Choices Abound for Attractive & Smart Facade Design

Mindful specifications to satisfy aesthetics, comfort, and sustainability

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Sponsored by AZON, Cascade Architectural, Inpro, and Vitro Architectural Glass

By Andrew A. Hunt

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The Lowdown on Low-E Facades

Large or jumbo glass is a favorite choice for building facades, but it has historically created some challenges with solar heat gain and comfort for occupants. Architects today can achieve the look they want on sprawling glass-dominant facades while still satisfying demanding performance expectations. Gone are the days of specifying large expanses of glass with the understanding that energy performance will be compromised.

Source: Measure Guideline: Energy-Efficient Window Performance and Selection. Building America Program, Energy Efficiency and Renewable Energy, U.S. Department of Energy. November 2012.

This graph shows how different types of low-e coatings can reduce the amount of infrared solar radiation. Note that by choosing different types of low-e coatings, more or less heat energy can be allowed to pass through the window. This is important when choosing window coatings in colder climates where additional solar heat gain may be advantageous.

To satisfy increasingly stringent energy codes (such as ASHRAE 90.1) and green building standards (such as LEED), low-e coatings can have a significant impact on energy efficiency by reducing the amount of solar heat transmitted into the building, even to the point of permitting specification of smaller HVAC systems.

Low-e glass coatings were developed to minimize the amount of ultraviolet and infrared light that can pass through glass without compromising the amount of visible light that is transmitted. A microscopically thin transparent coating allows low-e glass to reflect exterior heat in warm temperatures and hold in heat during cold temperatures, making buildings light, bright, and energy efficient.

GETTING BRIGHT ON LIGHT

A little understanding of basic science is helpful to appreciate how low-e coatings on glass facades can greatly impact the energy performance of a building. Visible light is a small fraction of the entire large light spectrum of energy that surrounds us every day. Microwaves, radio waves, x-rays, and ultraviolet are all forms of light that we cannot see. Also, outside the visible light spectrum is infrared, which is heat energy. Different materials can reflect, absorb, or manipulate the wave forms of light energy, and low-e coatings have a high reflectance to long-wavelength infrared radiation.

When applied to a pane of glass, the coating reduces long-wavelength radiative heat transfer between glazing layers by a factor of five to 10, thereby reducing total heat transfer between two glazing layers. Coating a glass surface with a low-e material and facing that coating into the gap between the glazing layers blocks a significant amount of this radiant heat transfer, lowering the total heat flow through the window. Low-e coatings may be applied directly to glass surfaces or to suspended films between the interior and exterior glazing layers.

There are many different types of low-e coatings available, but almost all are microscopically thin, virtually invisible metal or metallic oxide. The benefit is that low-e coatings can lower the U-factor of the window without interfering with the visibility. For example, uncoated glass has an emissivity of 0.84, while some advanced low-e coatings can reduce emissivity to 0.02.

Low-e coatings are broken down into three classifications: high, moderate, and low solar gain.

High-solar-gain low-e coatings typically have a solar heat gain coefficient (SHGC) value greater than 0.40 and are designed to reduce heat loss but admit solar gain. High-solar-gain products are best suited to buildings located in heating-dominated climates and particularly to south-facing windows in passive solar designs.

Moderate-solar-gain low-e coatings typically have an SHGC value of 0.25–0.40. Such coatings reduce heat loss, maintain high light transmittance, allow a reasonable amount of solar gain, and are suitable for climates with heating and cooling concerns.

Low-solar-gain low-e coatings typically have an SHGC value less than 0.25. These products are considered spectrally selective low-e glass and are used to reduce heat loss in winter and heat gain in summer. Low-solar-gain low-e coatings are generally specified in hot, sunny climates.

Spectrally selective coatings are optically designed to reflect particular wavelengths but remain transparent to others. Such coatings are commonly used to reflect the infrared (heat) portion of the solar spectrum while admitting more visible light. They help create a window with a low U-factor and SHGC but a high VLT.

A final consideration when evaluating low-e windows is the UV, or ultraviolet, protection the window coatings offer. Blocking UV light is important because it can protect furniture, art, carpet, and decor from the fading effects of UV exposure. Low-e windows can block more than 70 percent of the UV light coming through the window, but for projects such as galleries or offices with extensive art collections, finding a low-e coating with greater UV protection can be important.

Technological advances in coatings chemistry, such as the introduction of triple-silver- and quad-silver-coated glasses, have significantly improved performance over the past two decades. Today’s newest and most advanced low-e glasses are coated with extremely thin layers of silver that are applied to glass through the magnetron sputter vacuum deposition (MSVD) process, significantly improving performance. For example, when an MSVD coating is used on a 1-inch IGU with clear glass, it can block nearly 75 percent of the sun’s heat energy—while still allowing more than 60 percent of daylight to pass through. These low-e silver coatings can be applied to a range of glass substrates, specifically low-iron glasses for superior transparency and color fidelity as well as tinted glasses.

Smaller HVAC systems may be specified for buildings glazed with solar control low-e glasses, potentially reducing the associated up-front capital investment required. As a result, the initial facade investment may be repaid in a matter of months.

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Originally published in December 2019

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Choices Abound for Attractive & Smart Facade Design

Buyer's Guide

Azo-Core^™ High-Density Foam Thermal Barrier

Azo-Core^™ high-density foam thermal barrier by Azon for aluminum framing first made its debut in Europe. Thermal results of 0.25 U-value with double low-e insulating glass are achieved through actual testing. Beyond high thermal performance, Azo-Core maintains the structural aspects of the aluminum fenestration product (Quaker OptiCore^™ shown).

Azon
www.azonintl.com

Fabricoil^® Coiled Wire Fabric Systems

Fabricoil^® systems are utilized by architects and designers to achieve remarkable interior and exterior spaces. Fabricoil material is a flexible metal mesh that comes in a variety of materials, gauges, and weave sizes and is used for energy savings, solar shading, light diffusion, decorative applications, security, partitioning, and more.

Cascade Architectural
www.cascade-architectural.com

601 Joint System for Facades

The hinged cover or “pan” of the Inpro 601 system can be filled with glazing, metal panels, stone, or brick, disguising the expansion joint. The 601 is an ideal system for high-end exterior applications or conditions where the structural joint is located in a high-profile position on the building facade.

Inpro
www.inpro.com

Solarban^® Solar-Control Low-E Glasses

Trusted by architects for half a century, count on Solarban^® glasses to keep occupants comfortable and realize designers’ boldest visions. Solarban glass products feature some of the industry’s highest light to solar gain (LSG) ratios and can be combined with a wide array of tinted and low-iron glass options.

Vitro Architectural Glass
www.vitroglazings.com/solarban