How Moisturising Actually Works: Occlusives, Humectants, Emollients, and Ratios by Skin Type

Moisturising is talked about too abstractly. It answers one engineering question: water leaves the stratum corneum, and you have three moves - grab it, fill the gaps, or cap the exit. Those map to humectants, emollients, and occlusives. Healthy facial TEWL runs about 6 to 14 g per hour per square metre; dry or damaged skin rises clearly above that, and this number - not the slick feel after applying - is the honest test of whether moisturising works.

The corneum holds water through natural moisturising factor: free amino acids, pyrrolidone carboxylate, lactate, urea - two to three tenths of its dry weight, mostly from filaggrin breakdown. Over-cleansing, dry air, and age all shrink that reserve, so the point of moisturising is not to pour water in but to rebuild the structure that keeps and caps it.

Humectants: they grab water, but watch the source

Glycerin is the most studied humectant, effective around 3% to 10% in formula, and it also improves lipid phase behaviour and filaggrin processing. Urea does double duty: below 5% mostly humectant, 5% to 10% begins mild keratolysis, above 10% for thick heels and elbows. Sodium lactate and sodium PCA directly refill natural moisturising factor. Hyaluronate splits by molecular weight - large stays on the surface as a film, small penetrates deeper but in dry air can become an evaporation channel if overused. Panthenol at 1% to 5% humects and mildly soothes.

The precondition is ambient humidity. Below 40% relative humidity, a high-strength humectant alone can pull water out of the corneum and leave skin drier. The fix is a humectant followed by an occlusive or emollient to trap what it grabbed; in an air-conditioned room also lift humidity to 45% to 55%.

• Glycerin 3% to 10%, the most basic and steady
• Urea 5% to 10% for roughness and buildup
• Hyaluronates need a follow-up occlusive; alone they fall short
• In dry air, do not finish with a watery serum and stop

Humectants pour water into a sieve; occlusives plate the bottom.

Emollients: fill the gaps and feed lipids

Emollients slip into the spaces between corneocytes and press down the lifted scales, so the surface reads smooth and evenly reflective. Squalane is stable and low-clogging, good for most; fatty-acid esters feel light; vegetable oils vary by fatty-acid profile - safflower and sunflower, high in linoleic acid, suit barrier, while olive, high in oleic acid, raised barrier permeability in some work and is poor for sensitive or infant skin. The lipids that actually rebuild structure are physiological: ceramide NP, AP, and EOP with cholesterol and free fatty acid near a 3:1:1 ratio, which rearranges the bilayer fastest.

A quick call: if skin feels rough with fine lifted scales, lead with emollient; if it only feels tight but looks smooth, lead with humectant plus occlusive. A ceramide-complex cream at $16 to $48 is worth the spend during barrier repair.

• Squalane: general-purpose, even oily skin tolerates it
• Ceramide complex: first choice in barrier repair
• High-linoleic oils beat high-oleic ones
• Rough lifted scales: emollient before acid, safer

Emollients fill gaps; lipids rebuild the wall.

Occlusives: the only class that directly cuts loss

Petrolatum cuts TEWL by about 98% in classic in vitro work, is the most efficient occlusive, and sensitises almost never; medical petrolatum at $3 to $7 lasts a long time. Mineral oil follows; dimethicone and cyclopentasiloxane give a breathable, non-stuffy seal; beeswax and lanolin work but lanolin carries some sensitising risk. Occlusives add no water, so they only matter layered over a water-bearing step - a dry skin that applies petrolatum over a damp face after cleansing outperforms petrolatum on dry skin.

The 'seal method' suits winter nights, planes, heated rooms, and hands and feet; two to three times a week is enough. Acne and oily skins that seal the whole face easily breed closed comedones, so use it only on the mouth corners, under-eyes, and cheek sides. The thin corneum and few sebaceous glands of the eye and lip areas benefit most from an occlusive.

A common misstep is applying occlusive balm to already-dry, flaking skin and calling it fixed; without a humectant underneath there is no water to trap, so the seal just sits on top. The order matters: water or a humectant layer first, then the occlusive while the surface is still damp. In very dry rooms this single sequencing change resolves more tightness than buying a more expensive cream.

• Petrolatum: best-value occlusive
• Silicones: light, non-stuffy, good for combo-oily
• Whole-face seal caps at two to three times a week
• Breakout zones: avoid heavy occlusive, use a gel

Humectants pour water into a sieve; occlusive is the layer that plates the sieve's bottom.

