Glazing is an extremely important component in contemporary buildings, providing daylighting and views. Poor choices for glazing design impacts visual comfort, thermal comfort and escalates the building energy demand. In the tropical/composite climatic conditions of the Indian sub-continent, a fully glazed building with all its blinds down in the daytime is a common sight. Hence, it is important to use glass wisely with appropriate design and technologies.
Good glazing design requires a balance between daylighting and heat gain. But what exactly does this entail? In this webinar, Venugopal Ravi, design manager at Saint Gobain discusses the key technical parameters to arrive at the best-suited glass through theory and simulations. He also elaborates on technological innovations which are currently shaping the market with the help of case studies.
There are essentially three metrics associated with glazing, namely Solar Factor (SF), Visual Light Transmittance (VLT) and U-value. Let’s review this. When solar radiation strikes a glass surface, part of the energy transmitted through the glass as infrared radiation, another part is absorbed by the glass and the remaining is reflected onto the surroundings. The absorbed component is partly re-emitted into the room and partly to the exterior. Solar Factor (SF) is the total direct heat gain through radiation in the space from the transmitted and re-emitted components. The visible light penetration into the space is termed as VLT. A clear single glazed unit has a VLT of even 80%. Conductive heat gain depends on the U-value of glass. A single glazed unit has a U-value of 5.8 W/sq.m K while a Double Glazed Unit (DGU) has a U-value is 2.8 W/sq.m K.
Mr. Ravi stresses that lower SHGC (or Solar Factor) has a greater impact on decreasing the heat gain, compared to reducing the u-value of glas in a predominantly hot climate like India. The webinar looks at various glass types ranging from a conventional DGU glass to a high performance DGU with low-emissive coatings to understand the pivotal role of SHGC. Through calculations, it is demonstrated that as the SGHC goes from 0.73 to 0.18, the direct heat gain through the glass reduces by about 75%. In comparison, when u-value drops from 2.8 W/sq.m K to 1.6 W/sq.m K, the total reduction in the conductive heat gain is only about 40%.
Another important metric is the spectral selectivity. It is the ratio of the visual transmittance to the solar heat gain which describes the total daylight penetration and associated heat gain into a space. High spectral selectivity values indicate high daylight penetration and low heat ingress which further reduces the cooling requirment. Spectral selectivities for various glass could range from 0.9 to 2. Hence, designers in tropical climate should opt for glazing with low SHGC values whereas in cold climates, glass with higher SHGC would provide better thermal comfort through passive solar heat gain.
Technological advancements have brought forth a wide range of glass products to choose from. Mr. Ravi discusses three innovations by Saint Gobain: PrivaLite, PictureIT and SageGlass.
PrivaLite allows the glazing to be rendered opaque or transparent through electronic controls. This type of glass can be used as projector screens, partitions, facades & windows etc. PictureIT makes digital printing on the glazing possible. However, it is important to keep in mind that digital printing of the glazing affects the VLT, SHGC and other metrics. Sageglass enables dynamic tinting of the glass. On bright days, the VLT of the glass can be reduced to avoid glare whereas on gloomy days the VLT can be increased to allow how light to enter the space. It is essentially a laminated DGU with an electrochromic glazing sandwiched between the panels. When current passes through it, the tint of the system changes. The SHGC can be customized from 0.1 to 0.4 and VLT from 1% to 60%.
Multiple studies correlate adequate glare-free daylighting with good visual comfort, energy savings and psychological well-being. Hence, choosing the right type of glass as per the context and climate conditions is extremely crucial.
This webinar was conducted on 8th August 2019.
Venugopal.R leads the Design ACE team of Saint Gobain, supporting architects, developers and consultants decide on glazing for their buildings using state of the art technologies and software. He is a certified green building professional, EDGE Auditor and Glass Academy Trainer.
Yeah, SAGEGLASS can be connected to a building management system. The product comes with two sensors – one is to measure the sun angles and the second is to measure the available lux levels. Based on these readings it will change its tint.
Q2. Has SAGEGLASS been used in any project in India. If so, please share some names.
Yes, HQ of Monte Carlo Group in Ahmedabad.
Q3. What is the difference in manufacturing online and offline coated glass?
Saint-Gobain manufactures the basic glass, which is the clear glass, and tinted glass using a technology called Float Line Technology. Molten glass is cooled over a bed of molten tin and removed when the temperature reaches around 600℃.
In online manufacturing, when this glass is exiting this float, a chemical vapour deposition is done on the glass by the coating system. Limitations of the technology are – it’s an old technology, lacks variety in the coatings and very high internal and external reflection. Advantage is that its available easily and can be manufactured in large quantities.
In offline manufacturing, they cut the clear glass or the tinted glass and take them to a separate facility called Coater facility, where the coating is done using an advanced technology called magnetron sputtering. In this process, there are multiple target material (for example, silver is used as a target material for adding a performance layer) which are deposited on the glass. These coatings are in nanometers. The offline coating is more advanced technology and very high performance glasses (up to selectivity of 60% light transmission and 28% solar factor) can be made.
Q4. Which climate would require triple glazed glass?
The advantage of TGU is that much lesser U-values are achieved. TGU is more beneficial in some European and other colder countries where you need low U-values and high SF. Whereas in India, it is very rare. You can achieve a lot using a DGU.
Q5. What is the installation cost variation in a spectrally selective glass to a clear glass?
The installation cost between a spectrally selective and a clear glass is quite similar. The glass prices are different as there has been some value addition. Following are the various costs of the glass:
Clear glass – ₹ 700/sq m
Basic coated single-glazed glass – ₹ 900-1400/sq m
Basic coated double-glazed glass – ₹ 2000-3500/sq m
Q6. What is the working principle of the PRIVA-LITE Glass? How long does it take for the glass to change its working mode?
In PRIVA-LITE, the unique technology is that there is a lamination in between the glass surfaces. When electricity is passed through this lamination, the ions reorient themselves. This leads to the opaque and transparent faces. The change is instantaneous.
Q7. Apart from VLT and solar factor, what are the other important factors that determine glass selection?
The other main factors that determine glass selection are:
Design of the building: This includes the window-to-wall ratio and the orientation of the building.
Location: The latitude will determine the kind of glazing required.
External shade factor: They essentially block direct sunlight from reaching the windows and thus affect the requirements.