A compromise — #19 of 31 Tweed ranks #19 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.
Tweed measures 50/100 for breathability, 14% moisture regain, 82/100 warmth, at 350 gsm. Warmth-to-weight is the metric: 82/100 warmth at 350 gsm is a fair ratio for packable insulation.