MARK The Three-Layer System
The three-layer system is the foundation of outdoor clothing — and the skill is in managing it through the day, not just owning the layers.
Base, mid and shell — what each does, how to pick them, and how the system changes for winter.
The Layers
Base Layer — Wicks Moisture
Sits against your skin and wicks moisture away. Merino wool (150–200 gsm for spring/summer, 200–250+ for autumn/winter) for warmth and odour resistance; synthetic for faster drying and lower cost. Fit snugly but not restrictively. Avoid cotton at all costs — it absorbs moisture and accelerates heat loss.
Mid Layer — Traps Warm Air
Lightweight fleece for active use, thicker primaloft or down for colder conditions. Consider two weights: a light one for high-output days, a heavier one for cold-weather or lower-intensity activities.
Shell Layer — Wind & Rain
Protects from wind, rain, and snow while allowing moisture to escape. This is the layer where a real waterproof matters.
The Skill Is Management
You should be slightly cool when you start moving — within ten minutes your body heat will warm you. Vent or remove a layer before you start sweating. Add layers before you get cold. The most common mistake is wearing too much at the start and spending the rest of the day wet from sweat. Pay attention to zips and vents — pit zips, zip-neck base layers, and half-zip fleeces give fine-grained temperature control.
Layering for Winter
The Insulated Mid Layer
Winter demands a proper insulated mid-layer with 80–120g of synthetic or down fill. Down gives the best warmth-to-weight ratio but stops performing once wet; synthetic insulation like Primaloft or Climashield keeps working even when damp and dries faster — the more reliable choice for high-output days.
Static Moments & Insurance
For summit stops and lunch breaks, a heavier belay jacket (120–200g fill) lives in your pack and goes on when you stop. Below –5°C, hand warmers, thicker expedition gloves, and a balaclava stop being optional. Keep vents closed when exposed to cold, open them during hard efforts, and carry a spare insulated layer in a dry bag.
MX Summary: The three-layer system works only if you manage it — start cool, vent early, add layers before you get cold, and never let sweat soak you from the inside. Get that right and you’re warm and dry in almost anything the UK throws at you.