Double and triple glazing

Whenever the internal surface of exterior glazing is at a lower temperature than the mean room surface temperature and the internal air temperature, heat is lost by a combination of radiation exchange at the glass surfaces, air conduction and air convection currents inside and out, and also by conduction through the glass itself. This heat loss can be reduced considerably by the use of multiple glazing with air, partial vacuum or inert gas fill (Fig. 7.14).

Double glazing reduces the direct conduction of heat by the imposition of an insulating layer of air between the two panes of poorly insulating glass. The optimum air gap is approximately 16 mm, as above this value convection currents between the glass panes reduce the insulating effect of the air. The use (as a filling agent) of argon, which has a lower thermal conductivity than air, further reduces heat transfer by conduction. The use of krypton or even xenon within a 16 mm double-glazing gap in conjunction with low-emissivity glass can achieve a U-value of 0.8 W/m2 K. Similar reductions in conducted heat can be achieved by the incorporation of an additional air space within triple glazing. Thin low-emissivity films suspended within the cavity can further reduce the U-values of double-glazing units to as low as 0.6 W/m2 K. Typical U-values are shown in Table 7.2.

Window energy ratings

A European system of energy rating bands for complete window units based on the range A (best) to G (poorest) gives guidance to specifiers on energy efficiency. The ratings take into account a combination of the three key factors which affect performance; namely, U-value, solar gain and heat loss by air infiltration. The

Table 7.2 Typical U-values for single and multiple-glazing systems

Glass system

Single clear glass 5.4

Double clear glass 2.8

Triple clear glass 1.9

Double clear glass with hard low emissivity coating 1.9 (e.g. K Glass)

Double clear glass with soft low emissivity coating 1.8 (e.g. Kappafloaf)

Double clear glass with low emissivity coating 1.5 and argon fill

Double solar glass with low emissivity coating 1.2 and argon fill

Triple clear glass with two low emissivity coatings 0.8 and two argon fills

Double clear glass with hard low emissivity coating 2.4 mounted in timber or PVC-U frame (dependent on size of glazing unit)

The data relate to 6 mm glass and 12 mm spacing.

Fig. 7.14 Mechanism of heat loss through single and double glazing (after Button, D. and Pye, B. (eds) 1993. Glass in building. Butterworth Architecture)

solar heat gain and U-values relate to the whole unit not just the glazed areas. The leakage rate is taken for average conditions. The rating bands are colour coded from green (A) through yellow (D) to red (G) for ease of recognition and they compare the overall energy performance of the windows measured as the total annual energy flow (kWh/m2/yr). Some windows in the A band may give an overall positive energy contribution to the buildling, whilst G band windows contribute an energy loss in excess of 70 kWh/m2/yr.

An appropriately insulated and sealed PVC-U framed system with triple glazing, using 4 mm clear white glass for the outer and inner panes, 4 mm hard coat low-emissivity glass as the centre pane, and two 16 mm cavities filled with argon gas can achieve the highest 'A' rating. Typically, a timber-frame double-glazed window with 16 mm argon fill, corrugated metal strip spacer and soft coat low-emissivity glass would achieve a C rating, whilst a thermal-break aluminium frame double-glazed window with 16 mm argon fill, silicone rubber spacer and soft coat low-emissivity glass would achieve a D rating.

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