Fine-tuning the office environment - Window coatings can let the sunlight in and still keep the sun's heat out.


By Al Dueck and John Hockman
Article appeared in Canadian PRoperty Management

Office space has changed significantly in the past 20 years, and glass technology innovators have been doing their best to keep up. With more computers and more people, offices and HVAC systems inevitably feel the heat - literally. That's before the heat gain from sunlight is even taken into consideration.

Solar energy radiates in short wavelengths, and standard clear window glass is fairly transparent to it. Once solar energy gets in and strikes furniture and other objects in a room, the objects heat up and give off long wavelength energy. Even clear glass blocks some of this long wavelength energy, trapping it in the building. Early lowE window coatings were designed to trap even more heat inside a building.

Today, lowE coatings are available that permit even more natural light and heat to be captured and kept in the building. The AFG E2 and the PPG 500 are hard coat low E solar gain products. These trap heat inside the building. More importantly for commercial buildings, which often have to cool parts of the building even during cooler weather, coatings are now available to reduce the entry of the solar energy. Tinted glass has been used for decades to control solar heat gain, but usually with a big reduction in visible light transmittance; and the increased need for electric lighting levels even during daylight hours. The new lowE coatings allow a tremendous amount of sunlight into an office without the associated heat gain problems.

Building designers are able to manipulate a building's internal climate in this manner. For instance, AFG's Ti-R and Ti-AC and PPG's Solban are soft coat low E products, which provide a low solar heat gain coefficients (SHGC). A standard triple-glazed window will allow 60% of the solar energy to come through glass, while these special products can reduce that level to between 30% to 35%. At the same time these new coatings permit over 65% of visible light to enter the building, reducing the need to operate lighting systems, which further reduces the internal heat gain and building operating costs. This type of product is ideal for windows that face a westerly direction. Buildings with large glass areas facing west can become unusable due to the heat of the afternoon sun. Such a structure might have low airflow requirements for heating and much higher airflows required for cooling. This can lead to a difficult balancing act for HVAC designers and building maintenance staff. Selection of a coating product with a low SHGC at the design or retrofit stage can offset this problem.

Determining which type of product to use is the greatest challenge. It isn't as simple as looking at the direction in which a window faces and deciding which coating technology to use.

There are computer simulation programs that help managers select window applications. The Canadian Building's Incentive Program (CBIP) uses EE4, a software program that uses a detailed energy simulation program, US DOE 2 (Department of Energy) to predict energy usage. Using software such as this can aid in the selection of the most appropriate coating for the various windows in a commercial building, leading to large energy savings and improved occupant comfort. It is almost impossible to do that effectively without such a program.

As a final note, many people install window blinds thinking that it will solve their heat gain problems. But blinds still let the heat in through the glass, and once the heat comes through the glass it is in the space - it just takes longer to transfer into the rest of the room. Meanwhile, they block out the visible light. With the right coating, selected carefully, the sunlight comes in without the heat, which can reduce lighting costs as well as cooling costs.