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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.

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). 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.