On a warm humid day, why can the same 27 C room feel much worse once you start sweating?

Answer: Sweat evaporates poorly

Air holds less heat — Humid air does not feel worse because it holds less heat; if anything, moist air can carry slightly more heat energy. The important comfort bottleneck is what happens at wet skin. When the air is already loaded with water vapor, it accepts less additional vapor from sweat. That blocks one of the body's main emergency cooling routes.

Sweat turns insulating — Sweat sitting on the skin can feel sticky, but it is not mainly acting like a sweater. Sweat cools only when liquid water becomes vapor and carries latent heat away. If evaporation stalls, the sweat remains wet without buying much cooling. The misconception is useful because it separates being wet from actually losing heat.

Sweat evaporates poorly ✓ — Correct. Evaporation is powerful because it carries heat away as water vapor leaves the skin. High relative humidity narrows the vapor-pressure difference between skin and air, so sweat evaporates more slowly and cooling weakens. This is why humidity may feel like a minor detail in a cool dry office but becomes decisive during heat, exercise, or poor ventilation.