If you have ever bought a moisturizer because the label promised “deep hydration,” slathered it on faithfully every night, and still woken up to skin that felt tight and dull by morning, you were probably missing something. Not the moisturizer itself, necessarily, but an understanding of what it was actually doing and what it was not built to do on its own.
The skincare industry tends to use words like “hydrating” and “moisturizing” as though they mean the same thing. They do not, and the distinction matters enormously when you are trying to build a routine that actually holds moisture in the skin for more than an hour. At the heart of that distinction is a trio of ingredient categories: humectants, emollients, and occlusives. Each one does a different job, works at a different level of the skin, and becomes significantly more effective when paired with the other two. Understanding all three is the clearest path to understanding why skin flooding works.
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Humectants: The Water Magnets
Humectants are ingredients that attract water. They do this through a process called hygroscopy: they have a strong chemical affinity for water molecules and pull them toward themselves from two available sources, the environment around the skin and the deeper layers of the skin below the surface. When you apply a humectant to damp skin, it draws that surface moisture into the upper layers of the stratum corneum, plumping the cells and temporarily improving the skin’s ability to hold water.
The result of a well-applied humectant is visible almost immediately. Skin looks fuller, fine lines appear softer, and the overall texture becomes smoother. This is not a long-term structural change; it is more like the skin taking a long drink of water. The plumping effect is real and beneficial, but it is also temporary without something in place to keep that water from evaporating straight back out.
Common Humectant Ingredients
Hyaluronic acid is the humectant most people recognize by name, largely because of how aggressively it has been marketed over the past decade. It is genuinely impressive: a single molecule can hold up to a thousand times its own weight in water, and products formulated with multiple molecular weights can attract moisture at different depths within the skin. Glycerin is arguably more reliable in everyday use, cheaper to formulate with, and effective across a wider range of humidity conditions. Other worthwhile humectants include sodium PCA, panthenol, aloe vera, honey, beta-glucan, and urea, which doubles as a mild exfoliant at higher concentrations. Each has slightly different properties, but all share that core capacity to draw water in and hold it.
The Humidity Caveat
There is one important limitation that does not get discussed nearly enough. In very low-humidity environments, humectants applied to completely dry skin can actually draw moisture upward from the deeper layers of the dermis rather than pulling it in from the air. This is the reverse of what you want: the humectant is essentially borrowing water from Peter to pay Paul, and the skin can end up feeling drier than before. Applying humectants to damp skin sidesteps this problem neatly, which is one of the core reasons skin flooding specifies working on skin that is still wet from cleansing.
Emollients: The Smoothers and Repairers
Where humectants are about attracting water, emollients are about what happens to the skin’s surface and structure once that water is present. Emollients fill in the microscopic gaps between skin cells, the spaces in the lipid mortar of the stratum corneum, smoothing rough texture and making the skin feel soft and supple to the touch. They are the ingredient category most responsible for the sensory experience of a good moisturizer.
Beyond texture, emollients contribute something more substantive. Many of them share a structural similarity with the lipids naturally found in the skin barrier and can help restore the barrier’s organization over time with consistent use. This is a slower benefit than the immediate plumping of a humectant, but it is more durable. A skin barrier with a well-organized lipid structure is intrinsically better at retaining moisture than one held together with occlusives alone.
Common Emollient Ingredients
The emollient category is broad and includes both oils and butters as well as certain synthetic ingredients. Squalane, derived from olives or sugarcane, is a lightweight emollient that closely resembles the skin’s own sebum and is well tolerated by almost every skin type, including acne-prone skin. Shea butter, jojoba oil, and rosehip oil are richer options that suit drier skin types particularly well. Ceramides technically span both the emollient and barrier-repair categories: they are lipids that integrate directly into the stratum corneum and help restore its structural integrity rather than simply filling gaps temporarily. Fatty alcohols like cetyl alcohol and stearyl alcohol are emollients commonly found in cream formulations, where they contribute to a smooth, non-greasy texture.
Occlusives: The Sealers
Occlusives are the final piece of the puzzle, and they work differently from both humectants and emollients. Rather than attracting water or integrating into the skin’s structure, occlusives form a physical film on the skin’s surface that slows or blocks the evaporation of water outward. They are, in a very direct sense, a lid on everything that has been applied beneath them.
This makes occlusives the ingredient category most directly responsible for reducing transepidermal water loss. They do not hydrate the skin themselves; they preserve the hydration that humectants have drawn in and keep it from escaping while the skin absorbs and benefits from everything below the surface. Applied without humectants beneath them, a thick occlusive on dry skin mostly just seals in dryness. Applied as the final layer over well-saturated skin, it becomes genuinely transformative.
Common Occlusive Ingredients
Petrolatum, the active ingredient in plain petroleum jelly, is the most effective occlusive available and one of the most thoroughly studied ingredients in dermatology. It reduces transepidermal water loss by more than 98 percent under controlled conditions and is non-comedogenic in its pure form despite its reputation. Dimethicone is a silicone-based occlusive widely used in lighter formulations that want sealing power without heaviness. Beeswax, lanolin, mineral oil, and plant-based waxes like candelilla all offer varying degrees of occlusion. For those who find straight petrolatum too heavy or prefer a more natural approach, a few drops of a denser plant oil such as marula, castor, or sea buckthorn applied as a final layer provides a lighter but still meaningful sealing effect.
Why All Three Work Best Together
Each category has a clear weakness when used in isolation. Humectants attract water but cannot prevent it from evaporating. Emollients smooth and repair but do not dramatically reduce water loss on their own. Occlusives seal effectively but have nothing to seal in if the skin beneath them is already dehydrated. The categories are genuinely complementary, and the sequence in which they are applied is just as important as having all three present.
The logic of layering thin to thick follows directly from this. Humectants go on first because they need contact with damp skin to work properly and because their lightweight, water-based formulas would not penetrate through a layer of emollient or occlusive placed before them. Emollients go on next, smoothing the surface and beginning to organize the lipid structure. Occlusives go on last, forming that preserving film over everything that has been built up beneath. Remove any one layer and the system becomes noticeably less effective. Use all three in order and you have, in practical terms, replicated what a well-functioning skin barrier does for itself on a good day. Skin flooding is simply the disciplined application of exactly this logic.