Moisture Sandwich

The moisture sandwich is a sequence: dampen the skin, apply a humectant layer, optionally repeat, then seal with a moisturiser or occlusive. The point is to give the humectants a supply of water at the surface and then stop it evaporating.

The reasoning is sound and specific. Humectants like glycerin and hyaluronic acid pull water from wherever it is available. In humid conditions or onto damp skin, that source is the environment and the applied water. In dry air, onto dry skin, a humectant can instead draw water upward from the deeper layers of the epidermis, where it then evaporates — which is the mechanism behind the common complaint that hyaluronic acid made skin feel tighter. Sealing the layer is what converts a humectant from a pump into a reservoir.

Practically it matters most in low humidity, in winter, on a plane, and for anyone whose barrier is already compromised. It matters least in a humid climate where the air is doing the work anyway.

Two cautions. Damp skin increases penetration of everything, so this technique is for hydrating layers rather than for actives — sandwiching an acid or a high-strength retinoid increases irritation rather than comfort. And an occlusive on acne-prone skin should be chosen with that in mind; sealing the layer does not require petrolatum on every skin type.