Mid layer (fleece, wool jumper, primaloft)

The mid layer (mellomlag) is where the warmth comes from. The base layer manages moisture, the shell stops wind and water — the mid layer is what actually keeps you warm. It must also let sweat through from the base layer, so the moisture can carry on outwards.

The three types

On the market today there are three clear families of mid layer, and they do different jobs.

Fleece is a polyester fabric with plenty of air in its structure. Light, cheap, dries quickly, and insulates well for its weight. The big drawback is that fleece is completely open to wind — without a shell over you, the warmth blows out.

Wool jumper is the classic Norwegian solution. A thick islender, a lusekofte, or modern felted wool. Heavier than fleece, but copes better with wind on its own, holds warmth even when damp, and does not smell after days of use. The wool jumper is still a good choice on winter trips where you alternate between walking and standing still.

Synthetic puff (primaloft, climashield, and similar fills) resembles a light down jacket in form, but has a synthetic fill between two thin fabric layers. More warmth per gram than fleece, and — unlike down — it keeps its insulation when it gets wet. Primaloft as a mid layer is popular on damp Norwegian trips where a down jacket would drown.

Weight and warmth rating

For a rough guide to weight and use:

Thin fleece (100-weight, about 200–300 g) — summer evenings, ventilation, an extra layer under the shell in cool weather.
Thick fleece (200- or 300-weight, 400–600 g) — typical winter use on day trips, or as the main mid layer on a 3-season trip.
Wool jumper (400–700 g) — winter, camp life, an evening in the tent.
Light primaloft (300–450 g, 60–80 g/m² fill) — as a light puff under the shell.
Heavy primaloft (500–800 g, 100–133 g/m² fill) — as a mid layer in winter cold, or as a warm jacket on its own.

Fleece is the weakest for warmth per gram, primaloft the best, wool somewhere in between — but wool holds warmth in damp conditions better than synthetic and fleece.

Two thin layers or one thick

Two thin mid layers insulate better than one thick one of the same total weight, because the air pocket between the two layers gives extra insulation. You also regulate more easily: take off the outermost when you get warm on a climb, put it on when you stop. On a winter trip a typical combination is a thin fleece or wool top as the first mid layer, and a light primaloft or thicker wool jumper as the second.

When each layer

Summer in the lowlands: rarely any need for a mid layer while moving. A thin fleece or light wool jumper in the rucksack for breaks and the evening.
3-season, day trip: a thick fleece or wool top covers most of it.
Winter, day trip: two mid layers — one thin and one thicker — give the best regulation.
Winter, tent: a heavy primaloft or expedition wool jumper in addition, which you put on when you stop for the evening.
Wet conditions: prioritise wool and primaloft over down. Down collapses in the damp; primaloft and wool do not.

Materials and brands

Patagonia and Norrøna make quality fleece in recycled polyester. Devold and Aclima dominate the Norwegian wool-jumper market, together with Bergans. Patagonia’s Nano Puff and Black Diamond’s First Light are classic primaloft jackets; Norrøna Falketind PrimaLoft60 is a light Norwegian variant. Marmot and Mountain Equipment also have solid alternatives.

For most people a thick fleece at 600–1200 kr or a good wool jumper bought used from Finn is the sensible starting point — a specialised primaloft jacket comes on top once you know what kind of trips you actually do.

Next steps

Base layer — the layer beneath the mid layer; moves the sweat further out
Outdoor jackets — the shell over the mid layer that stops wind and water
Headwear — where much of the warmth leaks out when the mid layer does not cover the head
The layering principle — how the three layers work together
Choosing materials — wool, fleece and synthetic compared for moisture and warmth

Learn more

The layering principle → · Choosing materials → · Base layer → · Outdoor jackets →

Text: Snuitide (2026).