A solid choice — #13 of 31 Polypropylene ranks #13 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.
Polypropylene measures 40/100 for breathability, 0% moisture regain, 40/100 warmth, at 120 gsm. Warmth-to-weight is the metric: 40/100 warmth at 120 gsm is an excellent ratio for packable insulation.