Women's Heavyweight Base Layer

Fabric from about 260 to 400 grams per square meter defines this class, and 320 gsm merino is the figure you will see most often. Thickness at that level holds a deep cushion of still air, which is the only thing that keeps a motionless body warm. Most versions brush or grid the inner face to add loft, so the cloth measures noticeably thicker than a midweight of the same fiber.

Standing still is the condition it is designed around. A body at rest produces roughly a fifth of the heat it makes while walking briskly. The fabric has to supply what your muscles are not. That is why the same top that feels perfect on a touchline at 15 F turns into a sauna on a ten minute walk. Nothing about the weight adapts, and there is no way to vent a layer worn under everything else.

Mixed days that include real effort belong to a Midweight Base Layer, which this class cannot cover. Lightweight Base Layers exist for the opposite condition entirely. Indoor warmth at less money comes from Waffle Knit Thermals, which collapse when wet. Where this one has no substitute is the long cold wait, whether that means a hunting stand, a frozen lake, a winter camp or a building site.

Below 20 F with little movement is the window, and above 30 F with any activity you will regret it. Bulk is the practical limit on where it fits. A 320 gsm bottom will not go under slim trousers. It changes how a ski boot or work boot closes around the calf, so try boots on over the layer rather than after buying. People who are cold at their desks year round often buy this weight and then find it unwearable indoors.

Feel the inner face for depth rather than softness, since loft is what holds air and a flat heavy knit is just heavy. Full length hems and long cuffs earn their keep, because gaps undo the one thing this weight exists to do. Drying is slow at this weight, so budget a full day on a rail and never a radiator, which hardens elastic and felts wool. Two sets in rotation is the sensible arrangement for any trip lasting more than a weekend.