The U-value is the most important value for the user independent assessment of the energy requirement of a building
The heat transfer coefficient (U-value) describes how much energy flows through a building element, e.g. a wall
The lower the U-value, the better the insulation
Quantitative assessment of heat bridges is possible. Sources of mold can easily be detected

Heat Flux is the rate of heat energy that passes through a surface. Depending on the exact definition of heat flux, its unit can be expressed as either W/m2 or W.
Heat flux sensors can be used to understand the thermal properties of building envelope and is a primary variable used for U value measurements.