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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 useed.

The UK Building Regulations splits the U-value requirement for a door and window into two categories; those for a new-build property and those for an extension, alteration or replacement to an existing property.

Extensions, Alterations and Replacements

Doors and windows that are fitted into an existing property are required to be more efficient in preventing the loss of heat than those fitted into new builds.  For instance a new extension will be built to comply with the current UK Building Regulations however what of the existing house?  There are many properties with no caviities in the walls, no roof insulation and no floor insulation that were built when the Building Regulations were far less stringent.  In these properties the regulations state that the doors and windows installed should comply with lower U-values than those fitted into a new build in an attempt to increase the insulation of heat for the property as a whole.

New Builds

A new build dwelling will have been built with heat insulation in mind and all parts of the house will be insulated in compliance with the current Building Regulations.  This means that the house in general will achieve a much better standard of insulation than the average existing properrty.  The walls, roof and floors will all be well insulated and as such the doors and windows will not need to be quite so well performing to give better overall insulation for the dwelling.  Therefore the U-value requirements for doors and windows installed into a new build are not as low as those fitted into an existing property.