Ultra-running brand OMM (Original Mountain Marathon) has released what might be the world’s most breathable mid-layer. Billed as a ‘new concept in active insulation’, the Core Hoodie promises ‘near-total air permeability’ with ‘the warmth of a fleece but at less than half the weight’. Even the lightest breeze will create ventilation, enabling better thermo-regulation during your runs (or fast hikes). It all sounds rather extraordinary – but how does it work?

The Core Hoodie is made entirely from a PrimaLoft Active synthetic plume sewn onto a single-layer fabric mesh. Traditionally plume insulation is found loose between two outer layers. Here, however, the loose plume is knitted onto a single-layer mesh, creating an insulating layer at an insanely light weight – it weighs just 115g in the Core Hoodie (£90) version. That really is ultralight. A slightly warmer version, the Core+ Hoodie (£100) weighs 185g because it features more PrimaLoft Active insulation, while the Core Vest (£65) weighs just 72g.

Of course, if wind can blow straight through the hoodie, then it won’t keep you that warm and the insulating properties will be compromised. Consequently, the Core Hoodie is specifically designed to be used in conjunction with a minimalist windproof jacket. As OMM tells Outdoors Magic: “Paired with a shell, the Core is an incredibly effective insulator, but – thanks to near-total air permeability – the lightest breeze can take away that heat. This allows you to keep your Core layer on even when you have warmed up and you can thermo-regulate by removing or unzipping your low bulk shell (such as the Sonic windproof at 48g). The Core Hoodie is a true valley-to-summit piece. It works wet or dry with incredible wicking properties to ensure you won’t stay wet for long.”