Moisturisers and the skin barrier: what a balm can and cannot do

Brick and mortar, lamellar lipids and transepidermal water loss explained, then an honest account of what a balm actually achieves on skin.

Almost every claim made for a moisturiser depends on a single structure roughly a tenth of a millimetre thick. Understanding what the stratum corneum is, and what it is made of, makes it obvious why some products work quickly, why others feel good and change nothing measurable, and why a balm behaves so differently from a cream. It also makes clear where the honest limits of a tub of oil and wax lie.

Bricks and mortar: what the barrier actually is

The outermost layer of skin, the stratum corneum, is typically 10 to 20 micrometres thick over most of the body and considerably thicker on palms and soles. It is often described as bricks and mortar, and the model holds up well.

The bricks are corneocytes: flattened, dead, protein-filled cells, roughly 15 to 20 layers deep, each surrounded by a tough cross-linked envelope and packed with keratin. Inside them sits natural moisturising factor, a mixture of amino acids, pyrrolidone carboxylic acid, lactate, urea and salts that is intensely water-attracting and keeps the cells plasticised.

The mortar is the interesting part. Between the corneocytes is a continuous, highly organised lipid matrix arranged in stacked bilayers called lamellae. That lipid matrix is the actual barrier. Water crossing the stratum corneum has to travel a long, tortuous path through those lipid layers, which is why an intact barrier is so effective and why disrupting the lipids raises water loss immediately.

What the mortar is made of

Three lipid classes dominate, in proportions that are unusual and specific. Figures below are approximate, expressed by mass, and vary with body site, season, age and analytical method.

Note what is not in that list: triglycerides. Almost every plant oil used in balms is a triglyceride, and triglycerides are not a component of the lamellar barrier lipids. Sebum, which sits on the surface, does contain them, along with squalene and wax esters. So a plant oil resembles the skin's surface film more than it resembles the barrier itself, and this is the first reason to be cautious about "replenishes your skin's natural lipids" copy.

Transepidermal water loss, and what it does not tell you

Water diffuses out through the skin continuously, independent of sweating. That flux is transepidermal water loss, measured in grams per square metre per hour with an evaporimeter. Typical published values for undamaged forearm skin at rest in a controlled room fall in the region of 5 to 10 g/m2/h, but the figure is exquisitely sensitive to room humidity, air movement, skin temperature, recent washing, the instrument used and how long the subject has been sitting still. Comparing TEWL numbers between studies is generally not meaningful; comparing treated and untreated sites within one study is.

TEWL is a barrier measure, not a dryness measure. Skin can feel dry and rough with normal TEWL, and skin can have raised TEWL without looking dry. The two commonly used instruments answer different questions: an evaporimeter measures the flux out, a corneometer measures the electrical capacitance of the surface layer as a proxy for how much water is in it. A good product can move either, and moving one does not guarantee moving the other. This is worth knowing because most product claims quote whichever measurement moved.

The three routes a moisturiser can take

Given that structure, there are only a few available mechanisms, and they map onto the three functional classes described in occlusive, emollient and humectant.

1. Slow the loss. Put a hydrophobic film over the surface so water has an extra layer to diffuse through. This is occlusion, and it works within minutes.
2. Add or attract water. Supply water directly (only possible in a product containing water) or supply humectants that hold it. This is what an emulsion does and what an anhydrous product structurally cannot.
3. Supply lipids and smooth the surface. Fill the gaps between lifted corneocytes, restoring flexibility and light reflection. This is emolliency, and it is immediate and mostly cosmetic, though genuine physiological lipids can go further.

A balm takes routes one and three, comprehensively, and route two not at all. That is the whole story of the format, and it is set by the chemistry described in anhydrous formulation rather than by any choice a formulator makes.

What a balm can genuinely do

Several things, and they are not trivial.

It reduces water loss immediately and persistently, more persistently than an emulsion because nothing evaporates from it and the whole applied mass stays where it was put. It smooths the surface, which changes both feel and appearance within seconds. It reduces friction, which matters more than people expect on heels, hands and lips. It provides physical protection against wind, cold, water and repeated wetting, which is the mechanism behind the skin protectant category in the US drug monograph. And by holding water in the outer layers it plasticises the corneocytes, which is what makes skin flexible rather than tight.

There is also reasonable evidence that reducing water loss supports the skin's own repair processes, since the epidermis appears to respond to the water gradient across it. That argument is why petrolatum performs well in barrier-recovery studies despite being biologically inert.

What a balm cannot do

It cannot add water. If skin is dehydrated in dry air and you apply a balm to dry skin, you have sealed in a low water content.

It cannot rebuild the barrier's own lipid composition. Applying shea butter does not supply ceramides in the right ratio to cholesterol and free fatty acids, and work on physiological lipid mixtures found that incomplete mixtures could actually delay barrier recovery rather than help it. A triglyceride balm is not a lipid replacement therapy, whatever the packaging implies.

It cannot deliver water-soluble actives, it cannot change the shedding process the way a keratolytic does, and it does not treat any condition. What it does is buy the barrier time: it lowers the demand on a compromised barrier while the skin does its own work. That is a modest, real and useful claim, and it is the one worth making. For dry patches specifically, what the evidence supports for eczema-prone skin looks at the trials rather than the marketing.

Why lips are a different problem

The vermilion, the coloured part of the lip, is a genuinely unusual bit of skin and the reason lip products exist as their own category.

Its stratum corneum is markedly thinner than facial skin, which is part of why the blood beneath shows through as colour. It has no hair follicles and, across most of its surface, no sebaceous glands, so it produces very little of its own surface lipid film. Published comparisons of the vermilion with adjacent facial skin report a weaker barrier and higher water loss than neighbouring skin, which is consistent with that anatomy.

On top of the anatomy sit three behaviours that no other skin site faces. Saliva wets the lips repeatedly, then evaporates and takes surface water with it, and saliva also contains digestive enzymes. Lips are mechanically abraded by eating, drinking and talking. And they receive the highest reapplication frequency of any product, which means any irritant in a lip balm gets an unusually large number of exposures. Those three together explain most of what people describe as balm dependency; the mechanisms are worked through in does lip balm dry your lips, and the ingredient consequences in ingredients for chapped lips.

What follows in practice

Four things, all of which are cheap to do.

Apply to damp skin. Within roughly three minutes of washing or bathing, while a film of water is still present. This is the only route by which an anhydrous product participates in hydration rather than just retention.

Layer rather than choose. A humectant-containing cream underneath and a thin balm over the top gives you all three mechanisms. Choosing between a balm and a cream is often a false choice.

Use less than you think. All of a balm stays on the skin, unlike a cream, so the correct dose is small. How much balm to apply puts numbers on it.

Adjust for the season. Cold outdoor air holds very little water and indoor heating dries the air further, so the same product that felt heavy in July is correct in January. Cold weather skin covers the seasonal adjustment, and an overnight lip mask is essentially an occlusion strategy applied at the time of day when nothing will rub it off.

The barrier is a structure you can support and inconvenience but not rebuild from a tin. Most of what a good balm does is remove obstacles: less friction, less evaporation, less repeated wetting and drying, fewer irritants. That is a smaller claim than the industry usually makes and it is the one that holds up.

Reviewed and updated 6 September 2026. Spotted an error? Tell us and we will fix and log it.