-weight closer to a light insulated jacket than to a fleece, while staying breathable and moving moisture rather than holding it. In other words it is aiming squarely at the active-then-static problem: dump heat and sweat while you are working, trap warmth the moment you stop.

That is the exact swing this trip is built around, which is why it has my attention. On paper it is a better active midlayer for a week like this than a heavy fleece, because it should let me keep it on while I am actually moving without the overheating that drives me out of the Phalanx, and still have real insulation left when I stand still. The heavy fleece optimises hard for the static side. The Thernova is trying to cover both sides of the swing in one layer, and if it does, it is the more useful single piece for an active Arctic week.

What matters here is the reasoning and the spec, and the fact that I am close to buying it for precisely the job this piece is about. The men's version comes in three colours, Castor Gray, Coffee Bean and Black, and it is a 100% polyester construction with the PrimaLoft doing the insulating.

And the part I would rather say out loud than let you find out later: the Thernova is the one piece on this page we earn a commission on. Salomon is an affiliate partner, the Phalanx and the Delta are not. That does not change my reasoning for it, active insulation really is the right category for this swing, but you should know it, weigh it, and trust me a little more for having told you rather than less.

Fleece or active insulation: which side of the swing

So the real decision is a fabric-type decision, and it comes straight back to the swing.

A high-loft fleece like the Phalanx is a static-warmth specialist. Its lofted pile traps a lot of air, so it is warm and soft when you have stopped, and it breathes reasonably through its open structure. But it is a heat trap first, so on anything aerobic it overheats you fast, and it packs down poorly in a bag. If your trip is mostly standing around in the cold with short bursts of gentle movement, a warm fleece is the honest answer and the Phalanx is a lovely one.

Active insulation like the PrimaLoft in the Thernova is built for the swing itself. It carries down-like warmth for its weight, so it insulates properly when you stop, but its whole design goal is to breathe and shed moisture while you are moving, so you can keep it on through the effort rather than stripping it off every climb. It also packs smaller. The trade is that at the very coldest, most static extreme it will not match a thick fleece gram for gram of standing-still warmth. But on this trip the standing-still warmth is meant to come from the down jacket over the top, not from the midlayer alone, which tips the balance towards the layer that behaves better on the move.

That is the crux of it. If I were optimising purely for the aurora vigil, I would take the fleece. If I am optimising for a week that swings hard between snowmobile effort and dead-still cold, and I am letting the down handle the deepest static warmth, the active-insulation midlayer is arguably the smarter single choice. That is why the Thernova, not the Phalanx, is the piece I am on the edge of buying.

Where the midlayer sits in the -25 system

None of this decision makes sense in isolation, so here is the whole stack it lives in, because the midlayer only has to be as warm as the layers around it allow.

• Baselayer. Against the skin, moving sweat off me so I am never sitting in a wet film when I stop. For me that is a synthetic top and merino leggings, for reasons I go into in the baselayers for extreme cold piece, and there is a wider weights-and-fabrics breakdown in the base layers guide.
• Midlayer. This piece. The insulating engine room, chosen for the swing: active insulation to move in, or a warm fleece to stand still in, depending on the day.
• Down and shell. Over the top for the cold, static moments, and for the weather. The down does the heavy standing-still warmth so the midlayer does not have to, which is exactly why I can lean the midlayer choice towards the active side. That layer gets its own full treatment in the down jacket for extreme cold piece.

Read that top to bottom and the midlayer's job gets easier, not harder. It does not have to be the warmest possible layer, because the down is there for the deep cold. It has to be the layer I can keep on across the widest range of effort without overheating or freezing, which is the active-insulation argument in one line.

What I'm actually packing

Pulling it together, here is where my head is for the midlayer, still with a few weeks to commit:

• The active/mid layer, on the move and as my main insulator: most likely the Salomon Thernova, the active-insulation hoody, bought for this trip. If it earns its place through a few cold UK weeks first, it is the layer I keep on across the snowmobile, the sled and the fell climbs, breathing when I work and holding warmth when I pause. If I decide I want more outright standing-still warmth from the midlayer itself, the ThruDark Phalanx I already own steps in as the fleece I stop in, with the down over the top.
• Indoors at the villa: the Arc'teryx Delta, purely as the soft, light comfort layer for when the heating is on. Not outdoor kit, and not pretending to be.
• Over the top, for the dead-still cold: the down jacket, which is where the real aurora-night warmth lives, covered in its own piece.

That is a system, split by job, not a single hero fleece. The midlayer's role in it is to manage the swing, and the more I reason it through, the more the active-insulation route looks right for a week that is either flat out or frozen solid with very little in between.

As with the baselayers, I will report back once I am home: whether the Thernova earns the buy, how it actually held up across the swing at minus twenty-five, and whether I ended up reaching for it or the Phalanx when it truly mattered. That write-up is coming.

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