Technical Glazing Terms

Roof Glazing Thermal Performance

Roof Glazing Performance Changes

All Ug values given for architectural glazing are always given in its vertical position. When you change the angle that the glass sits at to use for roof glazing,  the thermal performance - and therefore the Ug value of the glass changes.

This change in thermal performance is important to note as it will affect the overall Uw value for structural glass roofs and rooflights and may make it difficult to achieve the requisite Uw values for a project if not addressed correctly.

The change in thermal performance of glass at tilted angles is due to the convection currents that occur inside all double or triple glazed units. When a glass unit is vertical the gas filling between the glass panels heats from the internal temperature of the building, this gas filling then rises and the heat escapes to the external of the glass panel causing the gas to cool again and sink. This movement of the gas causes a current in the gas filling and aids heat loss to the external of the glass panel.


Improve Insulation in Roof Glazing

When you tilt a glass unit the same thing happens but the vortices occur a lot more frequently across the glass unit as the convection currents only travel vertically. This means you get more convection currents which aids heat loss to the external of the glass panel and the thermal performance of the glass reduces.

For instance, if the thermal performance of a double glazed unit in a vertical position was 1.2 W/m2K when this unit is tilted on its side (to be used as a fixed rooflight for instance) the thermal performance will reduce quite dramatically to 1.7-1.8 W/m2K. This Ug value may not be good enough to achieve most prerequisite performance criteria for modern building regulations or other building guidelines.


Improving Thermal Performance

There are a number of ways you can enhance the thermal performance of your roof glazing, including specifying triple glazing within the glass specification. Although triple glazing will lead to improved thermal performances, it may also result in increased costs due to the need for glass lifting equipment. Crane hire may be required if the weight of the roof glazing is one third more than a double glazed unit.

The insulation should be looked at as a whole and in some cases, the thermal insulation of a space can be improved y other measures such as increased heating appliances or better wall insulation. We can improve this thermal performance for rooflights further by introducing a solar control coating to the structural glass unit.

If you would like to speak to IQ Glass about the details of your architectural roof glazing please contact us.