Balancing Health and Performance Benefits through Natural Lighting

Understanding how to specify glazing systems that balance access to natural light with thermal performance and building code requirements
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Sponsored by Oldcastle BuildingEnvelope®
By Juliet Grable

Light Shelves

Daylight typically penetrates a room at a depth that is one and a half to two times the window height. Light shelves are horizontal projections installed onto the interior of a window opening that redirect and redistribute natural light into the interior space. They are typically installed toward the head of a window, or between a window and a transom or clerestory. Light shelves are most effective on south-facing windows, and the top surface is usually bright white to encourage the reflection of light. The depth of the light shelf should not exceed 30 inches, or the height of the clerestory or transom above it, and it should be installed 7.5 to 8 feet from the floor to avoid creating an obstacle.21

Skylights

Some of our most inspiring buildings feature atria and skylight systems. Skylights are a particularly effective source of daylight because they bring in light from the brightest part of the sky into spaces not reached by perimeter windows. In addition, skylights don’t often cause glare because they are high above the occupant’s visual field. Skylights can drastically reduce electric lighting energy demand; however, solar heat gain must be controlled.

Holistic Design Strategy

Window and glazing choices should be considered holistically since they can impact so many elements of a building, including lighting design and HVAC sizing. Once the design team and owner agree on the design problem, window and glazing options can be evaluated. Issues to consider include heat gains and losses, visual requirements, including views, privacy, and the potential for glare, shading and sun control, thermal comfort, condensation control, ultraviolet (UV) control, sound transmission, daylighting, energy requirements, and aesthetics.

Types and Properties of Glass

When it comes to glass and glazing systems, there have never been as many options available to the architect as there are today. Architectural glass can be designed or “tuned” to reduce solar energy transmittance, control glare, and screen out UV radiation. For example, transmitted solar energy is reduced by the use of tinted or coated glass, colored interlayers, silk-screened glass, or a combination of strategies that absorb or reflect part of the solar radiation in the UV, visible, and near-infrared ranges. The absorbed energy is converted to heat, and a large portion is dissipated to the exterior.

Some manufacturers offer energy analytic applications that calculate a project’s specific energy requirements and performance needs. These applications can incorporate design changes and immediately calculate the impacts on performance. Before we look at specific applications, let’s review the types of glass and key principles related to energy performance.

Heat-Treated Glass

Glass is heat treated to increase its strength and ability to resist external stresses, such as temperature fluctuations. There are two general categories of heat-treated glass: heat strengthened and tempered. The rate of cooling the glass during the heat-treating process determines the strength of the heat-treated glass. Tempered glass is cooled rapidly, while heat-strengthened glass is cooled more slowly. Heat-strengthened glass is approximately twice as strong, and tempered glass is approximately four times as strong as annealed, or untreated, glass. Both types of heat-treated glasses can exhibit distortion that may slightly distort reflected and transmitted views.

Heat-strengthened glass is treated to have a surface compression of between 3,500 and 7,500 psi. It shows greater resistance to thermal stress and shock. When it breaks, the pattern of breakage is similar to annealed glass; it fractures into large shards and tends to stay in the frame. Heat-strengthened glass is not a safety glazing material; in other words, it can’t be used where safety glazing is required.

Heat-strengthened glass should be specified when additional strength is desired and when it is desirable for the glass stay in the frame if broken. Heat-strengthened glass is appropriate for most applications where the glass may experience temperature extremes (high thermal stress), but there are some applications (such as insulated spandrel conditions) where tempered glass will be required.

Fully tempered glass is treated to have either a minimum surface compression of 10,000 psi or an edge compression of at least 9,700 psi, or meet the requirements for safety glazing set by ANSI Z97.1 or CPSC 16 CFR 1201.

Tempered glass is extremely strong and is a safety glazing material due to its break pattern. It has better resistance to impact, thermal stress, and windloads, and it shatters into small pieces when broken, which reduces the risk of injury from impact. However, it typically does not stay in the frame once shattered.

Building codes define applications that require safety glazing. These include tub and shower enclosures, patio doors and entry systems, glass balustrades, and hazardous areas adjacent to walking surfaces. Tempered glass may be needed to meet the wind-load requirements for buildings in high-wind-load regions, such as hurricane zones.

Once the glass has been heat treated, neither heat-strengthened nor fully tempered glass can be cut or drilled.

Laminated Glass

Laminated glass is a type of safety glass that holds together when broken. It consists of two panels of glass held together by an interlayer, typically of polyvinyl butyral (PVB). When laminated glass breaks, the interlayer holds the broken glass pieces together. Laminated glass can be manufactured using annealed, heat-strengthened, or fully tempered glass. It is used in applications where impact or blast resistance is required, or where security is an issue. Because the interlayer(s) help dampen sound, laminated glass is also a good choice for reducing noise levels in noisy environments.

