Glass Options for Enhanced Building Design

Multiple glass options offer customized ways to suit different building needs
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Sponsored by Pilkington North America – NSG Group
By Peter J. Arsenault, FAIA, NCARB, LEED AP
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Safety Glass

There are many interior and exterior applications where codes and general practice require the use of safety glass that reduces the possibility of injury if broken. Tempered glass is commonly used, although the tempering process introduces distortion in the glass which can give the unwanted characteristic of visual waviness. Furthermore there is a small risk of spontaneous breakage when using tempered glass, which restricts its use in certain applications. Heat strengthened and laminated glasses are options to mitigate these issues. One thing to be aware of however, is that heat treating float glass to produce either heat strengthened or tempered glass requires a focused cooling process called quenching. The quenching air jets used in a tempering furnace cannot cool the glass with complete uniformity though. As a result some areas are cooled faster than others, resulting in differential shrinkage which creates areas of different compressive stress. This differentiation can create “quench marks” due to very slight changes in the glass density which affects the light passing through the glass. Quench marks should not be considered a defect; they are an indication that the glass has been heat treated. Appearance of quench marks is dependent on lighting conditions and they can be visible in transmission and reflection, and particularly when viewing the glass at an angle other than directly facing the glass. They are more visible in thicker, clear, heat treated, glass; when a lightly reflective coating is used; or when both lights of glass in an insulating glass (IG) unit are heat treated. They become particularly visible in any glass when polarizing sunglasses are worn. Therefore, if viewing clarity is the priority, designs which do not require heat strengthened or tempered glass can be preferable to those that do. It is always important to construct a mock-up of the intended glazing design to gain an understanding of any potential issues related to distortion or quench marks.

Visual Clarity – Anti-Reflective

There are many applications where glass is used and great visual clarity is the major design criteria. This might include retail displays, showrooms, museums, sports stadiums, zoo enclosures and exhibits. In these cases glass is commonly used to create a physical barrier but clear visual characteristics are needed without distortion. Since visual clarity is affected by the amount of visual reflections that appear on the surface of the glass, anti-reflective glass is available to overcome this issue. It is formed by using two pyrolytic coated surfaces in a single laminated glass product to minimize visible light reflectance to less than 2 percent compared to normal clear glass which is 8 percent. This allows more than 90 percent of visible light to transmit through the anti-reflective laminated glass with very low masking reflections. In addition, anti-reflective glass can block more than 99 percent of transmitted UV thus reducing the fading of interior fabric and furnishings. As a laminated glass product, it also offers the traditional benefits of enhanced security, improved safety, and damage protection. It is possible to get anti-reflective glass tempered if increased strength is required. One manufacturer produces an anti-reflective glass that offers a lower emissivity than normal glass; giving improved thermal insulation in addition to a reflection reduction. Additionally, anti-reflective glass can be laminated to offer a lower emissivity than normal glass; giving improved thermal insulation in addition to a reflection reduction.

Ordinary glass on the right can produce veiling reflections that obscure the view through to the other side. Anti-reflective glass on the left dramatically improves the clarity of objects beyond the glass.

Photo courtesy of Pilkington North America

Ordinary glass on the right can produce veiling reflections that obscure the view through to the other side. Anti-reflective glass on the left dramatically improves the clarity of objects beyond the glass.

Visual Clarity – Low Iron Glass

For situations where very pure color clarity, edge color of the glass may be seen, and/or high VLT are desired, low iron glass may be worth considering. Normally, all glass has traces of iron content which is responsible for the edges of a piece of clear glass appearing green. When glass is manufactured by deliberately reducing the amount of iron present, the green edge color is reduced in intensity and the view looking through the glass is very clear. It is therefore ideal for use where glass edges are visible or where a neutral color is desired. As its light transmission is 1 percent and 8 percent higher than clear float glass in 3 mm to 19 mm thickness respectively, it is perfect for applications where transparency and purity of color are desired. It can be used in anti-reflective glass, in insulated glass units, and for maximizing solar heat gain in colder climates. Low-iron glass can also be heat treated for safety and laminated for security. All of these traits make it a high clarity choice for storefronts and displays, furniture, solar collectors, photovoltaic panels, and special applications requiring thick glass such as bullet resistant glass, aquariums etc.

Low-iron glass shown on the left is more optically clear without the common green tint found in most conventional glass products as seen on the right, particularly along the glass edges.

Photo courtesy of Pilkington North America

Low-iron glass shown on the left is more optically clear without the common green tint found in most conventional glass products as seen on the right, particularly along the glass edges.

Special Building Design Applications

There are a number of building design situations where some very specific needs arise for glass. Manufacturers have responded by developing and testing glass products to meet those special applications.

