Understanding thermal performance of Glazed Facades

Recent developments in glazing systems provide optimized solutions for daylighting as well as heat gain. They are manufactured to transmit an adequate amount of light in the visible portion, while excluding unnecessary heat gain from the part of the radiation spectrum. Near-IR radiation should get reflected through low solar gain coatings especially in case of hot climate. Double glazed-reffective coated glass gives the best performance in these situations. They provide almost 10% better energy performance compared to single pane clear glass for a commercial building located in warm-humid climate.

Dark tinted glass absorbs considerable amount of visible light and changes its colour. Whereas if they absorb ultraviolet (UV) or near IR, there will not be any change in visual appearance. There are some heat-absorbing glasses to lower down the heat gain and control the glare. Tinted glass is used to absorb the radiation and reduce the heat transmission but more heat absorption by a tinted glass causes rise in its surface temperature. However, the absorption is not the most efficient way to reduce the heat gain.

Absorbed solar energy from the glazing emits back as long-wave far infrared energy. The emission of radiant heat is one of the most important heat transfer pathways especially in case of double glazed system. For example, coating a glass surface with low emittance (Low-E) material and facing that coating into the gap between the glass layers prevents significant amount of this radiant heat transfer and reduces the total heat flow. Due to its energy efficiency, daylighting and comfort benefits, low-e glasses are now widely used in the facade construction.

The three thermo physical properties of the glass that are usually given by manufacturers are; conductance (U-Value), solar heat gain coefficient (SHGC) and visual light transmittance (VLT). Depending on the climate, U-value becomes important for energy savings and for comfort. When considering U-value of a window as a whole, it refers to the centre of glass, edge of glass and frame respectively. Buildings with low U-value windows/ facades bring down the conductive heat gain in to the buildings. SHGC is the fraction of solar radiation that hits the glazing and passes through the glazing and heats the room.

The lower the SHGC the greater it shows its shading ability. VLT is the percentage of transmitted visual light through the glazing. When a good daylight is desired, high VLT is advisable. But the lower SHGC (eg. tinted-glass) will bring down the visual light transmission (VLT). So, proper combination of these two properties is required for a good selection of glazing materials. The best performing glass is with VLT 0.66 and SHGC 0.33 as specified by the manufacturer but the physical theoretical limit is roughly VLT 0.6 and SHGC 0.25. The performance of the facade doesn't depend on the glazing properties given by the manufacturer alone. The overall heat gain will increase when a glass is fixed on a metal frame. However, the glazing properties specified by the manufacturer always do not match with its performance in actual conditions. SHGC of the glazing has more importance than U-value in the determination of cooling load.

Effect of glazing on energy systemsWhile calculating the cooling load we consider several external and internal heat sources. A building with 10m2 floor area and with commonly used concrete construction gets 35-40% of heat gain through walls and windows. Solar heat gain through window tends to be the single most significant factor in determining the cooling load of a building. However, there are other parameters related to window design to be considered for optimizing its impact.

The HVAC equipment has to be selected carefully. Especially in fully glazed buildings, proper combination of control set points, glazing and solar shading are crucial for energy performance. Past research shows that for every 10C drop in set temperature, about 10% of additional energy is required. So, it is necessary to create comfortable indoors with optimal energy consumption.

There is a chance of overheating near fully glazed areas which causes non-uniform distribution of temperatures across the room. So, it is important to select an air-conditioning system that works to satisfy the demand using less amount of energy. Sizing of the system is also very important which is done through estimating the correct amount of heat gain. Lower SHGC glazing reduces the peak demand which leads to the smaller size equipment. An integrated approach is required to get benefits because of the interrelationship between window glazing, mechanical systems and lighting systems.

Issues related to facade designVarious parameters that influence the facade design are - the outdoor environment; building form and orientation; properties of opaque and transparent materials used; scale and surface properties of the surrounding buildings; window size and geometry. New technologies and some design guidelines are described below.