Laminated glass is also ideal as decorative glass, as colored interlayers can be added. Certain interlayers are good UV blockers, thus laminated glass is often used to prevent fading in homes, museums, libraries, etc.

Insulating Glass Units

Insulating glass units (IGUs) are preassembled units composed of two or more lites of glass that are sealed at the edges and separated by air or gas-filled space(s). Insulating glass, also known as double glazing, enables higher energy performance, as it reduces heat loss through the glazing. Insulating glass typically consists of two glazing lites, but some manufacturers offer triple or even quadruple glazing. The space between the glass lites can be filled with a gas, typically argon, which enhances its insulating performance. The glazing panes are held together by spacers and include sealants and desiccants. The sealants ensure that moisture does not enter the insulating space, and the desiccants ensure the insulating space remains dry. IGUs may incorporate just about any type of glass, including annealed, heat strengthened, tempered, or laminated, and the glass can be coated or treated to achieve specific energy performance goals. IGUs can also receive decorative treatments.

Because of its versatility, insulating glass is used in a wide variety of exterior applications, including vertical and sloped glazing, skylights, and both vision and non-vision, or spandrel, locations. It can also be specified where impact, bullet, and blast resistance are required when constructed with an appropriate laminated lite to the interior of the IGU.

Key Concepts Related to Energy Performance of Glazing

When specifying glazing systems, it is important to have a solid understanding of several key concepts related to energy performance, strength, and impact resistance. Terms related to energy performance include U-factor, SHGC, and VT.

U-factor is a measure of the heat flow through the window due to convection, conduction, and radiation. The higher the U-factor, the more heat is transferred or lost through the window; hence, low U-factors are desirable for reducing heating and cooling loads. U-factors for the entire unit may be significantly higher than the U-factor through the center of the glazing.

SHGC represents the ratio of the solar heat gain entering the space through the glazing to the incident solar radiation. SHGC is represented as a number between 0 and 1; the lower the SHGC, the less heat is transmitted. In general, a low SHGC is desirable when cooling loads are high, and a higher SHGC is optimal for projects that rely on passive solar heating.

VT represents the percentage of visible light entering the space through the glazing system. VT is measured on a scale of 0 to 1; the higher the VT, the more light that is transmitted. Clear glazing has a VT of about 0.90. An adequate VT is desirable for effective daylighting.

VT can be affected by tints and coatings. Tints are usually added for aesthetic reasons, but they can also help reduce solar gain and glare. Coatings are intended to impact thermal performance by limiting the transmission of infrared and near-infrared wavelengths. Because these coatings reduce the emittance of radiant heat, they are often referred to as low-emittance, or low-e, coatings. Standard glass has an emittance of about 0.80, while glass with a low-e coating might have an emittance as low as 0.018. Emittance is typically not represented as a separate value but is factored into the U-factor of the glazing product. Hence, low-e coatings result in lower—aka better—U-factors.

There are two types of coatings: sputtered and pyrolytic. Sputtered coatings are applied as thin metallic coatings to finished glass. Because the coating is applied to the surface of the glass, it may be susceptible to degradation through oxidation or scratching during handling or installation. Some sputter-coated products may be tempered or fabricated after the coating has been applied.

Pyrolytic coatings are applied during glass manufacturing. The coatings essentially become part of the glass, making it extremely durable. Pyrolytic-coated glass can also be heat strengthened or fully tempered.

Coatings can be applied to either side of a glass surface. For laminated glass, the coating can be applied to one of four surfaces: the outside or inside of each of the pieces of glass separated by the interlayer. Similarly, double-glazed insulating glass offers four surfaces available for coating. Some low-e coatings are not recommended to go against the interlayer due to the color shift of the low-e coating after laminating. Low-e coatings can be used to maximize solar heat gain (passive heating) or limit the amount of heat entering the building (solar control). It used to be the case that pyrolytic coated glass was typically used for passive heating, while sputter-coated glass was used to decrease solar heat gain. There are no longer strict delineations like this.

Coatings can be used to create spectrally selective glass. This refers to glass that transmits desirable portions of the energy spectrum and reflects others. This enables the glazing product to be “tuned” for specific energy flows and daylighting.

Dynamic glazing with electrochromic coatings is also becoming more available. Dynamic glass can change its performance characteristics in response to the quantity and quality of light hitting it. It uses low-voltage power to electronically tint the glass to the desired level; when the voltage is removed, the glass returns to its original state.

Low-e coatings are sometimes evaluated using a value called the light-to-solar gain ratio, or LSG. This is calculated by dividing the VT of a glazing system by the SHGC. A higher ratio implies a higher-performance glass.

 

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Originally published in Architectural Record

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