What's the difference between thermal conductivity and heat transfer coefficient?

There are 2 terms look almost the same but completely different issues, which is easy confuse, below a quick breakdown.

1.Thermal conductivity (λ) – also called k-value

Below you can better understand: Take a 1 meter thick piece of material. Make one side 1K warmer than the other. Thermal conductivity is how much heat passes through 1 sqm of the material in 1s, the unit is W/(m·K).

What's the affection? The material's composition, density, moisture content, and temperature.

What doesn't? Size, shape, and thickness. Thermal conductivity is purely about the material itself. So two pieces of the same material have the same thermal conductivity, no matter how big or small they are.

For glass, the typical value is about 1 W/(m·K).

2. Heat transfer coefficient (K or U-value)

This one's a bit more complicated. It measures how much heat passes through one square meter of a structure – like a wall, a window, or a glass panel – when the air temperature on each side differs by 1K. The unit is also W/(m²·K).

Here's how two different standards define it:

JGJ/T 151 says: For a window or curtain wall, the U-value is the heat transferred per unit time, per unit area, per degree of temperature difference between the indoor and outdoor air.

GB/T 22476 says: For insulating glass (IGU), the U-value is measured in the center of the glass – ignoring edge effects – under steady-state conditions. Same idea: heat per unit time, per unit area, per degree of temperature difference.

Unlike thermal conductivity, the U-value depends on more than just the material itself. It also cares about thickness, plus boundary conditions like radiative heat transfer and convective heat transfer. That's why two windows made of the same glass can still have different U-values.