Windows are one of the most important components of a

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1 design concepts Arjun Kamal MAKING BETTER INDOOR ENVIRONMENTS WITH ENERGY-EFFICIENT WINDOW DESIGN An insight into the various technologies used for constructing windows, and an attempt at understanding what goes into making energy-efficient windows Windows are one of the most important components of a building. They are responsible for the flow of light and air in and out of the building. They not only provide day lighting but also ventilation. In the last 50 years, there have been several radical changes in the field of development of windows. Thermal insulation performance has been improving continuously, and has been integrated with innovative techniques of regulating solar heat and light transmission. Today, we have high-performance windows with improved frame materials, multiple panes, gas filling and insulation, which are capable of drastically decreasing energy consumption, and thereby, the heating and cooling costs of a building. Such windows also provide acoustic insulation. While determining the energy efficiency of a window, a value that should be understood is the ability to resist the flow of heat measured in terms of U-Value (refer Figure 1). This value is the numerical inverse of the R-Value (which is used as an industry standard for rating insulation). Higher the R-Value, the more energy-efficient the window. Plain ordinary glass has an R-Value of 1, while an exceptionally energy-efficient window assembly has a U-Value of 0.07 (R=14) Fig. 1: U-Value, the numerical inverse of equivalent to that of a wall. R-Value 28 ARCHITECTURE - Time Space & People October 2014

2 Fig. 2: The various modes of heat transfer through fenestration The design and placement of windows can significantly influence the energy gain. Therefore, making the right decisions for them is vital to the creation of a sustainable building. The energy efficiency of a window is the overall effect of radiant heat gain, heat transfer through conduction or convection and air leakage. Some of the technologies for the construction of windows are as follows: i. Single Clear Glazing ii. Low E Single Glazing iii. Double Clear Glazing iv. Low E Double Glazing v. Vacuum-glazed super glass vi. Aerogel i. Single Clear Glazing : This type uses conventional glazing, which has been commonly seen for years. This permits maximum transfer of heat. Windows with such glazing may stop wind from entering the building. They are not energy efficient at all. ii. Double Clear Glazing: This type of glazing provides thermal insulation. It has two glass panes fixed at a little distance apart, thereby creating an air cavity in between. This air cavity acts as a barrier between the panes, thus reducing heat transfer through conduction as you can see in Figure 3. These windows have an insulation rating of R-2 (U=0.5) at the centre of the space between the two panes of glass. a) Schematic section showing conventional window with a Fig. 2: U-Value of a material explained single glass and the entry of solar radiation into the room b) Schematic section showing a conventional window with a double glazing and the entry of solar radiation into the room October 2014 ARCHITECTURE - Time Space & People 29

3 design concepts Fig. 4: Single glass vs double glazing Low-E Glazing: Low-E glass incorporates a coating that minimizes the heat, which is reradiated. This coating, called the spectrally selective glazing, was invented in the late 1970s. It is transparent, as the name suggests, for selective wavelengths of light whereas it behaves like a mirror for solar radiation having near infrared wavelength, reflecting it back to the exterior region. This type of radiation otherwise increases unwanted heat gain while allowing daylight transmission. This glass has an advantage over other tinted and solar reflective glazing, as it provides a higher level of visible light for a specific amount of solar radiation. With its clear appearance and decent thermal properties, this low-e coated 30 ARCHITECTURE - Time Space & People October 2014

4 Fig. 5: Air-filled cavity between two glass panes in a double-glazed window Fig. 4: Low-E double glazing glass is suitable for both single and double glazing. In the latter, the glass on the outer side has a low-e coating. Gas-filled windows: Double clear glazing that comes with an air cavity is definitely a lot better than a window with just a single pane of glass. However, there is a catch. The air present in the former type creates a convective loop of hot and cool air revolving which lowers the efficiency of the window. Light air on the hot side of the window moves up, while on the cold side, air moves down because it is heavier. This results in a thermal conveyor belt sort of thing, which helps seep heat in or out of the room. But this can be easily fixed. Inert gases, such as argon or krypton, which are heavier than air have lower potential to create such convective loops. Hence, their insulating values are much higher. Therefore, windows with these gases as a filling between the glass panes, are much more energy efficient. Vacuum-glazed super glass: High-insulation vacuum glass has been developed by researchers at US-based Guardian Industries with a phenomenal R12-R13 insulation rating. Usually, R12 rating is for a typical insulated brick Fig. 6: Gas-filled windows and plaster wall. Glass typically has an R1 or R2 rating. Therefore, Guardian Industries made a breakthrough with a glass that provides as much insulation as a thick insulated wall. This glass has a 0.25mm space between two panes of vacuum-sealed glass. This vacuum does the magic of mitigating the heat transfer. There is no matter, gas or air, Fig. 7: Graphic representation of aerogel which can move up or down in this vacuum, or transfer heat. Aerogel: Due to its phenomenally low density and thermal conductivity, Aerogel has an edge over all the other materials. It is translucent and looks like solid smoke. Not surprisingly, it is sometimes called frozen smoke, solid air and blue smoke. This material was October 2014 ARCHITECTURE - Time Space & People 31

