AGG - 2006 - March/April - Lettting the Sun Shine

Volume 20 • Issue 2 • March/April 2006

Letting the Sun Shine
Options in glass coatings for solar control
By Don McCann and Sara Theis

Two methods of manufacture exist for high-performance glass coatings. With pyrolytic deposition, layers of metal are applied to molten glass during the manufacturing process. The coating then becomes physically embedded into the glass surface, thus they are commonly referred to as “hard coats.” In the second method, magnetic sputter vacuum deposition (MSVD), metals are sputtered onto finished glass in a large vacuum chamber. These products are commonly called “soft coats.”

High-performance glass coatings have never been more popular because of increased requirements of energy codes and environmental consciousness driving sustainable design in commercial buildings, as well as the aesthetic attributes they offer.

The selection of glass coatings continues to evolve and broaden. Today, a plethora of coated products exists and understanding their differences is necessary to specify the appropriate product for a project.

The term “coated glass” is a generalization for what has become a broad category of glass products that combine optically thin layers of metal to offer improved solar-energy performance, provide exterior color or hue, and influence occupant comfort.

Glass coatings can be divided into three broad categories: Low-emissivity (low-E) which are generally high light transmitting, low reflectance; hybrid low-E which are high light transmitting/slightly reflective; and reflective which are very low light transmitting/highly reflective. Within each category different methods of manufacture play a role in the level of performance the coating will achieve.

Making a Deposit

Two methods of manufacture exist for high-performance glass coatings. With pyrolytic deposition, layers of metal are applied to molten glass during the manufacturing process. The coating then becomes physically embedded into the glass surface, thus they are commonly referred to as “hard coats.” By nature, the pyrolytic deposition process and the limited number of metals that can be used in the process restrict the level of solar-energy performance that can be achieved. In addition, pyrolytic low-E coatings are not offered in combination with all colored glass substrates. These products are generally used where the desire exists to improve energy U-values compared to non-coated glass.

In the second method, magnetic sputter vacuum deposition (MSVD), metals are sputtered onto finished glass in a large vacuum chamber. These products are commonly called “soft coats.” This process allows an expanded list of metals to be used, resulting in improved product U-values and overall solar performance. In addition, this method offers increased flexibility to apply the coating to a variety of colored glass substrates, further enhancing solar performance and offering more aesthetic options.

For example, if a clear uncoated monolithic lite of glass is upgraded to an insulating unit with a vacuum deposition low-E coating on the #2 surface, the solar performance can be optimized by up to 50 percent (based on the use of Solarscreen 2000 low-E coating by Viracon and a southern climate).

Looking at the Differences

Let’s look at the differences between the three categories of glass coatings to help in specifying the right product for the application.

The low-E category of coatings was first available in pyrolytic form. The overall performance of low-E coatings has since been enhanced with the introduction of the vacuum deposition process. This family of coatings is characterized as semi-transparent and flat in appearance providing generally ultra-high visible light transmission, low reflection and reduced heat transfer. They are available in a wide range of thermal performance contributing to a building’s sustainability, as well as extensive color/hue options for the desired aesthetics. The vacuum deposition low-E coatings cannot be exposed to the elements; therefore, they must be encapsulated within insulating or laminated units.

Hybrid Low-Emissivity Coatings

Within the past five years, hybrid low-E coatings have entered the commercial marketplace. These products are manufactured via the vacuum deposition process and are required to be encapsulated within insulating glass units. They are referred to as “hybrid” because they combine the best performance attributes of a low-E coating with a vivid, crisp exterior appearance and low interior reflectance yet also reduce solar heat gain to a level that was not previously achievable with standard low-E coatings.

For example, by applying a hybrid low-E coating to the #2 surface of an insulating unit, solar performance can be improved by as much as 40 percent compared to a standard low-E coating (based on the use of Solarscreen 2000 low-E coating on clear glass compared to Viracon’s VRE1-38 based on SHGC).

Reflective Coatings

In comparison to low-E coatings, reflective coatings offer a highly reflective, mirror-like exterior/interior appearance that reduces heat gain through high solar reflection. They were originally introduced as vacuum deposition coatings and are now available as enhanced performing pyrolytic products as well. The metal options that have been used range from basic stainless steel to real 24k gold, offering a large palette of reflected colors.

Who’s on First

The surface orientation of the coating is important for controlling heat transfer. To review, in a standard insulating unit, there are two lites of glass with four surfaces that are numbered starting from the exterior façade. In most commercial applications, the coating is fabricated on the #2 surface. The reason for this placement is to put the coating as close to the exterior of the building as possible, in return, reducing the amount of heat gain into the building.

An example of the importance of surface orientations occurs when the coating is moved from the #2 surface to the #3 surface. When this happens, the shading coefficient will increase, offering less solar performance. This causes the temperature of the glass surface to increase—sometimes dramatically. Calculations with Lawrence Berkley Laboratories (LBL) Windows 5.2 estimate a surface-three placement of a low-E coating can impact the glass temperature by as much as 12 percent, increasing the surface temperature by up to 13ºF, thus placing a larger burden on the HVAC system and impacting the comfort level of building occupants

Most commercial buildings have an issue with heat gain, which places stress on the HVAC system. Generally, heat gain comes from artificial lights, equipment in the building and people inside the structure; but the majority comes from the sun in the form of infrared heat. To further enhance energy control and the overall performance of the glazing, the coatings can be combined with insulating units and silk-screened patterns; the glazing may also introduce the safety features of a laminated component.

It is apparent that fabricated glass products, and specifically high-performance glass coatings, have become a critical component in energy conservation and occupancy comfort. As energy codes for buildings become more stringent and Leadership in Energy Efficient Design (LEED) or green sustainable design are influenced into more building specifications, the demand for these glass products will continue to escalate.

Don McCann, CSI, AIA Allied, is manager of the architectural design group at Viracon. He presents (AIA accredited educational seminars and has spoken worldwide on a variety of architectural glass topics. Sara Theis is a product specialist at Viracon. She has been with the Owatonna, Minn. based company since 1998 working in various capacities within the sales and marketing departments.