Orangeries

Thermal Qualities of Glass 


In the last few years the number of options in glass specification has increased exponentially with literally thousands of combinations being possible and yet the basic requirement for heat insulation has remained paramount.

 

The unit of measurement recognised by the building industry in defining the amount of heat lost through a material or an item used in the building envelope is the U-Value. Measured in W/m2K (Watts per metre squared, Kelvin) the value is indicative of the amount of heat that passes through one m2 of a material or building component in a time unit during a temperature differential of 1 degree Kelvin between the interior and exterior air. It therefore follows that the lower the value the better the thermal insulation of the material / component.

 

Two different U-Value figures are commonplace within the glazing industry; the Ug, which is the U-Value of the glass itself (or the ‘centre-pane’ U-Value as its otherwise known) and the Uw value, the overall U-value of the window i.e. glass plus frame.

The Ug, the centre pane value, is a constant figure dependant upon which glass is relates to. The Uw, the overall value, is difficult to define as it relies upon a complex equation that takes into account the amount of glass, the amount of frame, the thickness of frame and the type of gaskets (amongst other factors) to arrive at the end figure. All these factors will be different for each size, shape and configuration of window and of course the resultant U-Value will change accordingly. For this reason the centre pane U-Value is most commonly used.

 

As a rough guide the following may be used:

 

Uw Value from 2.0 to 1.7 – standard performance

Uw Value from 1.7 to 1.4 – good performance

Uw Value from 1.4 to 1.0 – very good performance

Uw Value from 1.0 to 0.8 – high performance

Uw Value below 0.8 – Exceeds PassivHaus standard – exceptional performance.