Why is glass such an amazing material and the subject of so much debate in building design? This likely has little to do with the material itself and more to do with what glass does for a building and the building occupants. Glass creates tremendous value for the users on both sides of the window.
Glass is a transformational material. By looking through a window, you connect to the other side of the glass without considering anything about the window or glass itself. However, that piece of glass can accomplish dozens of different functions and the user may never know. Just to name a few features; the glass could be tempered, heat-strengthened, laminated, part of an insulated glazing unit, hurricane-rated, turtle-code friendly, fire-rated glass, dynamic glass, light-emitting glass, heated glass, bird-friendly glass, power generating glass (BIPV – Building Integrated Photovoltaic), privacy glass, switchable glass, sound-reducing acoustic glass, structural interlayer glass, blast-resistant glass, bullet-resistant glass, anti-reflective glass, anti-glare glass, EMI shielding glass, transparent electrical conductive glass, anti-microbial glass, self-cleaning glass, or just a piece of annealed monolithic glass. Within this framework, there are literally dozens of ways that the window creates value for the occupant or user. However, the casual observer sees almost no difference. Fundamentally, this is the key reason we’re celebrating glass with the International Year of Glass 2022. Glass creates a massive value proposition for many purposes but is most times invisible within the application.
So, which purposes are primary or the most often considered in construction? For buildings, the two key points typically center on heat/cold management and daylight. These are the important features of buildings that are most often discussed in design and performance codes. Glass and window details are often the most defining characteristics to a building, but understanding how to mutually serve the aesthetic, performance, and cost drivers of windows are the fundamental issues for design and installation.
The benefits of sufficient daylight within buildings have been exhaustively studied. Windows connect occupants to the outdoors and studies show that people are now spending >85% of their time indoors in certain climate regions. This daylight access and connectivity with the outdoors have shown significant benefits. In buildings with sufficient daylight access, children in schools are shown to perform better on tests and cognitive work, hospital patients are shown to recover more quickly, and office occupants show improved output and productivity. Globally, the standards of required daylight levels and window light transmission levels differ dramatically, but in general, the overall daylight transparency of windows has increased over the last 20 years due to the benefits of daylight.
In addition to daylight management, we also consider how windows in buildings help manage both heat and comfort levels. This is typically broken up into two categories – solar management and thermal management. In more common terms, glass and windows are designed in a way to 1) Filter out unwanted heat, color, or characteristics of the solar spectrum or heat transfer and 2) Insulate the window to minimize heat transfer through all mechanisms of convection, conduction, and radiation. These issues can have extremely nuanced details in how we design and manage windows, especially once you start to consider things like framing, overhangs, shading, blinds, etc. However, let’s look at some basics.
- The sun’s energy gets transmitted into buildings primarily through ultraviolet rays, visible daylight, and infrared light transmission.
- The sun’s energy will either transmit through the window, get absorbed into the glass, or reflect off the glass. The sum of these three mechanisms always equals 100.
- Filter – In general terms, we try to filter out the sun’s UV and IR rays and allow transmission of the visible daylight. This term is known as spectral selectivity – we filter out only a portion of the solar spectrum. This can be done with the glass substrate as some glass naturally reduces solar transmission. This can also be done with Low-e and other coatings. To do this, we reduce transmission by increasing either absorption or reflection. For instance, a Low-e coating can reduce IR transmission by making an IR reflective coating stack.
- In addition to managing the sun’s energy, we also manage general heat transfer such as long-wavelength infrared (IR) heat. This is typically how heat is lost from a building in cold climates as it escapes through radiation, conduction, and convection through glass and windows and other materials in design. The more heat loss, the lower the insulation (R-value).
- The key points for making a window a better insulator are 1) Low emissivity (or Low-e) coatings, which reflect heat back to its heat source, 2) Using more than one piece of glass (insulated glazing units – IGU) which creates insulation pockets between the glass, 3) Low conductive gas between the glass panes (argon is commonly used in the insulation pocket).
These terms can also have interdependent characteristics. In colder, heating-dominated climates we often design the windows to have high solar transmission (passive solar heating – free heat from the sun) but have high thermal management (high R-value, insulation properties). In warmer climates, reducing the solar loading (blocking most of the sun’s energy) tends to be the main driver for design. There are complex performance evaluations, specialty terms for these metrics, and code drivers globally that dictate the types of glass and windows which are typically allowed, but these are the fundamental drivers around design.
By using these basic elements, we can manage the daylight and heat loss/gain performance metrics of a particular window design. The NSG Group has a wide range of products, processing, technologies, and capabilities globally to manage these fundamental issues around windows. In addition to the base performance of windows, we also see ever-expanding demands of added functionality to include safety ratings, structural strength, or additive technologies like fire-ratings or blast protection to a particular design.
This article is part of NSG Group’s coverage of the International Year of Glass 2022. Throughout the year NSG Group will present monthly themes to recognize the importance of glass in terms of its scientific, economic and cultural roles. For more info visit: https://www.nsg.com/en/iyog-2022
Article courtesy of NSG Group