Optical and Solar Properties of Glazing- Part 1

Glass has many benefits that make them a popular choice among architects for building facades. They are aesthetically pleasing in a cityscape and create an inviting entrance for a building.

Glazed walls create the impression of an open and airy space and improve the overall mood created in a building. Glass also allows a lot of natural light and heat to enter into a building thus allowing to effectively reduce heating and artificial lighting cost.?

While glass offers all the above benefits, these same properties of glass can also create negative atmosphere in a building. Therefore, it is important to be careful while specifying the right type of glass suitable to the location, occupancy type and utility of a project.

In this article I discuss the thermal and optical properties that affect the selection of glass.

Windows and glazing can be understood to be the least insulating element of a building assembly. Excessive use of glazed area can in fact lead to greater energy consumption and occupant discomfort due to glare. Therefore, windows must be carefully designed to control the solar gains to suit different climatic regions and orientations.

Solar energy incident on any glazed surface comprises of light and heat. 55% of solar energy incident on Earth comprises of heat and slightly over 40% is visible light.?This simply means it is really not possible to completely separate the light and heat out of the solar spectrum.

Normal clear glass is almost completely transparent to high frequency solar radiation (visible and infra red), but is a barrier to low frequency. As solar radiation strikes the facade, the solar heat energy passing through the glazing tends to warm up the internal surfaces by absorption, and these internal surfaces become heat radiators. However, the heat re-emitted is low frequency radiation of?which the glass is a barrier.?

This is called green house effect.

?The major thermal/energy parameters of glazing are:

1)????U-Value

2)????Solar Factor/ g value/ Solar Heat Gain Coefficient

3)????Shading Coefficient

4)????Solar Transmittance

5)????Solar Reflectance

6)????Solar Absorbance

Solar Factor (SF) or G-value: is the amount of solar energy which passes through a glazed area of a building when compared with the total incident energy upon the glazing. This is also called SHGC (solar heat gain coefficient). This gives a measure of the capacity of the glass to allow/ block heat energy from passing through. See image above.

U-Value:?One of the most commonly used terminology, is the amount of heat passing through the glazing, per unit of surface area, for a difference in temperature of 1 °K on each side of the glass. This property is related mostly to conductivity of the material. Air is a good insulator and this is why the u-value of an insulated glazing unit is much better than that of single glazing. Image below shows a treated DGU glass, the total transmission has been reduced to 21% compared to the 87% of clear 3mm glass.

image credit: pharosglass.com

?Shading coefficient (SC): a term commonly heard in the industry is the ratio of the solar factor of the evaluated glazing unit to the solar factor of a 3 mm single glazing.?SC= SF/0.87

?This simply means that a 3mm single glazing allows 87% of the incident solar energy to pass through.

image credit: johnsonswindowfilm.com

The main light/ visual parameters of glazing are:

Light Transmittance (LT)

Light Reflectance (LR)

Color Rendition Index (CRI)

Light Transmittance (LT): is the fraction of the visible spectrum of sunlight, which is transmitted through a window door or skylight glazing. LT is expressed as a number between 0 and 100%. Glass with higher LT transits more natural light into a space.

Light Reflectance (LR): When light meets a glass surface, some of the light is reflected, depending on the angle of incidence and the refractive indices of the glass and the medium the light is coming from (e.g., air). LR is the fraction of the incident light that is reflected from the glass surface.

imagecredit:clipground.com

Light reflection is important since it is a measure of how effectively images are reflected off a glazed surface. This has a downside as well... high light reflection also means high glare. Imagine driving through the interstate highways during the summer months and all the glazed buildings along the road reflecting light onto the drivers eyes. This consideration is particularly of importance in Middle Eastern countries where the summer solar irradiation is higher than in other parts of the world. Hence light reflectance is a parameter which needs to be careful chosen based on the location of a building.

Color rendition index (CRI): (also called color rendering index), is a quantitative measure of the ability of a glazing to reproduce the color of various objects faithfully in comparison with an ideal or natural light source. It is expressed as a scale of Scale is 1 – 100.

In commercial glass, CRI indicates the effect the specific glass configuration has on the appearance of objects viewed through the glass. CRI has a direct impact on how well the users of the building see the outside environment, and therefore has a direct impact on affecting the mood of the inhabitants. Working in an office for an 8-5 job, the last thing an employee would appreciate, is to look out of windows for a few minutes of relaxation and see grass with a strong brown/blue tinge...!!

image credit: continuingeducationbnpmedia.com

Clear glass has as an inherent slightly green transmission color due to the chemical composition of its key constituent, i.e. sand.

Engineers and architects need to be aware of how the solar characteristics of glass has an impact of the usage of glass as a fa?ade material and how to choose the right parameters for a project. In part 2 of this article I will be discussing the deciding factors and values to bear in mind while designing a glazed fa?ade.