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.