Fire Resistance with Glass

For building areas that require viewing through glass but also high levels of fire protection, then fire-resistant glass should be considered. It is specifically designed to limit conductive and radiative heat transfer with product performances ranging from 20 to 120 minutes. These glass products must always be used as part of an approved fire resistance or fire protected framing assembly. Fire-resistant glass consists of multiple laminates of float glass and a special transparent intumescent interlayer, which is totally compatible and optically homogeneous with the glass. When exposed to fire, the pane facing the flames fractures but remains in place. As the heat penetrates the glass, the interlayers react by foaming to form a thick, opaque, resilient and tough insulating shield that blocks the conductive and radiant heat of the blaze. As a design tool, it allows natural light and unobstructed views in fire rated walls, openings and doors while restricting the spread of heat, smoke, flames and hot gases. It can be combined with a full range of other glass products to address security, bullet and hurricane resistance or visual clarity requirements. These products are typically tested and classified independently by organizations such as Underwriters Laboratories (UL). Ultimately, it addresses health safety and welfare by reducing fire damage to people, property and valuables.

Fire-resistant glass can be used to spread daylight and create open visual contact in areas that require fire resistance ratings in buildings.

Photo courtesy of Pilkington North America

Glass can be laminated with special PVB to improve acoustic attenuation and to achieve specific noise reduction requirements.

Acoustic Control

Some buildings are subject to higher levels of noise than others while some building uses are simply more sensitive to noise affecting the occupants / users of that building. Either way, unwanted noise coming from things like roadway traffic, railways, aircraft, factory operations, music, or other activities needs to be limited and controlled. Methods of creating sound insulating walls are common and fairly well known, but if those walls have windows, the glass needs to provide acoustic control as well. Noise control glass is the ideal choice in situations like this. By using a polyvinyl butyral (PVB) interlayer laminated between two layers of selected float glass, manufacturers offer a high quality product that produces excellent noise reduction without compromising on light transmittance or impact resistance. The acoustic performance can be varied by combining different thicknesses of glass with the PVB interlayer to achieve a specified rating. In this way noise control glass offers the opportunity to achieve project-specific noise reduction requirements. It is generally available in a variety of sizes and can be fabricated to meet any needed safety ratings. It can be used singly or in double or triple pane Insulating Glass Units (IGUs). Depending on the configuration and thickness of the glass, test results have shown that noise control glass can achieve STC ratings between 31 and 39. The use of acoustic PVB reduces mid-range frequencies and tests with an STC in the low 40s. Attenuation of mid-range frequencies is important as this includes sounds such as urban road traffic, railway traffic, music and factory noise.

Self-cleaning Exterior Glass

Buildings of many types contain glazing that is difficult to reach for cleaning or simply require cleaning because of the surrounding environment and that often creates ongoing high maintenance expenses. In response, a rather innovative glass product has been developed that uses the power of the sun to clean itself. This self-cleaning glass can dramatically reduce or eliminate window cleaning, while still offering good visual clarity and an unspoiled exterior aesthetic. This glass uses UV energy from the sun, which is abundant even on cloudy, overcast days, to keep windows clean naturally in several steps. First, a photocatalytic process generated by additives in the coating of the glass loosens dirt and gradually breaks down organic residue so it doesn’t adhere to the glass. Next a hydrophilic action on the glass surface causes rain to sheet on the glass, carrying dirt away with minimal spotting or streaking. Under most conditions, natural rain is sufficient to keep the window clean, and a quick spray with a hose will achieve the same result even in prolonged dry weather. Since the coatings used in this self-cleaning glass are pyrolytic and an integral part of the glass surface, they aren’t susceptible to peeling, separation or disintegration over time. In addition, they are not damaged by liquid glass cleaners so there is no need to be concerned about re-coating or retreating the glass. Combined in an insulating unit with an inboard lite of either low-e or solar control glass, all aspects of glazing performance in a building can be addressed and maintained. Self-cleaning glass is commonly available in clear or blue colors and a range of thickness from 1/8 inch up to ¼ inch.

Bird-Safe Glazing

As the use of glass in building facades increases, so does the importance of bird-safe glazing. Birds in flight are unable to use visual signals such as window frames to distinguish the location of glass. The reflection of vegetation, the sky, and other buildings are deceptive and can result in bird collision. Alternatively, the transparency of windows can be seen as an unobstructed flight route and have the same result. Opaque patterns or decals, applied by film, frit, or etching, have previously been used to deter birds from colliding into windows. These patterns are visible not only to birds but also to humans and may be a nuisance and obstruct views. However, the use of a patterned UV reflective coating is now an available alternative that makes the glass visible to birds while remaining hardly noticeable to humans. This is because birds are able to see light in the ultraviolet spectrum, which is not visible to the human eye. Whether using an opaque or transparent solution, a two-by-four pattern rule of thumb should be followed when trying to ensure bird safety. The two-by-four pattern rule states that the vertical distance in the pattern must be 2 or fewer inches apart and horizontal distance must be 4 or fewer inches apart.


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Originally published in July 2018