Midlayer Guide: Fleece, Active Insulation & How to Choose

The biggest mistake people make with midlayers is treating them as something to put on and leave on. In a well-managed layering system the midlayer goes on and off multiple times during a mountain day, being added at transitions, removed on sustained climbs, swapped for a down layer or heavier belay jacket at a belay in extreme cold. Getting comfortable with this active management, and knowing when to pre-empt rather than react, is as important as the specific midlayer you choose.

Fleece is the traditional mountain midlayer and has been for decades because it does its job reliably across a wide range of conditions. Fleece is a knitted synthetic fabric that insulates through trapped air in its fibres, retains insulating ability when damp, and dries quickly. Wind cuts straight through most open-knit fleece, so a shell layer is required in exposed conditions, but within a complete layering system it remains one of the most versatile and cost-effective midlayer options available.

Fleece weight, measured in gsm, is the primary variable determining warmth and breathability. Lightweight fleece (100–150gsm) suits high-output activity and mild conditions. Midweight fleece (200–300gsm) is the most common all-round option, covering general mountain use across the majority of conditions most people encounter. Heavyweight fleece (300gsm+) is for cold conditions with lower output such as camp layers, belaying, and cold-weather downhill skiing, where warmth is the priority and breathability demands are lower.

A note on Octa® fleece specifically: Octa® is a recent fleece technology developed by Toray using an octagonal cross-section fibre that produces exceptional warmth-to-weight ratio and dramatically lower moisture retention than standard fleece. It dries faster, weighs less for equivalent warmth, and breathes better during movement. The Jöttnar Eldar uses Octa® fabric and represents a step up in technical performance over standard fleece. For high-output mountain activity where you want fleece rather than active insulation, it is a very capable option.

Active insulation represents a fundamental shift in how midlayer performance is achieved. Where traditional fleece and synthetic insulation are optimised for warmth, active insulation is optimised for the transition between movement and rest. Polartec® Alpha™ is the leading active insulation technology. It uses a low-density, air-permeable insulation structure that breathes during movement, allowing excess heat and moisture to escape while still providing warmth when you stop. The result is a layer that avoids the overheating problem of traditional midlayers during high-output activity without sacrificing warmth at the transitions where you need it.

In practical terms: with a conventional fleece midlayer, serious uphill effort requires either removing the layer - fiddly, interrupts momentum, creates a wet base layer exposed to cold, or accepting that you may overheat and arrive at each transition saturated with sweat! With Alpha™, the layer manages the transition itself. It breathes aggressively enough during effort to prevent overheating, and insulates effectively enough at rest to keep you warm without needing to add a layer. The Jöttnar Asger Hooded Jacket and hoodless Asger-HS are built on Alpha™ construction for exactly this use case. The Asger Alpha™ Gilet focuses on providing core warmth with minimised bulk.

Alpha™ is most valuable in the specific context of variable-output mountain activity such as ski touring, climbing, general uphill/downhill trekking and any use where you cycle repeatedly between high exertion and stationary rest. It is less differentiated for consistently low output activity where a good fleece covers the ground at lower cost. Alpha™ is also versatile enough to function as a standalone outer layer in dry conditions due to the wind resistant outer we build into our Alpha™ based jackets. The premium over standard fleece is real.

A synthetic insulated jacket used as a midlayer provides more warmth than fleece or Alpha™. The trade-offs are breathability and packability: synthetic insulation breathes less efficiently during high-output movement and does not self-regulate across the exertion-rest transition the way Alpha™ does. This makes it best suited to deployment at rest and then removal during movement. Use as the insulation layer in colder conditions where activity levels are lower, longer static periods are anticipated and warmth is the dominant requirement.

Down as a midlayer can be also used for transition warmth and its lightness gives it an advantage over synthetic insulation. A packable down jacket worn over a base layer or active insulation under a shell provides the highest warmth-to-weight ratio of any midlayer option. The down layer lives in a pack and deploys at any stop, then packs away immediately when movement resumes. Like synthetic insulation, down doesn't breathe adequately during high-output movement and must be removed on the move. 

The Jöttnar down range products are commonly used as midlayers in this way, put on over a base layer at belays, transition points and stops, removed during movement. The gilet format of the Asta is particularly practical as a belay layer because it adds core warmth without the bulk of a full jacket under a shell. For full detail on down insulation, fill power and the down range, see our Complete Guide to Down Insulation.

A softshell jacket sits at an interesting point in the layering hierarchy. It’s more than, or at least different to other true midlayers, but less than a hardshell. Wind-resistant, highly breathable, with some water resistance, and some secondary light insulation properties, it can function as a versatile midlayer under a hardshell in cold or severe conditions, or as a standalone outer layer in dry, active conditions where a hardshell's full weather protection is not needed but wind resistance is. The Jöttnar Sigvard is a technical softshell with four-way stretch and a PFC-free DWR finish and is versatile enough to serve both roles. It is not a replacement for a hardshell in sustained rain, but as a dynamic all-conditions piece it occupies ground that neither open fleece nor a full hardshell fills as well. This was how we layered as Royal Marines during extreme cold weather / Arctic and mountain training. The military, breathable, softshell equivalent (called a ‘windproof) was worn over a base layer with a hardshell held in reserve and then placed on top of the windproof in rain, sleet and heavy snow.

Where softshell fits in practice: on dry, cold, windier days where a full hardshell is too warm and too heavy but open fleece would be cut through by the wind. As the outer layer on fast-moving routes in changeable but predominantly dry weather. As a high-breathability midlayer under a hardshell in conditions cold enough to need both wind protection and very light insulation. Understand softshell as a distinct category, with its own specific use cases rather than a generalised substitute for either fleece, down, synthetic insulation or hardshell.

A note on layering order with a softshell: if wearing a softshell under a hardshell, the softshell's wind resistance slightly reduces the hardshell's breathability from the inside. In high-output conditions this can create a moisture management issue. Use the softshell as the outer layer on the move and add the hardshell only when weather demands it.

One final point: most outdoor and mountain users end up with more than one midlayer. A fleece or softshell for general walking days, an Alpha™ layer for versatility, a packable down layer or heavier weight synthetic insulation jacket for belay and transition use. These are not redundant; they are complementary tools for different conditions and different activity types. Building the range over time, starting with the midlayer that matches your most common use case, is a more practical approach than searching for one layer that does everything equally well.