Best glass type for overhead glazing

With the myriad of glass types available now, it is often confusing what to choose in terms of safety, thermal and solar performance and balancing cost with the benefits on offer for overhead glazing. Richard Burgess, managing director of Lonsdale Metal Company, shares his experience.

Above all, safety is the critical factor in overhead glazing and the summary below summarises what type of glass complies with current Building Regulations.

Up to 5m above floor level, toughened, laminated or wired glass is suitable for single glazing, while for double glazing, the inner pane must be toughened or laminated, and the upper pane should be toughened in either instance.

Between 5m and 13m, and for single glazing, toughened glass not more than 6mm thick, and panes not larger than 3m2, should be used for single glazing, which should be laminated or wired. For double glazing, the inner pane must be toughened or laminated, and the upper pane should be toughened in either instance.

For glazing heights above 13m, single glazing should be either laminated or wired, and the lower pane on double glazed units must be laminated.

From time to time, it is desirable to use wired glass in double glazed combinations. However, there is a high risk of breakage due to thermal stress and advice should be sought from the sealed unit manufacturer.

It is not recommended to use toughened glass in single glazing or to the inner pane of double glazed units over swimming pools, food and drink preparation units, or any areas where the small broken pieces could cause subsequent injury due to contamination. In these situations, laminated glass or polycarbonate would be the best choices.

More or less without exception, the outer pane of a double-glazed unit should be toughened. The inner pane may be toughened or laminated depending upon location and height above ground. Generally, two sheets of toughened glass provide the most economical solution for domestic projects where the roof light is no more than 5m above floor level. If the height exceeds this, then laminated glass should be used for the inner pane.

When it comes to performance, the following are features to consider:

Low-e coating offers improved thermal efficiency retaining more heat within the building.

There are two types available: standard hard coat finish or the more popular soft coat, which offers a typical centre pane U-value of 1.2W/m2 when combined with an argon filled cavity.

Tinted anti-sun glass. Body tinted toughened glass can be used as the outer pane combined with a laminated low-e soft coat glass to give the added benefit of reducing solar heat gain in the summer months at reasonable cost. This is most commonly used in blue, but green, grey and bronze colours are also available. Typically, this provides a 50% reduction in solar transfer and 50% light transmission.

Neutral solar control glass has a special coating that provides improved solar control and appears more neutral in colour, albeit with a slight grey/green tint. This is often described as 70/30 meaning it offers a 70% light transmission, but only 30% of the sun’s heat enters the building.

All of the above glass type can be combined in a triple glazed units, achieving both improved sound insulation and U-values as low as 0.60W/m2K. Consideration, however, must be given to the additional weight and handling during installation.

The most economic glass tends to be 4mm-6mm thick, which can rarely be installed wider than 1,000mm, hence most domestic roof lantern and conservatory installations feature glazing bars every 600mm to 900mm.

However, with the current trend for flat rooflights, single pane rooflights can be installed up to 3m x 6m in one pane of glass. This calls for specialist manufacturing and installation by experienced companies with the necessary handling equipment and expertise. Much thicker toughened and laminated glass is required so as not to deflect, and care must be taken to ensure compliance with building regulations and local, design wind and snow loads.