The glazing of highperformance windows

Triple glazing optimized for insulation can achieve Ug-values of 0.5 to 0.8 W/m2K. The value depends on the depth of the gap between the panes (8 mm to 16 mm) and which gas filling is used. Normally the noble gas argon is used. For very narrow spaces (8 mm to 10 mm), krypton is used, although this is rather expensive. Note that U describes the U-value in the centre of the glazing surface area. The heat lost at the edges is not included in this number and is significantly higher. These heat losses have to be considered by a two-dimensional thermal bridge calculation according to ISO 10077 (2000), which gives the ^g-value.

The g-value (total direct and indirect radiated heat gain through the window compared to the amount of solar radiation striking the exterior surface) for common triple glazing is about 40 per cent to 60 per cent, depending on the coatings used and the glass transmittance. A rule of thumb based on simulations is that the glass 'g' value multiplied by a factor V should be equal or greater than the glass U-value:

If this rule is fulfilled, a south-oriented - and not shaded - glazing should provide a positive energy balance during the winter. This criterion may be checked easily with the data given from the manufacturer. It is important to check the energy balance for the whole house before making a final decision (PHPP 2004).

Low-emissivity coatings

Highly insulated glazing typically has three panes, with two of the panes having a thin, soft metal coating. This low-emissivity (low-e) coating reflects the thermal radiation but transmits the visible light. The coating thus lets the sun shine in while keeping the resulting heat within the house. Different layers of the glazing may be coated, as can be seen in Figure 9.5.3.

Normally, the surfaces 2 and 5 (counted from the outside) are low-e coated, which leads to typical g-values of 52 per cent (Euronorm 410) and Ug-values of 0.5-0.6 W/m2K (Euronorm 673). Coating

Source: www.passivehouse.com

Figure 9.5.3 Different configurations of high-performance glazing: (a) double-glazing coating on surface 3; not suitable for passive houses; g = 64 per cent; Ug ^ 1.1 W/m2K; glazing rebate made of aluminium; (b) triple-glazing coating on surface 2 + 5; g = 52 per cent; Ug = 0.6 W/m2K; thermally separated glazing rebate - for example, of armed polycarbonate; (c) triple-glazing coating on surface 3 + 5; danger of rupture; use hardened safety glass for the middle pane; g = 54 per cent; Ug = 0.6 W/m2K; thermal separated glazing rebate - for example, of armed polycarbonate

Source: www.passivehouse.com

Figure 9.5.3 Different configurations of high-performance glazing: (a) double-glazing coating on surface 3; not suitable for passive houses; g = 64 per cent; Ug ^ 1.1 W/m2K; glazing rebate made of aluminium; (b) triple-glazing coating on surface 2 + 5; g = 52 per cent; Ug = 0.6 W/m2K; thermally separated glazing rebate - for example, of armed polycarbonate; (c) triple-glazing coating on surface 3 + 5; danger of rupture; use hardened safety glass for the middle pane; g = 54 per cent; Ug = 0.6 W/m2K; thermal separated glazing rebate - for example, of armed polycarbonate surfaces 3 and 5 yields a slightly higher g-value (54 per cent); but the middle pane must be hardened safety glass to prevent breakage from heat stress.

Condensation on windows is a potential problem and is more frequent on windows inclined skywards (Fritz et al, 2000). This can be observed on car windows that are frosted in early mornings after a clear night sky. This phenomenon may be less frequent with conventional double glazing because its greater heat loss may prevent frost. With superior glazing, frosting is more likely. Condensation can easily occur on the exterior surface because it will become colder since less room heat flows out. Such exterior condensation is a sign of good glazing insulation quality and not a defect.

Hard low-e coatings on the exterior surface of glazings also significantly reduce the radiation heat losses. Soft coatings cannot be used on external surfaces since they scratch easily. Hard-coated glass is preferably used with inclined roof windows. Hard coatings have, however, a somewhat higher emissivity than soft coatings and therefore cannot achieve the same low U-value as windows with soft coatings.

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