5 design concepts developed by NASA for aerospace purposes. Therefore, it is rather expensive with respect to a common user of construction materials. It has been in use for exclusively translucent purposes. Researches and experiments are still in progress to make them suitable for transparent application and mass production at low cost. Once this happens, it will take the lead as the most effective solution for energy efficiency in the fenestration industry. Another thing to be considered while choosing windows for energy efficiency is the frame material. Frames constitute almost one-third of the overall bulk of a window. Therefore, you can very well understand their role in the insulation, longevity and strength of a window. The material that you ll use for the frames, determines the heat which would be transmitted by conduction through it. The more heat a material absorbs, the more it will radiate. If it is a bad conductor of heat, it is good for the window, because it will allow the least possible heat transmission. Wood is considered the worst conductor of heat, but has a drawback. Moist wood can easily decay. However, by applying suitable coatings, this shortcoming can be easily overcome, thereby leading to a longer life of your window frame. A term that you should consider while studying the window frame material is its U-value (explained earlier), of course, and the phenomenon known as thermal bridging. It is a fundamental law of nature heat always travels from high concentration to low concentration. If there is a good conductor of heat present between these two areas of varying concentration, it acts as a bridge. It allows for better transfer Fig. 8: Modes of heat transfer through fenestration of heat. This phenomenon is known heat readily, making it a poor choice for as thermal bridging. While achieving an energy-efficient window. energy efficiency, we have to stop For reduced heat gains and losses, this bridging to the maximum extent hybrid materials incorporating both possible, using materials in the form of metals and non-metals are used for insulators. making spacers. In comparison to Spacer: This is a hollow strip of aluminium, very less heat is conducted material located between the two through these materials. window panes of a doubly glazed Insulation: Despite using the window. Such strips maintain a uniform most energy-efficient materials for barrier between the two panels of the your windows, you may not reach the glass in a window. final goal of energy efficiency without Till now, aluminium was being incorporating insulation and sealants. used as spacer material. However, this Tiny gaps in the window frames and material is a very good conductor of the overall assembly allow heat to heat. Hence, it absorbs and transmits enter and leave. Therefore, when it is hot outside your house and you have put your ACs on, the heat outside will seep through these gaps into your room. This reduces the efficiency of your ACs as they will have to cool the extra air that enters simply because nothing was there to stop it! Similarly, if it is warm inside and the outdoor environment is cold during winters, heat energy will flow from inside to the outside. Sealants are available in the form of foams, which can be sprayed along the gaps of the window frames. However, due care should be taken while installing windows. Even the tiniest gap Fig. 9: Section of a window showing spacer overlooked during installation can 32 ARCHITECTURE - Time Space & People October 2014

6 Fig. 10: Low-expanding urethane foam helps insulate rough openings of windows environments, which will affect the overall development of a country. The types of windows listed above hold crucial importance in terms of energy reduction in buildings. Aerogel, though expensive today, holds the maximum potential in the fenestration industry to achieve energy efficiency. Who knows, maybe even better revolutionary technology than the aerogel is in the process of being developed, that can change the face of energy efficiency in the field of fenestration forever. bring your effort, to achieve energy efficiency, to nought. This is where the sealants come in. As the name suggests, they seal these gaps, blocking infiltration. Buildings account for one of the largest consumers of energy in the world. With energy- efficient windows, this consumption of energy can be reduced to make better built Arjun Kamal, B.Arch; Assistant Professor, Apeejay School of Architecture and Planning; Pursuing M.Arch. in Landscape Architecture. Illustrations: Courtesy, author. October 2014 ARCHITECTURE - Time Space & People 33