Even though the primordial objective of using the secondary glazing unit for an older building is to enhance the windows’ thermal performance by reducing heat conduction and draught-proofing, secondary glazing can offer numerous benefits.
Heat loss coming from the room and through the window especially during summer season is considered complex since 3 major mechanisms are in play:
• Through uncontrolled air leak that can either take warm air from the interior or bring in cold air outside. This is known as air infiltration and it can occur even though the windows are closed.
• Through the colder surface in the window which absorbs infrared radiation coming from the room.
• Through conduction and convection, from the warm room temperature to the colder surface of the frame and the glass.
Heat Loss through the Window Frames and Glass
It doesn’t matter whether heat leaves your room by radiation, conduction or convection, the lost heat passes through the frame and glass as conduction. Take note that the glass is considered as the most conductive portion of the window; however, heat could also be lost through the window frame but at a lower rate.
According to experts, single glazing is considered as a poor thermal insulator aside from the fact that it can readily conduct heat. The usual 4mm-thick glass could have a U-rate of 5.4W/ m²K. Thermal loss for single-glazed windows is largely dependent on the total glass area, conductance of the frame material that you’ve used as well as the quality of the glazing and framing materials. The typical rate for timber-framed single glazed windows is 4.8W/ m²K according to best secondary glazing London.
For optimum thermal performance, remember that the most favorable airspace between the panes is 16 to 20mm. In addition, the larger air space permits convection current to develop within the spaces and this would result to more heat being lost. Also, the placement of secondary units is more often than not dictated by window reveals and can frequently be fitted within a distance of 100mm from the main glazing.
On the other hand, a significant percentage of the thermal advantage of secondary glazing has something to do with the decoupling of the window frame from the prime timber moving parts. This would reduce the U-rate to more or less 2.5W/ m²K. Usage of low E or low-emissivity glass for your secondary glazing can further enhance the thermal function to lower than 2.0/ m²K. In order to maintain the figure, it’s crucial that you keep the coating totally sanitized. The standard is actually visually clean.
Air Leakage and the Amount of Heat Being Lost
The main cause of heat loss is the gaps that are present around the window and for a large window, the heat that is lost due to conduction through the window glass tends to be far greater. Because draughts, that is caused by air infiltration and convection could make the occupants feel colder, it is suggested that the heating should be turned up and whenever possible, leave it running for quite some time.