Volume 40,   Issue 6                            June  2005

BetweentheLites

Energy Matters
   
Looking at Windows from a "Green" Perspective

by Randy Brown

Today, energy-efficient windows are key elements in the development of any environmentally responsible “green building.” Architects, developers and building owners have additionally recognized that the meaning of “green” includes windows that promote a healthy environment for building occupants by combining daylighting, natural ventilation, noise and ultraviolet (UV) protection, as well as condensation and mold-resistance.

These green fenestration attributes are promoted and certified by Leadership in Energy and Environmental Design (LEED), a rating system developed by the U.S. Green Building Council (USGBC) (see related article in the February 2005 USGlass, page 60). 

Thermal Efficiency 
Today’s windows are often loaded with features that deliver many environmental, energy-efficient and economic benefits.

Green windows are designed to improve energy efficiency by controlling the forms of heat transfer. Heat or thermal heat energy can be transferred through a material in three manners. 

• Conduction is simply the process where heat is transferred through materials that touch one another;

• Convection is where gases or liquids circulate to transfer thermal energy; and

• Radiation transfers heat energy at a distance through high frequency waves such as visible light, UV or microwaves. The heat transfer accomplished by greenhouses and microwave ovens are examples of radiation. 

Virtually all commercial buildings and residential structures that are more than two or three stories use aluminum windows for strength, durability and code compliance.

Unfortunately, aluminum is a good conductor of heat and therefore may compromise the goals of energy efficiency. Early versions of aluminum windows readily conducted thermal energy. Newer aluminum windows, though, often have thermal break frames, which greatly reduce the conduction problem. A thermal break is a non-metallic resin or plastic material installed in the metallic window frame that physically separates the interior part of the window from the exterior part. 

In the Airspace
The air space between the glass lites in an insulating glass unit usually is filled with air dried by desiccant. Inert gases can be substituted for air in this sealed space as an energy-efficient option. Inert gases are much denser than air and therefore slow the heat transfer process by reducing convection. Argon is the most commonly used gas in this application. Others, such as Krypton and Xenon, are more expensive and typically only used in special, large curtainwall or window wall applications.

An often-overlooked energy efficiency item with insulating glass systems is the type of spacer used. Older technologies used aluminum box sections, but these conducted heat readily between the glass and the spacer, and created unnecessary thermal stresses on and premature failures of the hermetic seal. 

State-of-the-art window designs often incorporate warm-edge spacers. These systems do not have direct metal-to-glass contact, but separate the window glass from the spacer structure with less heat-conductive materials such as butyl sealants or plastics. 

During installation, installers must avoid creating any thermal “short-circuits” to the aluminum window thermal break system. New windows are secured in place by metal screws and anchoring devices that, if installed incorrectly, can bridge the frame’s thermal break and create an unwanted pathway for heat to transfer from the building interior to exterior or vice-versa. 

Protecting Human Health 
Studies have shown that buildings with operating windows contribute to individuals’ physical and psychological comfort by permitting individual control over ventilation in different sections of the building. Windows with warm-edge spacers used in the insulating glass system offer optimum protection from undesirable humidity that could lead to unhealthy, mold-breeding conditions on damp surfaces.

LEED for Existing Buildings 
The green building movement has been led initially by architects and builders, following the certification protocol of LEED for New Construction (LEED-NC).

In 2002, the USGBC initiated its LEED certification program for existing buildings (LEED-EB). Since there are nearly 100 times as many existing commercial, industrial and institutional buildings as there are new buildings built every year, the impact on energy savings and the environment will have an enormous impact on building operators and occupants across the United States. 

The impact of high-performance operating windows on achieving LEED points is significant. In the area of Energy and Atmosphere, LEED-EB awards a building up to 10 points for achieving an Energy Star® score ranging from 63 to 99. In the LEED-EB realm of Indoor Environment Quality, one point is awarded for increasing outdoor air ventilation, and another for providing individual ventilation control. One point each is awarded for daylighting 50 percent or 75 percent of “critical visual” workspaces and one point each for achieving “views” from 45 percent or 90 percent of occupied spaces. Under the program, buildings are required to provide a permanent monitoring system. This helps to ensure optimum performance of the building, in turn benefiting not only building owners, but occupants as well. 

The Author:
Randy Braun is director of commercial business development for Edgetech I.G. Inc., located in Cambridge, Ohio. 


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