A compromise — #21 of 31 Cotton ranks #21 of 31 fabrics for warmth without weight. It is a middling fabric for this. Ranked on warmth relative to weight, with a packability bonus. The top pick is Cashmere.
Cotton measures 75/100 for breathability, 8% moisture regain, 35/100 warmth, at 160 gsm. Warmth-to-weight is the metric: 35/100 warmth at 160 gsm is a fair ratio for packable insulation.