By Amy Roberts
Energy efficient buildings contribute to greater utility savings for building owners and facility managers with potential pass-through to tenants. The building envelope’s fenestration systems — its curtain wall, storefront, windows, doors, skylights, and other glass and glazing components — typically provide significant opportunity to contribute to energy efficiency and savings.
Fenestration’s influential impact on a facility’s energy goals have concentrated on improving thermal performance by minimizing conductivity, the heat energy transferred through the building envelope. In simple terms: Keep the cold outside in winter for heating-dominated, northern climate zones. Keep the heat outside in summer for cooling-dominated, southern climate zones. Maintain a steady, comfortable interior temperature year-round.
The insulating glass (IG) unit is the primary solution in all commercial fenestration applications seeking improved thermal performance. An IG unit consists of two or more lites of glass separated by insulating and structurally supportive spacers to create a hermetically sealed cavity that can be filled with dry air or an inert gas — typically argon. An IG unit’s performance can be further increased by adding low-emissivity (low-e) or reflective coatings, tinted glass, and insulating methods. Increasing the number of glass lites and cavities, such as with triple IG units, also can upgrade performance.
Practically speaking, the ability to increase energy efficiency by lowering fenestration’s conductivity may have reached its upper limit. Now, glass and fenestration manufacturers are digging deeper to wring out incremental energy savings by focusing on radiant heat transfer through solar heat gain optimization. Advanced glass coatings have moved to center stage.
Controlling Solar Energy
Incident solar radiation consists of three types: visible light (VL), infrared (IR), and ultraviolet (UV), the latter of which can cause fading of window treatments, artwork, furniture, and carpets. In many commercial buildings, the ideal energy-conserving glass would let in most of the visible portion of the solar energy — reducing lighting requirements in the facility — while blocking most of the UV radiation and IR heat energy.
The IR solar radiation that is transmitted through architectural glass is expressed as a fraction between 0 and 1. This fraction is called as a solar heat gain coefficient (SHGC). The lower the SHGC is, the less solar energy it transmits. Generally, a low SHGC is desirable in warm climates, especially through south-, west- and east-facing fenestration products, and a higher SHGC is desirable in colder regions. For comparison, clear glass registers an SHGC of 0.90 or greater.
Low-e coatings are a relatively recent innovation that has become the premier choice for improving thermal performance by allowing certain wavelengths of solar radiation to pass through more easily than others. This helps to manage solar heat gain in both warm and cold climates. Emissivity is a measure of a material’s ability to re-radiate absorbed IR radiation, so low-e indicates an ability to reduce heat absorption. In general, highly reflective materials have a low emissivity, and duller, darker colored materials have a high emissivity.
Low-e glass features a nearly invisible, microscopically thin coating of metal or metallic-oxide, such as silver or tin oxide. This coating is applied to a surface of one or more panes of an IG unit to deliver an emissivity as low as 0.04. This allows for the reflection of as much as 96% of IR radiation back toward the source.
The placement of low-e coatings within an IG unit also matters, depending on climate and the energy efficiency objective. The IG can be fine-tuned for heating-dominant or cooling-dominant climates by varying the glass surface to which the coating is applied. The glass surfaces are numbered, with #1 representing the exterior or “weather” surface. Figure 1 (below) represents a double-pane fenestration product with surfaces from 1 to 4. Similarly, a triple-pane product would have surfaces from 1 to 6.
In northern, heating-dominated climates, a low-e coating on the internal surface of the interior pane (#3) or the interior surface (#4) of an IG unit reduces heat loss by reflecting heat back into the interior, while reflecting the HVAC system’s heat energy back inside. (Before selecting a #4 surface product, the manufacturer should be consulted regarding any special considerations of use.)
In southern climates which are cooling dominated, a low-e coating applied to surface #2 reflects or rejects the solar heat back outdoors, lowering solar heat gain and keeping the room cooler. Some versions can block nearly 80% of the sun’s radiant energy, while transmitting more than 50% of the available sunlight.
Some IG units have low-e coatings on two surfaces. For example, a triple-pane IG unit with low-e coatings applied to the #2 and the #5 surfaces of an IG unit could reduce the thermal performance, as measure at the center-of-glass, by 31% compared to a unit with a single-coated lite.
For climates that get cold in winter and hot in the summer, dual-pane IG units with a silver-based low-e coating on surface #2 can be specified with a coating on the #4 surface to reflect escaping heat back into the room. Spectrally selective low-e coatings — which combine the best qualities of low-e, tinted and reflective glass — are also good for mixed climates where both heating and cooling are needed. They help restrict the heat energy from the sun, but allow in much of the light. Applying different combinations can give a broad range of SHGC between 0.78 and 0.17.
Different types of low-e and other coatings can be combined or applied to different IG glass surfaces to achieve a targeted performance level.
One combination, recommended for use in cold climates, is coated with a microscopically thin, optically transparent layer of silver that is sandwiched between layers of anti-reflective metal oxide coatings. An invisible, protective coating is applied to ensure durability and long life.
Low-e coatings also can have multiple layers. For example, one version adds a third layer of silver coating, resulting in a clear coating that blocks even more solar radiation, reflects heat and provides a high visible transmittance, which enhances daylighting. Such a combination actually outperforms tinted glass as often used in warm climates.
Another option is use of laminated glass with anti-reflective coatings on both surfaces #1 and #4, which reduces interior and exterior visible light reflectance to less than 2%, while blocking more than 99% of UV radiation.
Perhaps surprisingly, other types of coatings that can be combined with low-e to maximize performance include those that can contribute to a facility’s electric power supply through building integrated photovoltaic applications. Others can lower maintenance costs through coatings that help to break down organics and pollutants with help from the sun’s UV rays, helping glass stay cleaner for longer.
For most current applications, a double-pane IG unit with appropriate low-e coatings can meet most building’s energy savings goals. However, attitudes and requirements are changing quickly, and more efficient products are being demanded by owners, occupants and municipalities.
As glass and fenestration plays an ever-increasing role in the overall appearance of commercial landscapes, custom-tailored IG units can incorporate various glass types, thicknesses, coatings or tinted substrates to create buildings that are energy-efficient, economical and attractive. Glass and fenestration manufacturers can help in selecting products uniquely suited to meet a facility’s performance, aesthetic and financial goals.
Roberts is FGIA Director of Canadian and Technical Glass Operations. She oversees the Fenestration and Glazing Industry Alliance (FGIA) Canadian standards and regulatory building/energy codes, as well as the Insulating Glass Certification Council/Insulating Glass Manufacturers Alliance (IGCC/IGMA). She has more than 20 years of industry experience in glass and IG manufacturing, and in both residential and commercial window manufacturing.