A compromise — #18 of 31 Jersey ranks #18 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.
Jersey measures 65/100 for breathability, 8% moisture regain, 40/100 warmth, at 180 gsm. Warmth-to-weight is the metric: 40/100 warmth at 180 gsm is a fair ratio for packable insulation.