How does a vacuum flask work?
Short answer
A vacuum flask prevents heat from entering or leaving by stopping the three modes of heat transfer: conduction, convection, and radiation. A vacuum between the double walls leaves virtually no air molecules to conduct or convey heat. A reflective coating reduces thermal radiation, while a tight-fitting stopper minimizes air movement at the neck.
about a 20-second read
Stopping the three paths of heat transfer
Heat naturally moves from warmer to cooler areas through conduction, convection, and radiation. Inside a vacuum flask, conduction and convection are almost completely eliminated because air between the walls is evacuated to create a vacuum. Without gas molecules, neither direct contact nor moving air currents can transport thermal energy. Thermal radiation, the third path, is slowed by reflective surfaces that bounce heat waves back toward the liquid.
Essential components of the flask
Inner vessel and outer vessel
The flask consists of two separate containers placed one inside the other, made of glass or stainless steel, joined together only at the neck.
Double walls with a vacuum gap
The space between the inner and outer containers is evacuated of air. This insulating void prevents heat from conducting between the inside and outside of the flask.
Reflective coating
The surface of the inner container is coated with a thin metallic or silver layer that reflects infrared radiation back toward the contents or outward.
Insulating lid
Because the neck is the only junction between the inner and outer walls, the stopper is made with dense, poorly conducting materials to prevent heat loss at the top.
Invented for liquefied gases
The vessel was invented in 1892 by Scottish physicist and chemist Sir James Dewar to store extremely cold liquefied gases for laboratory research. Dewar never patented his invention, allowing German glassblowers to later commercialize the technology for daily home use under the Thermos brand.
Does not generate heat or cold
A common misconception is that a vacuum flask actively heats or cools beverages inside it. In reality, it is entirely passive and functions solely by delaying temperature change, keeping hot items hot and cold items cold.
Summary
A vacuum flask maintains a liquid's temperature by combining a vacuum between double walls with a reflective surface and an insulating stopper. The vacuum stops conduction and convection, the mirror surface reduces radiation, and the stopper seals the neck against heat loss.
Sources
- Vacuum Flask - HyperPhysics — Gsu (Accessed 2026-09-12)
- Vacuum flask | Thermal Insulation, Heat Retention, Double-Walled — Britannica (Accessed 2026-09-12)
- Hur fungerar en termos? - På Fjället — Pafjallet (Accessed 2026-09-12)
- How do Thermos vacuum flasks work? - Explain that Stuff — Explainthatstuff (Accessed 2026-09-12)
- Vacuum flask - Wikipedia — Wikipedia (Accessed 2026-09-12)
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