An interesting and very useful article on glass selection for buildings by Thomas Hens, Technical Adviser, AGC Glass Europe.
Glass is a multifaceted material that has been used widely for centuries to embellish facades and interiors. In contemporary architecture, glass plays an even more important role. Used alone, it can add a personal touch to any facade. Of course, this would be impossible without the wide range of new products and innovations to have emerged on the market in recent years. But is everyone aware of these latest developments? And just how do you select the right kind of glass for a given project? These are precisely the questions that this article will attempt to answer. Over four issues, we will be discussing the various factors affecting your choice of glass with a view to delivering a safer, more economical and more environmentally friendly project, or simply one that improves living conditions.
When choosing the ideal glass, you must take into account a whole raft of factors, chief among which are the insulation value (Ug value), the solar factor (g value), light transmission (LT) and appearance (colour, reflective properties). Glass choice really does have a significant impact on a building’s energy performance, meaning you should ideally choose the right kind of glass as early as the design phase. Glass must also meet different requirements for residential and office buildings, so the appropriate glass type must be selected separately for each given project.
Glass choice is vital in establishing a comfortable climate inside office buildings, where lights and electronic equipment generate enormous amounts of energy and heat and free solar gains are generally surplus to requirements. Solar gains are also hard to predict and particularly significant where facades are constructed entirely of glass, making air conditioning extremely difficult. Therefore, office buildings should always be fitted with glass offering the lowest possible solar factor (g value), and hence low light transmission (LT) properties. Roughly speaking, half of solar energy is visible light while the other half is made up different forms of radiation (UV rays, infrared rays, etc.). When glass retains a large proportion of solar energy (low g value), it also retains a lot of visible light. In other words, it has a low light-transmission rating. Since g values and light transmission are interrelated, a compromise must always be made between the two factors. In office buildings, g values are much more significant than light transmission because of the constant presence of artificial lighting.
The situation is a little different for residential buildings, where light transmission is instrumental in preventing the need for artificial lighting. Free solar gains are welcome but must be accompanied by effective external solar protection. The insulation value (Ug value) of the relevant glass is also an important consideration, just as in office buildings. However, a distinction must be made between new constructions and renovation projects. In existing buildings where glass with a low insulation value is replaced by glass with a very low Ug value (e.g. triple glazing), the dynamics of hygrometric regulation in the
building can change. The coldest surfaces in the building are no longer the windows, but rather the external walls. Without sufficient ventilation, this means mold is likely to appear – generally in corners. Therefore, during partial renovations, it is important not to focus solely on the Ug value of the glass. Of course, in complete renovations and new builds, where entire buildings are optimally insulated, very low Ug values are particularly beneficial. In practice this means that, for minor renovation work, triple glazing is less beneficial and double glazing is the more logical choice, while in new builds either high-performance double glazing or triple glazing can be used. To achieve effective external solar protection, we recommend a triple-glazing unit that combines free solar gains with a very low Ug value, e.g. AGC Glass Europe’s Thermobel Tri glazing, which has been certified for use in ‘passive houses’.
One final key factor when choosing glass type is which way the facade faces, in both office and residential buildings. For north-facing facades, you should choose a glass with high light-transmission properties that lets through as much daylight as possible, e.g. Thermobel Top N+. For other facades, you should opt instead for solar-protection glass such as Thermobel Energy N or enhanced solar-control glass like Thermobel Stopray (glasses marketed by AGC Glass Europe). To avoid any variation in colour, we recommend using the same kind of glazing for glass mounted on the same facade surface. However, you can use two different types of glazing on two different facade surfaces without any issues, since the two glazings will never be viewed from the same angle, so any differences will barely be visible.
Taking into account all these factors will help you to avoid several issues, such as overheating, and ensure that conditions inside the building remain much more comfortable. Intelligent design and optimal glass choice will enable you to radically reduce the amount of energy a building consumes. Selecting the right type of glass is about more than just appearance and insulation values. Choose wisely, as your decision might have a significant impact on the building’s energy envelope and the comfort of its occupants.
Ir. Thomas Hens (Technical Advisor, AGC Glass Europe)
For more information on AGC Europe products, visit http://www.yourglass.com