Klairs Skin School

What Is Transepidermal Water Loss (TEWL)?

Transepidermal water loss, or TEWL, sounds technical but explains a lot about why skin feels dry. Understanding it makes hydration make more sense. What is transepidermal water loss?Transepidermal water loss is the natural process of water evaporating from the skin's surface into the air. Some TEWL is normal, but when the skin barrier is compromised or the air is very dry, water escapes faster, which leaves skin feeling tight, dry, and less comfortable. How do you reduce excess water loss?You reduce excess water loss by supporting the barrier and sealing in hydration: layer humectants, then follow with a moisturizer that helps slow evaporation. Barrier-supportive ingredients like ceramides help too. A Rich Moist Barrier Capsule Cream is an example, and our barrier guide explains more. Why does it matter for your routine?Understanding TEWL is why sealing hydration matters: humectants draw in water, but without a moisturizer on top, more of it can evaporate, especially in dry air. That is the reasoning behind finishing a routine with a moisturizer. FAQ What is transepidermal water loss?It is the natural evaporation of water from the skin's surface into the air. Some is normal, but a compromised barrier or dry air speeds it up, leaving skin dry and tight. How do you reduce TEWL?Support the barrier and seal in hydration by layering humectants and following with a moisturizer, ideally with barrier-supportive ingredients like ceramides. Does dry air increase water loss?Yes. Low-humidity air, including heated indoor air, speeds up evaporation from the skin, which is why skin feels drier in winter. Why seal hydration with a moisturizer?Humectants draw water in, but a moisturizer on top helps slow its evaporation, so more of the hydration stays in the skin. Why sealing mattersTEWL is water escaping from skin. Support the barrier and seal hydration with a moisturizer, and skin stays comfortable, especially in dry air.