Modern glazings control radiation by addition of various thick coatings and colour tints that influence the absorption, reflection, transmission and emission as functions of radiation frequency. Gas fillers (argon, krypton, xenon) and special materials (poly carbon, fibre glass) are used to control conduction. These Insulating glass units (IGU) are hermatically sealed, multiple- pane assemblies consisting of two or more glazing layers held and bonded at their perimeter by a spacer bar and with a cavity space filled with air or argon gas.

Other recent smart window technologies include electrochromic, thermochromic, photochromic, liquid crystal device and suspended particle device windows. Frit glass enables the designer to use patterns on the facade They can help in reduce heat gain by using high performance ceramic coatings. Frames are also available with different materials and composites. They are manufactured with thermal breaks to bring down the heat enter into the buildings.

Window geometry and size are key factors which can influence the indoor mean radiant temperature (MRT). A large window area implies a large hot surface. Tall and narrow windows are better than square windows; they should preferably be located apart at a distance not less than one half of the smaller window dimension.

The high intensity of direct sunlight in the tropics provides a very significant potential of utilizing natural light without glare and excessive heat gain. Avoiding glare is the main challenge of daylight design with glazed facade. Although buildings with high VLT values are susceptible to glare, it can occur even with a small windows and very Low VLT values. Full glazing causes too much daylight at least in the perimeter areas.

A well designed building will provide natural lighting in to the deep floors. In this context, window geometry and percentage of area on facade are very important factors. Taller windows give greater penetrations, and the broader windows give better distribution of light. For a given penetration, a number of small windows properly positioned along the same or the opposite walls will give better distribution of illumination than a single large window. Buildings with small punctures/ openings can be a source of glare due to excessive contrast between the bright window and adjacent dark wall.

Bringing the natural light into the building will reduce the lighting energy consumption. The area of the opening/size is important for that. Numerous studies have shown that there are no daylighting or energy benefits with window to wall ratios over 60%, and in most cases an area of between 25 and 40% is optimum. Natural light can be integrated with electrical lighting using controls in achieving optimal lighting as well as energy savings. According to a model study done by Lawrence Berkeley National Laboratory (LBNL), Berkeley, higher window to wall ratios (WWR), with sunshades and daylighting controls requires less energy than lower WWR without daylighting controls.

When a tinted glass is selected for less heat gain, the visual light transmission value will also come down. In such cases, the whole window can be divided into two shelves a daylight window and a view window. Thus, lighting and thermal requirements can be balanced with good light transmission properties for daylight window and low heat transmission for view windows. Another practice is that use of different glazing for window and spandrel separately. Spandrel glazing will be insulated from inside to cut down the overall heat gain of the facade.

Surrounding surfaces also influences the facade design. For example, when reflective glazing is used on any neighbouring building, it will act like a mirror and intensifies the sun's effects and over heat building patios. Green cover around the building will reduce such affect where as large water bodies again can cause reflective glare. Solar heat gains vary by orientation which influences the energy use in hot climates. It is important to follow a few basic rules in design when determining perimeter areas like avoiding the East and West facing zones. Shading is necessary in the South facing zones and the North facing is acceptable even without shading. As the heat gain comes down, orientation has a lesser impact. Differences in energy use due to orientation can be minimized not only by selecting proper glazing but alsoby external shading Overheated spaces near the unshaded windows lead to non-uniform distribution of temperatures inside the building and cause thermal discomfort to the occupants. Shading should be considered as an integral part of facade design in order to balance the daylight requirements and reduce solar gains.

In this context, fully glazed buildings where the above discussed problems are prominent, sun control films are the one of the possible retrofitting options. Recent developments in solar film coatings for window glass show substantial solar heat reduction due to the direct beam and diffuse components. These films when stuck on window glass provide a substantial heat reduction; which indicates the lesser energy expenditures while people enjoy the natural light and maintain the goodvisual contact with outside environment. However, care hasto be taken when one wants to apply film on glass. Film, when stuck on the glass will increase the absorption of the glass and cause discomfort due to the surface temperature.