Thermal Knits: What to Check Beyond GSM

Warmth Comes From Trapped Air

A fabric does not warm you — it slows the heat leaving you, by holding still air near the skin. That is why loft matters more than mass. A lofty brushed knit at 260 GSM traps more air than a dense flat knit at 320 GSM, and wears warmer despite weighing less.

So GSM alone will not rank two thermal qualities. Thickness at a stated pressure, and how the surface is finished, tell you more.

Brushed, Sueded, Sheared, Polar

The finishing vocabulary gets used loosely, so confirm what you are buying:

• Brushed (single or double) — wire rollers raise fibre ends on one or both faces; double-brushed is warmer and softer, and loses a little dimensional stability
• Sueded — a light abrasion for hand-feel rather than warmth
• Sheared — raised pile cut to an even height, which is what keeps polar fleece looking uniform after washing
• Polar fleece — raised, sheared and anti-pill treated, the heaviest of the group

Ask which face is brushed. A garment with the brushed face inward feels warm; the same fabric sewn the other way is a different product.

Pilling Is the Complaint You Will Get

Raised surfaces pill. It is the single most common thermal-knit return, and it shows up after washing, not on the swatch card.

Ask for a Martindale or ICI pilling rating after wash — a rating on unwashed fabric is close to meaningless for a brushed quality. Grade 3–4 or better after five washes is a reasonable line for mid-market activewear.

Moisture Still Matters in Winter

Thermal base layers fail when they hold sweat: damp fabric conducts heat away and the wearer gets cold in the stops. For active winter use, a brushed polyester or a poly-rich blend that wicks and dries will outperform a heavier cotton-rich knit that feels warmer in the shop.

Match the fabric to the activity, then check pilling and shrinkage after wash — those two numbers predict most of what customer service will hear next season.