Why does this happen?
All Ug values given for architectural glazing are always given in its vertical position. When you change the angle that the glass sits at 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.
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.
In order to achieve a thermal performance (Ug value) of below 1.4-1.5 W/m2K IQ Glass would recommend specifying triple glazing for all structural glass roofs or rooflights.
Using Triple Glazing
A basic triple glazed unit will achieve a Ug value of 1.2 W/m2K when it is installed at a horizontal angle to be used as a glass roof or structural roof light.
We can improve this thermal performance for rooflights even further by introducing a solar control coating to the triple glazed structural glass unit. This would reduce the Ug value of the glass at a horizontal angle to 0.8 W/m2K.
This means that IQ Glass can still achieve far below the requisite Uw value of 1.5 W/m2K for all roof elements, depending on the fixings and finishes of the specific project.
If you would like to speak to IQ Glass about the details of your architectural roof glazing please contact us.