Anyone who hikes in the mountains knows the eternal insulation dilemma. Too thin and you are shivering on the saddle, too thick and ten minutes into the climb you are walking in your own steam. Mammut Loopinsulation Active is the Swiss brand’s answer: an insulation layer that stays closed when you stop and opens up as soon as you move. The jacket where this new insulation shows its strengths most clearly is the Aenergy Air IN Hooded Jacket.

Last week we published our review of the Sender IN Hybrid Jacket Women. Tester Chiara Fischer praised its warmth and freedom of movement, but also spotted the catch: between 10 and 16 °C and at a decent pace, the jacket quickly became too warm. That is not a design flaw. The Sender is built to hold on to heat. That is exactly where the story of the Aenergy Air begins.

What is Mammut Loopinsulation Active?

Loopinsulation Active is a synthetic insulation made half from Mammut’s recycled rope offcuts and half from recycled polyester from PET bottles. The difference from the original Loopinsulation lies in its construction. Where the original forms a dense fibre layer, the Active version is built like an accordion. When you stand still, the structure stays closed and the fibres trap the heat. When you move, the material stretches, the structure opens and excess heat and moisture can escape.

Remarkably, that breathable construction does not come at the expense of warmth. At a fill weight of 80 g/m², Loopinsulation Active reaches a clo value of 2.01, compared with 1.63 for the original. Gram for gram, the Active version is significantly warmer. Mammut also measured humidity at the skin during half-hour sessions of fast climbing and descending. With the Active version it was noticeably lower right against the skin, exactly where you feel clamminess first.

Why standing still and moving call for two different jackets

A classic insulated jacket does one thing well: it keeps heat in. On a cold summit that is exactly what you want, but once the trail climbs again that advantage turns against you. The heat your body produces has nowhere to go, you start to sweat and the moisture lingers. Stop again and you cool down faster than you would like. The result is the familiar ritual of on, off and on again.

Loopinsulation Active is designed to break that ritual. Instead of one jacket for the break and another for the climb, you get one layer that adapts to what you are doing.

Aenergy Air IN Hooded Jacket: built for pace

Because Loopinsulation Active is warmer per gram, Mammut can use less insulation in the Aenergy Air IN Hooded Jacket: 60 g/m² in the body and 45 g/m² in the hood, side panels and sleeves. For comparison, the Rime IN Hooded Jacket with original Loopinsulation uses 80 g/m² in the body. Less fill means less bulk, more freedom of movement and a jacket you happily keep on.

The light, stretchy face fabric helps keep air flowing once you move. The hood fits under a helmet and the front pockets stay accessible with a hip belt or climbing harness on. These details tell you exactly who this jacket is for: people who have no intention of stopping along the way. Mammut calls it a put-on-leave-on layer. You put it on at the trailhead and take it off when you are back at the car.

Who is the Aenergy Air for, and who is it not for?

The Aenergy Air IN Hooded Jacket is at its best for fast hiking, climbing and ski touring: activities where effort and rest alternate quickly and you would rather not keep opening your pack. If you mainly want warmth during long breaks, on a cold morning in the city or on a gentle snowshoe walk, the Sender IN Hybrid remains the more logical choice. For one light all-rounder on changeable days, the Rime IN Hooded is the obvious pick. The best jacket is the one that matches how you move.

A jacket with a past

Mammut has been making climbing ropes since 1862. In 2018 an analysis showed that rope production and sales accounted for 13 percent of the brand’s CO₂ emissions. That became the starting point for Loopinsulation. Rope offcuts are processed purely mechanically, without chemicals, glue or water. By September 2025, 41 tonnes of rope waste had been given a second life this way. According to Mammut’s calculation using Higg MSI 3.7, emissions are 48 percent lower than with virgin polyester insulation. That figure has not been verified by a third party, but the principle stands: the rope that once held someone on a wall now keeps you warm.

With Loopinsulation Active, Mammut is not launching a new technology. It is adapting an existing one to a different kind of mountain day. A day on which you do not stand still.

Quick answers

• What is the difference between Loopinsulation and Loopinsulation Active?
  Both are made from 100 percent recycled material: half rope offcuts, half rPET. Original Loopinsulation is a dense layer built mainly to retain heat. Loopinsulation Active has a stretchy accordion structure that opens up when you move and is warmer per gram.
• Is the Aenergy Air IN Hooded Jacket warm enough for winter?
  As an active layer, yes. It is designed to be worn while moving. For long stationary periods in severe cold, pair it with a hardshell or choose a warmer jacket such as the Sender IN Hybrid.
• Can I wear the Aenergy Air under a helmet and harness?
  Yes. The hood fits under a helmet and the front pockets remain accessible with a hip belt or harness on.
• How sustainable is Loopinsulation Active?
  The insulation is made entirely from recycled material and processed mechanically without chemicals, glue or water. Mammut calculated a 48 percent CO₂ reduction compared with virgin polyester. The calculation has not been externally verified.