What's the difference between a waterproof and a breathable pillow protector?

Waterproof protectors use a membrane backing that blocks liquids and sweat but can trap heat and feel crinkly to some sleepers, a complaint that recurs in verified-purchase reviews. Breathable cotton, bamboo viscose, or microfiber protectors prioritize airflow and a quieter feel but offer less liquid protection. Pick based on whether spill/sweat defense or cool, silent comfort matters more to you.