Ratios by skin type

Dry skin: all three. Within three minutes of washing, a glycerin-and-panthenol layer, then a ceramide cream, with a night seal in winter. Oily skin: humectant-led, an oil-free gel or silicone lotion, low oil content, occlusive only on local dry spots; oily skin also dehydrates, so tightness is not the cue to strip more oil. Combination: a thin lotion all over, a second cream layer on cheeks and under-eyes. Sensitive: the shorter the ingredient list the better - glycerin, squalane, ceramide, panthenol, nothing high in fragrance or essential oil.

Texture tracks oil content as a quick shop guide: gel is near oil-free, lotion about 10% to 20% oil, cream about 20% to 40%, balm and ointment higher. When a brand's same line shifts season to season along this gradient, you control variables better than swapping the whole routine.

Age shifts the ratio as much as skin type does. From the forties, sebum and natural moisturising factor both fall with hormonal change, so even lifelong oily skins often need more emollient and a night occlusive they once avoided; the dry-skin playbook starts applying. Conversely, a teenager with acne needs a humectant-led, non-comedogenic texture and must not be talked into rich creams that clog. Read the skin in front of you, not the age on the chart, and move along the oil gradient rather than assuming one formula fits a decade. When trialling a new texture, test it on the inner arm for a day; a sudden flare there predicts a facial one and costs far less to discover.

• Dry: humectant + ceramide cream + local seal
• Oily: oil-free gel or light lotion, oil only local
• Combo: zone layering, no need to unify the face
• Sensitive: the shorter the INCI, the safer

Read the skin, not the age, when choosing the ratio.

Body sites are not one problem

The periorbital corneum is under half the facial average thickness and short on sebaceous glands, so it shows dry lines and fine flaking first; a thicker, simple eye cream or a dab of face cream works, kept off the lash line. The mouth and nasolabial folds chafe from wiping and lip-licking into an eczema-like flake; the day fix is repeated petrolatum or a ceramide balm. The neck has few sebaceous glands but moves constantly and creases transverse; give it facial-class amount, not a casual sweep. Hands wash many times a day and need the most frequent reapplication.

Body sites differ more: the shin and elbows are thick and sebaceous-poor, first to show ichthyotic scale in winter, where a 5% to 10% urea or 12% ammonium lactate body lotion fits; torso and upper arms usually need only a standard body lotion.

• Periorbitals: thick texture, simple, off the lash line
• Mouth flaking: petrolatum several times through the day
• Neck gets facial-class amount
• Shins and elbows: urea or lactate products

The eye and the shin obey different rules.

How to tell moisturising is enough

Three self-tests. One, wash and apply nothing for five minutes; tightness, sting, or shine signals over-cleanse or under-moisturise. Two, watch the base makeup: flaking and patchy pickup within two hours usually means low corneum water, not the foundation. Three, watch the night recovery: if you wake drier than you went to bed, the night seal is insufficient or the room too dry. Perceptible improvement comes within two weeks; barrier markers steady around four.

Also tell apart conditions mistaken for dryness. Symmetrical red, weeping, sleep-breaking itch may be eczema or contact dermatitis; widespread ichthyotic scale from childhood with family history needs a genetic keratosis work-up; persistent mid-face redness with telangiectasia suggests rosacea. Moisturiser alone will not settle these; a clinician can add topical anti-inflammatories.

When even a correct emollient fails after four weeks, suspect a disease rather than a weaker product: persistent weeping or crusting, intense itch that breaks sleep, or a spreading red rash with scale needs a clinician, who may prescribe a topical anti-inflammatory. Moisturiser is support, not treatment, for eczema or allergic contact dermatitis, and pressing on with thicker creams only traps heat and delays the right diagnosis. The four-week line is a useful tripwire: past it, identify before you emolliate further.

• Five-minute nothing test for tightness after washing
• Base makeup flaking within two hours is low water
• Improvement felt by two weeks, steady by four
• Weeping, severe itch, wide scaling: see a doctor

Four weeks of failure means identify, not emolliate harder.

Moisturising earns repeating because it is both the fix for a problem and the floor under every other treatment: a steady barrier gives actives more tolerance room and speeds recovery from acids and lasers. In execution, remember three things - humectants grab water but need a follow-up occlusive, emollients fill gaps and feed lipids, and occlusives are the only class that directly lowers water loss. Set the ratios by skin type, the amounts by site, the texture by season, and spend the rest on observation: the tightness test, the base-makeup read, the morning state. Those three signals tell you the direction. Once this layer is steady, much of what gets filed under sensitive, dull, or rough simply disappears. Finally, judge moisturiser by behaviour, not by feel alone - a product that leaves a glamorous slip for an hour but ends the day tight has failed the only test that counts, which is whether transepidermal water loss stayed down and the skin stayed comfortable.