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U Value

Every component plays a part in the performance of the windows and doors. Heat gain and loss occur due to conduction, convection, radiation and air leakage.

The U-Value of a window describes transfer of heat by conduction, convection and radiation.

…see Heat Gain for Radiation

The R-Value is the inverse to this. U is the flow of heat and R is the resistance to flow.

Tip: Window manufacturers try to advertise a low U-Value (high R-Value). Be careful. This may only be the value at the centre of the glass (Ug). Don't settle for high glass values. Look for "whole-window" values (Uw) of U-3.0 or lower. U-values influence heat loss more in cold climates because the difference between indoor and outdoor temperature is much higher than in hot climates.

The Air Infiltration and the U Value are comprehensive measures to value the thermal performance of a window. By following the guidelines and including elevation and climate into your project plan for windows & doors, you will be able to achieve a comfortable and energy efficient result.

Important: The values are only one aspect of the product quality. An intelligent choice includes environmental aspects, life cycle assessment, life expectancy and operational comfort.


Another way that heat moves through windows. In a cold climate, heated indoor air rubs against the interior surface of the window glass & frame. The air is cooled, becomes more dense, and drops to the floor. As this stream of air drops to the floor, more warm air rushes to take its place at the glass surface. The cycle, called a convective loop, is self-perpetuating. Sitting on a sofa, you recognize this convective movement as a cold uncomfortable draft and raise the thermostat.


Movement of heat through a material. Low conductive glass, like double glazing with minimum air gap of 12mm and low conductive frame, like timber, minimises conduction.

Heat Loss through high conductive aluminium-only frame

Double glass and low conductive frame minimise convection.