Glazing and Energy Codes
Architects and building owners face growing challenges in balancing aesthetics and daylighting design needs with increasingly stringent building and energy code requirements. This month’s discussion will focus on ways to reduce heat loss and heat gain to comply with commercial energy codes. Thermal energy performance of glass can be improved several ways; many of them are just now beginning to be incorporated into the commercial segment.
Warm Edge Technology (Conductive heat loss)
Several products have been introduced that will help reduce conductive heat loss through the edge of insulating glazing units (IGUs). Warm-edge technology will also help reduce condensation that typically occurs around the edge of glass near the frame. Below are some of the different types of insulating glass (IG) spacers available.
1. Metal spacers: Made from stainless steel or aluminum. Dessicant consists of tiny beads which absorb any moisture trapped in the unit during manufacturing. Stainless steel offers better performance than steel.
2. Hybrid spacers: Changing metal spacers from a tube to a U-shaped channel reduces the flow of heat through the spacer.
3. Thermal break spacers: Thermal barrier technology creates a warm-edge IGU that reduces thermal conductivity.
4. Foam and thermoplastic spacers: Non-metal spacers include a foam material that has dessicant entrained within it and thermoplastic spacers consisting of a single-component polyisobutylen with included desiccant material.
Argon and Krypton Gas Fill (Convective heat loss)
An improvement that can be made to the thermal performance of IGUs is to reduce the movement of air between the panes of glass. Typically, the space is filled with air or flushed with dry nitrogen just prior to sealing. In a sealed IGU, air currents between the two panes of glazing carry heat to the top of the unit and settle into cold pools at the bottom. Filling the space with a less conductive, more viscous, or slow-moving gas minimizes the convection currents within the space, conduction through the gas is reduced, and the overall transfer of heat between the inside and outside is reduced.
Argon is inexpensive, nontoxic, nonreactive, clear and odorless. The optimal spacing for an argon-filled unit is the same as for air, about ½-inch (11-13 mm). Krypton is nontoxic, nonreactive, clear, and odorless and has better thermal performance, but is more expensive to produce. A mixture of krypton and argon gases is also used as a compromise.
Low-E Coatings (Radiant heat loss)
A great deal of winter heat loss (and summer heat gain) is due to radiation. In winter, low-E coatings help “reflect” heat energy back into the building. They also increase the surface temperature of the interior glass. This is very important when considering human comfort levels. People lose body heat in four ways:
• Conductive heat loss between the air and exposed skin;
• Convection heat loss due to air moving across the skin (think wind chill factor);
• Evaporative heat loss due to moisture on the skin evaporating (you feel hotter on a humid day because the skin cannot evaporate as much moisture), and
• Radiant heat loss due to the human body being warmer than the surrounding surfaces. More than 50 percent of body heat loss is due to radiation. The warmer we can make our surroundings, the less heat we will radiate to those surfaces and the warmer we will feel. This is referred to as “Mean Radiant Temperature.” Increasing the surface temperature of the glass will result in a higher mean radiant temperature and ultimately a greater feeling of human comfort.
In summer, and year round for most commercial buildings, we want to limit the amount of solar radiation entering the building, which increases air conditioning loads. Low-E coatings are very effective at minimizing the amount of solar radiation entering the building. In order to meet the 2012 Energy Code, areas of the southern U.S. in Zones 1, 2 and 3 will need to have a fixed glazing system solar heat gain coefficient (SHGC) of 0.25 or less. This can best be achieved with IGUs and low-E coatings. The days of ¼-inch single glazing in storefront and curtainwall are pretty much in the past.
• DOE Energy Efficiency and Renewable Energy, www.eere.energy.gov
• Windows for High Performance Commercial Buildings, www.commercialwindows.org
• Efficient Windows Collaborative, www.efficientwindows.org
• Online Code Environment and Advocacy Network, www.bcap-ocean.org