Flannel Weight (GSM)

Flannel is not a weave but a finish: the fabric surface is brushed (napped) to raise soft fibers that trap warmth. Quality and warmth track weight — a lightweight flannel around 150 GSM is a three-season shirt, while a heavy 250+ GSM flannel is a genuine cool-weather layer. Better flannels use combed or ring-spun cotton and are napped on both sides; cheap ones thin out and pill after a few washes as the nap wears away. On a spec sheet, flannel weight plus fiber quality predicts both warmth and how many winters it survives.