What is a U-value (Uw)?

The U-value is a measure of heat transmission through a material, and window performance is measured in this way. The lower the U-value, the greater a window’s resistance to heat flow and the better the insulating properties.

In order to get an accurate u-value of a window, the various parts of the window needs to be distinguished and tested. This structure is separated into two separate thermal insulation features, one being non-transparent (window frames and sashes) and the other transparent (glazing unit). This subdivision determines the u-value (coefficient of heat transmission) of the window. The formula below is used for technical approvals granted by the Building Research Institute (ITB) for systems of windows doors:

Usually the glass will be the lowest and best performing of these values (Uw).

And the Aluminium Frame will usually be the highest of these values (worst performing) because metal is a good conductor (Uf).

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