Humectants need a lid
Hydrating serums have a strange reputation. Half the people using them describe skin that looks plumper by the afternoon, and the other half say the stuff does nothing, or leaves their face tighter than before they started. Both groups are usually telling the truth. The difference is what they put on top, and what the air around them was doing.
What a humectant does
A humectant pulls water into the upper layers of your skin and helps hold it there. It takes that water from two places: from deeper in your own skin, and from the air around you. Glycerin is the reference example and the most-used moisturizing ingredient there is, appearing in roughly 69 percent of the skincare products in a catalog of over a hundred thousand. Hyaluronic acid, urea, sodium PCA and panthenol all belong to the same category.
When it works against you
The direction of that flow depends on the humidity around your face. Above roughly 40 percent relative humidity, a glycerin layer moves water into your skin. Below about 30 percent, the same layer can pull water out of it instead and hand it to the air.
Rooms with the heating running sit in that low band. So do air-conditioned offices and plane cabins. Put a hydrating serum on in one of those, apply nothing over it, and you can end up worse off than if you'd skipped the step. It is one reason people say hyaluronic acid does nothing for them. The serum was often fine. It just had nothing over it, in air that was thirstier than the person wearing it.
Molecular weight matters here too. Large hyaluronic acid, around 1000 to 1400 kDa, stays on the surface and forms a smoothing film. Smaller fragments, roughly 20 to 300 kDa, cross into the outer layer. Formulas carrying both sizes tend to be the most useful, and the effect compounds with repetition: measured on real skin, residual hyaluronic acid sat at eight times baseline a day after a single application and thirty-one times baseline after seven days of daily use.
Damp skin, then a lid
Two rules cover almost all of it.
Apply humectants to skin that's still damp. Straight out of the shower, or after cleansing while your face is barely towelled, there's water on hand for the layer to hold. On a bone-dry face it has to source that water from your own skin.
Then seal. An occlusive doesn't add water at all; it lays a film on the surface that slows water escaping, so it works best over something hydrating rather than alone. Petrolatum and mineral oil sit at the strong end of that range, silicones and plant butters in the middle, plant oils and light esters at the gentle end. Emollients like squalane sit alongside them, softening the surface and slowing evaporation without a greasy finish. If you can't tell whether a jar you own has anything sealing in it, the Skin Bliss app can scan the label and lay out the full ingredient list, so you can look for those oils and silicones yourself.
What a sealed layer looks like
In the morning: hydrating serum on damp skin, then a lightweight moisturizer over it. A gel-cream is plenty. You want something on top, not a wall.
At night: the same order with more weight behind it. Serum on damp skin, a richer cream over that, and in a cold snap or on a stretch of very dry skin, a thin layer of balm over everything to hold it overnight. That last move is what slugging is, and the next few steps in this course go into when it earns its place and when it backfires.
The order is the point. Water first, then the thing that keeps it there. A heavy cream over nothing is a lid on an empty pot.