Flat Rooflights

Glass Guide

Low E Coating

All of the Overhead Glazing rooflight products feature double glazed units with a special coating called Low E, which stands for low-emissivity.

This coating is transparent and incredibly thin, made of metal, and it serves two purposes in terms of energy efficiency. Firstly, it reflects a majority of the heat back into the room, helping to minimize heat loss through the rooflight. Secondly, it allows sunlight, along with its heat and light, to pass through the glass, which is known as passive solar heat gain. By having these properties, the demand for energy to heat your home is reduced.

low-e rooflight

Argon Gas Cavity

A double glazed unit is simply two panes of glass with a sealed air gap in between. Usually, the gap contains normal atmospheric air from the factory where the unit was made. However, for better energy efficiency, the air is replaced with argon gas. Argon gas is a colorless, odorless, non-flammable, non-reactive, and inert gas that has a thermal conductivity 34% lower than air. This significantly reduces the heat transfer between the inner and outer panes of the double glazed unit as compared to using air alone.

If the argon gas escapes or if the double glazed unit is initially manufactured with insufficient argon gas, the unit’s performance will be greatly affected. In fact, when the argon gas concentration drops to 80%, the additional thermal benefits of the gas are lost.

At Overhead Glazing, we ensure that our double glazed units have high argon gas fill rates and low leak rates. We achieve this by purchasing units only from suppliers who have invested in the latest double glazed unit production lines. These production lines seal the two glass panes together within a chamber filled with argon gas. Moreover, these suppliers are independently audited to ensure full compliance with the BS EN 1279 standard. This standard defines the regulations for gas filling and gas leakage rates in double glazed units.


Warm Edge spacer bar

The spacer bar in a double glazed unit is the perimeter frame that separates the two glass panes. It is bonded to the glass with primary and secondary seals to create a sealed cavity for argon gas. The spacer bar is hollow and filled with a desiccant to absorb moisture and prevent condensation.

In the past, double glazed units used aluminium for the perimeter seal because it was lightweight, strong, and cost-effective. However, as building regulations demand better thermal performance, aluminium spacer bars have been replaced with “warm edge” spacer bars made of low heat-conductive materials. These materials keep the edges of the sealed unit warm, reducing heat loss and resulting in lower energy bills. They are now used in all Overhead Glazing Roof light products.


Toughened glass

The standard construction of Overhead Glazing Roof lights includes two panes of toughened glass within the double glazed unit. The thickness of each pane depends on the size required, typically being either 4mm or 6mm.

Toughened glass is created by subjecting annealed glass to a thermal tempering process. The annealed glass is heated to a temperature above 600°C, and then its surfaces are rapidly cooled while the inner portion remains hot. This differential cooling creates compressive stress on the surface of the glass, which is balanced by tensile stress within the body of the glass. These opposing stresses give toughened glass its enhanced resistance to breakage compared to regular annealed float glass. In the unlikely event of toughened glass breaking, it tends to fracture into small, regular fragments, rather than long, dangerous shards that can cause injuries. Toughened glass also withstands breakage better under stresses caused by temperature variations within the pane.

The toughened glass used in Overhead Glazing Roof lights is manufactured to meet BS EN12150 standards, specifically for thermally toughened soda lime silicate safety glass. It is important to note that toughened glass should not be used above swimming pools or in locations where the glazing is more than 13m above floor level. If you have any doubts about the suitability of our products for your installation, please contact our technical team for clarification. Only Overhead Glazing’s “Walk on” structurally bonded rooflight products are considered non-fragile.

Glass type

Solar Control Glass

Solar control glass is a high performance coated product that reflects and radiates away a large degree of the sun’s heat while allowing daylight to pass through the rooflight. The indoor space stays bright and much cooler than would be the case if normal glass were used.

Solar control glass incorporates invisible layers of special materials on the glass which have the dual effect of allowing sunlight in, while repelling solar heat.  In addition, solar control glass units incorporate both Low-E and solar control properties, to maximize insulation in cooler periods and solar control properties in summer.

Overhead Glazing Roof light products featuring Solar control glass also incorporate argon gas cavities, warm edge spacer bars and both the internal and external panes of the double glazed unit are toughened glass.

Solar controlled options are often on an extended delivery period.


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