What it costs: curtain wall glazing

Specification of glass for curtain walling applications can be complex. Numerous functional requirements need to be taken into account and the competing demands of different performance requirements have to be balanced in order to achieve best value.

Benefits can be achieved by optimising the thermal and solar performance in order to reduce the heating and cooling loads in the building, and surface finish to the glass to reduce cleaning requirements.

Solar control and thermal insulation

To achieve optimum thermal performance in the British climate, glass curtain walling should, in the winter, allow the sun’s light and heat to enter the building and keep heat generated by the space heating system within the building. In the summer, it should minimise solar heat gain and limit sunlight to prevent glare.

Good thermal insulation will keep the building cool. Using insulated glass units can reduce U-values to about 1 W/m2K with subsequent energy cost savings. Key factors that improve the solar control and thermal insulation of insulated glass units include:

• Use of low-emissivity glass to reflect infra-red energy. In winter, low-E glass reflects heat from the building back into the space and in summer it reflects the external sunlight back to the exterior
• Filling the insulated glass unit with a gas of lower thermal conductivity than air. Gases used typically are argon and krypton. Krypton is more effective than argon at reducing heat loss but is much more expensive; it tends to be used for smaller pane spacing. Gas is introduced to the insulated glass unit cavity to at least 90% concentration. Over time, it dissipates, reducing the insulating effect from a rate of 0.5% to about 0.1%
• The optimum cavity width of the insulated glass unit for thermal insulation is about 16mm. Cavities larger than this allow the gas inside the unit to circulate, so losing heat by conduction as well as convection
• Most insulated glass units comprise two panes, but triple or even quadruple-glazed units are available to give lower U-values
• Solar control is also achieved by treating the glass to absorb heat and filter light to reduce glare. Thicker glass tends to absorb more solar energy. Different coloured glass will have different light transmittance and shading properties and should be selected to achieve the desired environment for the users of the building.

Insulated glass units should be to BS EN 1279 for assurance of long-term performance to resist moisture penetration and for assessment of gas leakage rates for gas concentration tolerances.

Self-cleaning

A very thin titanium dioxide coating is applied to glass during manufacture to promote self-cleaning in situ. The coating promotes two processes which help keep the glass clean:

• The coating reacts with UV light to become photo-reactive or start a photo-catalytic reaction which decomposes organic materials and traces of sealants or deposits from the glazing manufacturing and installation
• The coating affects the way rainwater flows down the surface of the glass. The glass surface becomes hydrophilic rather than hydrophobic, so water “sheets” down the glass, providing an even flow of water rather than water droplets that form unsightly streaks. The sheet of water is conducive to removing dirt.

Activation of the coating will depend on the orientation of the glazing; the more sunlight falling on the surface, the quicker the system reacts.

Self-cleaning glass does not mean cleaning is unnecessary. It may become less frequent and should be less difficult, as dirt does not adhere so firmly to the glass. A hose down or scrubbing with pH-neutral, non-abrasive cleaning agent and a cloth should suffice. Other cost benefits may include less need for artificial lighting as the glass allows more natural light to pass through.

Coated glass to BS EN 1096 will have been tested to demonstrate the resistance of the coating to attack by simulated natural weathering, resistance to abrasion, and to confirm that the glass’ light transmittance and solar transmittance is not affected by long-term exposure to solar radiation.

Other performance issues for glass used for curtain walling that the specifier needs to bear in mind include sound control, fire protection, safety, security and appearance.