Glazed facades: Myths and facts

Can glazed buildings be energy efficient? What is the optimum percentage of glazed surface that a building should have? In the dialogue of energy efficiency in buildings, glazed buildings are always believed to be energy inefficient and glass is considered as the single largest contributor to increase in energy consumption. But, let us take a pause... think rationally... and ask ourselves a simple question... can we avoid glass?

Imagine a building without windows and glass in which you do not know whether it is day or night or it is sunny or raining. Glazing has a large role to play in a building and research has proven that daylight has major positive impact on productivity and human health. On one hand glazing and glass allows us to connect with nature, lets in daylight , facilitates natural ventilation(when left open) and on the other hand also lets in heat which takes a lot of energy to be removed . While use of lesser windows/glazed surfaces would require that we turn on lights during daytime and increase energy use for lighting and air-conditioning ,over usage of glass also leads to added heat and glare and increase in air conditioning loads. So the challenge remains on usage of optimum quantity and quality of glass in a building. We thus need to understand on how heat is transferred through glass and correlation between energy consumption and usage of glass.

Glass allows in heat mainly in two ways: through conduction gains and through direct/diffuse radiation gains. Conduction gains happen because of temperature difference between outside and inside and direct gain happens due to incident solar radiation on glass. Typically, solar heat gain component through glass is several times higher than conduction gain on a hot and sunny day. Solar heat gain through windows is a significant factor in determining the cooling load of buildings.

The origin of solar heat gain is the direct and diffuse radiation coming from the sun and the sky (or reflected from the ground and other surfaces). Some radiation is directly transmitted through the glazing to the building interior, and some may be absorbed in the glazing and indirectly admitted to the inside. While conduction gain can be altered through use of double/triple glazing with air or gas fills, solar gain can be controlled through number of panes, simple shading devices and with use of advanced coating and films on glass. Solar heat gain of glazing ranges from above 80% for uncoated clear glass to less than 20% for highly reflective coatings on tinted glass. It is also true that use of coatings and films reduce daylight penetration in a building . Visible transmittance is the amount of light in the visible portion of the spectrum that passes through a glazing material.

A higher VT means there is more daylight in a space which, if designed properly, can offset electric lighting and its associated cooling loads. Visible transmittance is influenced by the glazing type, the number of panes, and any glass coatings. Visible transmittance of glazing ranges from above 90% for uncoated clear glass to less than 10% for highly reflective coatings on tinted glass. Earlier, windows with lower solar gain (with tints and coatings) also had reduced visible transmittance. However, new generation high-performance glass with low-solar-gain low-E coatings have made it possible to reduce solar heat gain with little reduction in visible transmittance.

Key properties of glass that impact energy consumption are its u-factor, solar factor and visible light transmission. Percentage of glazing to be used in a building is determined by its orientation, daylight needs, function of space adjoining the space, depth of space, aesthetic needs and ventilation requirements. TERI studies prove, that in our climate, optimum window to wall area ratio in a typical day use commercial building is about 20-40%, that allows usable daylighting and optimises energy gains. Glare control plays a major role, if we want to use the glazed surfaces as sources of daylight to reduce our lighting needs. A closer look at the fully glazed buildings during daytime reveals that blinds and curtain are drawn, thus largely negating the purpose of use of glass. Advanced building energy simulation tools are available that can help us to design buildings with optimum glazed surfaces and choose properties of glass and shading devices that allow required glare free daylight with moderated heat ingress.

Properties of glass are of very less significance when windows are kept open e.g. in residential buildings during daytime. Shading, position, orientation, and size of windows have big impact in such cases.

The Energy Conservation Building Code 2007 (ECBC) of India has prescriptive standards for use of glass which is particularly of significance in commercial buildings. The code allows a maximum limit of 60% of glazed area, but 40% is the recommended optimum upper limit. This is amply proven by the fact that solar factor of glass be used in buildings that have glass area beyond 40% is more stringent. It must also be understood that the ECBC is an energy code and not a design code. The designer has various options to use the code provisions to the best of his/her requirements. It is incorrect to blame the code for over usage of glass in buildings.

GRIHA (Green rating for Integrated Habitat Asessment: www.grihaindia.org ), the indigenously developed rating system by TERI with support from MNRE has very effectively integrated the ECBC requirements. A GRIHA compliant building cannot exceed prescriptive glazing requirements of ECBC (a feature unique to GRIHA only) and GRIHA also mandates compliance with stringent energy performance benchmarks and daylighting provisions, that "forces" designers to use less glass, shaded glass and better glass.

Some common tips for opening/glazing selection or design are:

1. Orient a building properly so that most areas that require daylighting are facing towards north or south (in northern hemisphere).
2. Design floor plates with less depth; place buffer spaces such as staircase, toilets, stores etc towards east or west.
3. Shade the east and west properly.
4. Shading of south facade is comparatively simpler than shading east and west walls.
5. Choose window - wall ratio based on daylight requirements of adjoining spaces. If the windows are planned to be kept open to allow ventilation, window-wall ratio is determined also as per ventilation requirements.
6. U-factor (overall heat transfer coefficient) for a fenestration product (including the sash and frame) should be low, SHGC (solar heat gain coefficient) for a fenestration product (including the sash and frame) should be low for a hot climate, and the visible light transmittance should be high.
7. Pay attention to infiltration losses which may be a silent burden on energy consumption.
8. The maximum permissible WWR on a facade should not exceed 60%; however as per TERI research it is proven that 20-40% is optimum glazed area for day use commercial buildings.
9. Balance the conflict between glare and light.
10. Integrate daylighting with artificial lighting through use of daylinked controls.
11. Use shading devices such as chajjas, vertical/horizontal projections to control glare.
12. External shading devices are much more effective than internal shading devices.
13. Use light shelves, light pipes to bring daylight to deeper spaces.
14. Window size and glazing selection can be a trade off.
15. Big windows require better glazing.
16. Dark glass need not necessarily provide good solar control.
17. Don't count on glazing alone to reduce heat gain and discomfort.

Finally, in commercial buildings with high internal loads of people, equipment, computers, the heat gains through envelope may be considerably lower than internal heat generated. In such cases focus should be given on removal of heat build up internally..in such cases ,many a times it is found that tighter and high performance envelope may add to energy consumption, if adequate measures are not taken.

To conclude, glass is not alone to blame for high energy consumption...use glass judiciously and it can do wonders!!!!!