Excellent choice — #2 of 31 Alpaca ranks #2 of 31 fabrics for warmth without weight. It is one of the best fabrics for this. Ranked on warmth relative to weight, with a packability bonus. The top pick is Cashmere.
Alpaca measures 58/100 for breathability, 14% moisture regain, 88/100 warmth, at 220 gsm. Warmth-to-weight is the metric: 88/100 warmth at 220 gsm is an excellent ratio for packable insulation.