Occlusives do not add water, they refuse to let it leave

Everything here earned its place by changing an outcome. Nothing about occlusive layers is included to round the number up.

The mechanism in one sentence

Skin constantly loses water upward through the surface, a process called transepidermal water loss that continues every hour whether you notice it or not. An occlusive is a material that forms a continuous hydrophobic film across the surface and physically slows that evaporation. It supplies no water of its own, which is why the timing of the application decides whether it helps or does nothing.

Applied over damp skin it holds water that is already there; applied over dry skin it preserves a dry surface faithfully. Every argument about occlusives comes back to that distinction, and most disappointment with them comes from ignoring it.

What counts as one

Petrolatum forms the most complete film of the common options and is chemically inert, which is why it appears in medical dressings. Mineral oil, lanolin, beeswax, shea butter and heavier silicones such as dimethicone all form films of varying completeness.

Plant oils are only weakly occlusive, since their molecules are smaller and the film they leave is far more permeable than petrolatum. Cosmetic grade petrolatum is refined to remove the aromatic compounds people worry about, and that refining is a regulatory requirement in many markets. The choice between them is mostly about texture and how a heavy film feels rather than about efficacy differences you would notice.

Humectant, emollient, occlusive

A humectant such as glycerin attracts and binds water, an emollient such as squalane fills the gaps between surface cells, and an occlusive seals the lot. Most moisturisers combine all three, and the ratio between them is what separates a light gel from a thick balm. Dehydrated skin usually needs the humectant, rough skin usually needs the emollient, and skin losing water fast needs the occlusive.

After four to six weeks, reaching for a heavier cream when the actual shortage is water produces a greasy surface that still feels tight underneath. Naming which of the three your skin is short of is more useful than any product recommendation.

The slugging question

Applying a thin layer of petrolatum as a final night step suits dry, compromised or wind-exposed skin because it holds water in for hours. It should sit over a moisturiser on skin that is already hydrated, since a seal over nothing preserves nothing.

People whose skin congests easily may find a full occlusive layer unhelpful across the whole face, and using it only on dry patches is a sensible compromise. Trapping an active such as a retinoid or an acid beneath petrolatum increases its effect and its irritation potential in equal measure. Keep occlusives and strong actives on separate nights unless a doctor has specifically suggested otherwise.

Where occlusives shine

Cold, dry air with low humidity is where transepidermal water loss climbs fastest, which is exactly where a sealing layer earns its place. Cracked hands, lips and heels benefit most, since those areas have few oil glands and are constantly re-exposed. Overnight is the natural window because water loss rises during sleep and nothing is going to rub the layer off.

On a compromised barrier, aircraft cabins and centrally heated rooms create the same low-humidity conditions and respond to the same fix. The cheapest tub in the pharmacy performs as well here as anything with a designer label on it.

Patch test anything new, and introduce one active at a time so you can tell what did what.

Where they are the wrong tool

Hot humid weather is the classic mismatch, because water loss is already low and a heavy film simply sits there feeling unpleasant. An occlusive over sunscreen can disturb the film that the sunscreen relies on, so keep them at opposite ends of the day. Using petrolatum on skin that feels tight without any water underneath produces the frustrating combination of greasy and dehydrated.

After four to six weeks, open, weeping or infected skin is a medical situation, and what to put on it is a question for a doctor or pharmacist. An occlusive is a tool with one job, and it does that job perfectly as long as you supply the water yourself.

Everything above, in order of what to do first

1. The mechanism in one sentence. Skin constantly loses water upward through the surface, a process called transepidermal water loss that continues every hour whether you notice it or not.
2. What counts as one. Petrolatum forms the most complete film of the common options and is chemically inert, which is why it appears in medical dressings.
3. Humectant, emollient, occlusive. A humectant such as glycerin attracts and binds water, an emollient such as squalane fills the gaps between surface cells, and an occlusive seals the lot.
4. The slugging question. Applying a thin layer of petrolatum as a final night step suits dry, compromised or wind-exposed skin because it holds water in for hours.
5. Where occlusives shine. Cold, dry air with low humidity is where transepidermal water loss climbs fastest, which is exactly where a sealing layer earns its place.
6. Where they are the wrong tool. Hot humid weather is the classic mismatch, because water loss is already low and a heavy film simply sits there feeling unpleasant.

The takeaway

Wet the skin first, then seal it, because a lid over an empty jar keeps nothing.

Skin changes slowly. Anything promising otherwise is selling the promise.