Thermal underwear forms the first layer in a ski clothing system. Its job is to move sweat away from the skin, trap insulating air, and keep the microclimate stable during lifts, descents, and breaks. A well-chosen base layer helps maintain warmth without overheating.

Material choice guides performance. Merino wool provides warmth-to-weight, breathability, and natural odor control; it insulates when damp and feels soft next to skin. Synthetic polyester or polyamide dries very fast, resists abrasion, and often weighs less. Blends combine quick-dry strength with merino comfort.

Warmth levels vary by fabric weight and knit. Lightweight pieces suit high-output skiing and milder days. Midweight balances insulation and moisture control for most resort conditions. Heavyweight or grid-backed knits add extra warmth in deep cold. Fabric weight (gsm) and knit structure influence drying speed and breathability.

A close, ergonomic fit works best under ski wear, limiting bulk and aiding moisture transfer. Useful construction details include 4-way stretch, flatlock seams to reduce chafing, and raglan or offset shoulder seams that sit well under backpack straps. Crew neck and half-zip tops help manage venting; long sleeve designs pair naturally with leggings.

Bottoms matter as much as tops. Full-length leggings provide all-round warmth, while 3/4 length legs keep cuffs clear of ski boots to avoid wrinkling. A wide, comfortable waistband and articulated knees support freedom of movement. Reinforced or seamless gussets add durability.

Thermal underwear ranges from single tops and bottoms to coordinated sets. Many fabrics include odor control treatments, brushed interiors for softness, or mapped mesh zones that enhance breathability without sacrificing warmth where it is needed most.

Simple care preserves performance. Wash cool, avoid fabric softeners, and line dry to maintain loft, elasticity, and wicking. Merino benefits from gentle detergents designed for wool.

Layered under ski midlayers and shells, thermal underwear works with ventilation zips to regulate temperature through changing weather. OutdoorXL is a store and webshop with an extensive choice from leading brands, offering a broad range in the Sale > Ski > Ski wear > Thermal underwear category, so matching material, weight, and fit to conditions is straightforward.