Why does silk feel dry against the skin even though it absorbs water vapor, according to the concept of moisture regain?

Answer: Water vapor enters the fibers themselves, so the cloth can feel dry while holding moisture.

Silk feels dry because its fibers push water vapor away instead of taking it in. — This explains dryness by making silk repel vapor, which sounds plausible for a smooth fiber. But silk has a measurable moisture regain, so it does take up water vapor rather than shutting it out. The dry feel comes from where that vapor goes, not from total rejection.

Water vapor enters the fibers themselves, so the cloth can feel dry while holding moisture. ✓ — Correct. Moisture regain measures water vapor held by a fiber relative to its dry weight, not a wet film on the surface. Once vapor is inside the fiber, the cloth can contain moisture without immediately feeling damp.

The surface should feel wet, because vapor must form a film before entering the fibers. — This gets the idea of absorption partly right, because water does end up in the fabric. But moisture regain counts vapor taken into the fiber, not liquid sitting as a visible layer on top. A surface can look and feel dry while the fiber itself holds water vapor.