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More than 25 per cent of the heat from most buildings escapes through the windows – 90 per cent through the glass itself. But Phil Brown says thermal-insulation glass can keep heat in.

Window © iStock
© iStock

01 April 2019 | Phil Brown 


You may think the solution is as simple as installing double glazing, however, we’ve come a long way since standard double glazing was at the forefront of thermal performance. Advanced, thermally insulated glazing solutions are now available, which offer FMs a cost-effective way to make buildings warmer and more energy-efficient.

To understand how thermal insulation glass units keep the heat in, it is important to look at exactly how losses are measured. 

Calculating U-values

U-values measure how effective a material is an insulator; the amount of heat lost through a window is determined by its U-value. U-values are expressed as W/m2K, referring to the amount of energy that will pass through a square metre of material for each degree of temperature difference from one side to another – i.e. the amount of heat that passes through glass when it is colder outside than inside. The lower the number, the better.

Low-emissivity glass has lower U-values than traditional glazing, due to a near-invisible coating applied to the inner pane of a double-glazing unit (surface 3, counting from the outside).

A low-e coating works by reflecting warmth back into internal spaces while still letting in heat from the sun. This increases the effect known as ‘passive solar gain’, which allows sunlight to heat buildings.

Thermal insulation

Thermal insulation is typically provided by double-glazing units containing one pane of low-e glass, with an inert gas such as argon – which has a much 

lower thermal conductivity than air – in the cavity. 

It is possible to achieve even higher levels of thermal insulation by specifying double glazing with a low-e coating on both surfaces 2 and 4. These advanced units can achieve a U-value as low as 0.9W/m2K, which was previously only achievable with triple glazing. 

Triple-glazing units, with two panes of low-e glass and appropriate cavity widths, can achieve U-values as low as 0.5 W/m2K. But this may not always be the best choice for all applications, as there are other important factors to consider. 

A question of weight

The additional pane that makes triple glazing more expensive also requires larger and heavier frames than double glazing, making its use challenging for certain projects or when retrofitting older properties. 

Vacuum insulating glass 

An alternative solution, particularly for heritage and listed buildings, is vacuum insulating glass technology – using a vacuum rather than air or argon between the two panes. Vacuum insulating glass allows the thermal performance of a traditional double-glazing unit to be achieved in an ultra-thin format, which can even fit into single-glazed frames.  

This is useful to FMs looking after heritage or conservation buildings, where the building’s original window frames can mean that double glazing is not an option.

A more sustainable future

As many as 97 per cent of the buildings in Europe are considered ‘inefficient’, according to Building Performance Institute Europe (BPIE), mostly because of single or outdated double glazing.

As government regulations on energy efficiency in buildings tighten, thermal insulation glass can help FMs keep properties compliant while cutting costs and ensuring a brighter future.  

Phil Brown is European regulatory marketing manager at Pilkington UK Ltd, part of the NSG Group