A solid choice — #9 of 31 Flannel ranks #9 of 31 fabrics for warmth without weight. It is a good fabric for this. Ranked on warmth relative to weight, with a packability bonus. The top pick is Cashmere.
Flannel measures 58/100 for breathability, 12% moisture regain, 80/100 warmth, at 260 gsm. Warmth-to-weight is the metric: 80/100 warmth at 260 gsm is an excellent ratio for packable insulation.