A compromise — #17 of 31 Organza ranks #17 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.
Organza measures 72/100 for breathability, 8% moisture regain, 18/100 warmth, at 40 gsm. Warmth-to-weight is the metric: 18/100 warmth at 40 gsm is an excellent ratio for packable insulation.