by Karol Kazmierczak, ASHRAE, NCARB, LEED AP
Since glazing is the most advanced and expensive part of many façades, it warrants a good design that goes more than skin-deep.
Glass can be engineered to provide natural light, limit occupant discomfort, and make energy use more efficient, while maintaining the appearance desired by architects. The ‘coolness factor’—balancing the transmission of heat and light—remains the most important and, ironically, least-known performance characteristic of architectural glass.
In an era of widespread curtain walls and sloped glazing merging into vertical planes, the definition of what constitutes a ‘window’ can be thought of mainly in the context of code requirements. Unfortunately, it can seem like the International Building Code (IBC) treats the window as too obvious to specifically define. To make matters more complicated, the code introduces the noun “glazing,” which, depending on context, could be interpreted as a synonym of ‘glass’ or ‘window,’ but not always.
One of the frequently quoted industry standards, American Architectural Manufacturers Association/Window and Door Manufacturers Association/Canadian Standards Association (AAMA/WDMA/CSA) 101, Standard/Specification for Windows, Doors, and Unit Skylights, defines a window as:
an opening constructed in a wall or roof and functioning to admit light or air to an enclosure, usually framed and spanned with glass mounted to permit opening and closing.
IBC seems to agree with the assessment that a window’s primary function is to provide natural light, and establishes a bottom threshold:
Section 1205.2–Natural light. The minimum net glazed area shall not be less than 8 percent of the floor area of the room served.
This command would have been difficult to meet several hundred years ago, when glass was prohibitively expensive, production limits were severe, and real estate tax was charged per window in some places. (This is why some Parisian housing was purposefully built without windows.) Now, the requirement seems simple, but it is still frequently misconstrued. Strictly read (and against the apparent code intent), it allows opaque, glazed spandrel areas to be counted against the eight percent floor area demand. This can be problematic.
A good example would be a building designed with a visible light transmittance (VLT) in the single digits (e.g. five percent) for vision glazing in windows sized to comply with the minimum eight percent window-to-floor ratio mandated by code. This is akin to wearing two pairs of sunglasses. Such a design may be understandably objectionable to building occupants, regardless of the demonstrated building code compliance.
The aforementioned eight to 10 percent code requirement was developed with hindsight in days when dark-tinted glass was only used in vitrages, and the glass installed in windows was invariably clear, with the visible transmittance roughly 80 percent. Therefore, substituting five-percent VLT glass would logically require a net-glazed area approximately 16 times larger, making the interpolated window-to-floor ratio close to 130 percent. This is seldom feasible in typical construction.
Codes, terminology, and coolness
As its name suggests, VLT is the percent of total visible light (wavelength ranging from 380 to 780 nanometers [nm]) coming through the glass system—it is from zero to 100 percent, the lower the number, the less visible light. This is the only glass characteristic that can be verified by the traditional way architects choose the material: by comparing samples.
A better way can be found by trying to achieve the Indoor Environmental Quality (EQ) Credit 8.1, Daylight & Views, under the U.S. Green Building Council’s (USGBC’s) Leadership in Energy and Environmental Design (LEED) program. This entails a minimum two percent daylight factor (DF) in 75 percent of all occupied space for critical visual tasks.
The daylight factor is a measure of expressing the daylight availability in a room. It is a percentage ratio of the instantaneous illumination level at a reference point inside a room to what is occurring simultaneously outside in an unobstructed position, calculated under an overcast sky (excluding any direct sunlight). Using the same computer model, it is easy to analyze glare, as well as natural and artificial illumination as an additional benefit. Ultimately, an architect could specify VLT based on the DF analysis, and then correlate it with other necessary physical glass characteristics, which is explained later in this article.
Of course, when not properly specified, glass that brings in copious light can also bring in copious heat. This can mean an increase in the building’s energy consumption, as air-conditioning systems kick in to prevent occupant discomfort.
Another of the glass benchmarks allowing comparison of different glass types, solar heat gain coefficient (SHGC) illustrates glass transmittance of solar radiation, as opposed to the U-factor, which is for thermal transmittance. According to the European Standard (EN) 410, Glass in Building: Determination of Luminous and Solar Characteristics of Glazing, cited by the National Fenestration Rating Council (NFRC) standards, SHGC is the glass transmittance in range from 300 to 2500 nm. This range embraces ultraviolet (UV) rays, visible (VIS) range, and near-infrared (NIR) range, which is also called the ‘solar’ or ‘short infrared’ range.
The architectural glazing industry spends a lot of effort in increasing the ‘coolness’ of glass, illustrated by the proportion of the SHGC to the VLT. This is the most important characteristic of modern architectural glass, and part of the historical quest to bring natural light deeper into buildings while avoiding occupant discomfort.
