What dry skin is missing
Dry skin is often short on water in its outermost layer, the stratum corneum. This layer is made of flattened skin cells held together by fats that help keep moisture in and irritants out. When that barrier is disrupted, water escapes more readily, leaving the surface rough, tight, or flaky.
A seal slows water loss—it doesn’t add water
Vaseline is petroleum jelly, an occlusive: a substance that forms a barrier over the skin and reduces water loss. That can help protect the surface while the barrier recovers, but the jelly itself does not hydrate the skin by supplying water. Applied to already-dry skin, it can trap the little moisture present without replacing what has been lost.
That is why the order matters. A moisturizer containing water can add moisture to the outer layer; ingredients such as glycerin help hold onto it, while skin-compatible fats help support the barrier. Applying petroleum jelly over a moisturizer can then slow that added water from evaporating. It works as a finishing seal, rather than as the drink itself.
Why it can still help
Petroleum jelly is not useless for dryness. By reducing further water loss, it may protect vulnerable skin and give its barrier time to recover. But if the skin feels dry, the practical distinction is simple: add moisture first, then seal it in. The lid helps most when there is something underneath to keep.
