A Clear Design Goal
Today’s windows provide more than just a view. They have to protect, and even help to green, modern commercial spaces.
Joshua Early, TRACO
Windows in a commercial building provide a view to the outside world, but today are also becoming an integrated part of the overall building design and must satisfy more than just the building occupant. Architects, owners, facility managers, contractors, and historic-landmark experts all have higher expectations for windows. This increasingly includes a requirement that windows also contribute to today’s green building practices.
|More than just providing a view to the outdoors, today’s windows help protect building and occupants.|
Windows also have to satisfy a higher level of security against terrorist threats; provide protection against horrific weather; still provide natural daylight, energy savings, long-term performance; and offer architectural aesthetic appeal. Window research-and-development teams are exploring technologies such as dynamic glazing, photovoltaic designs, and automatic controls to meet a demanding and evolving commercial window market.
According to the online encyclopedia Wikipedia, green building is the practice of increasing the efficiency with which buildings use resources, i.e., energy, water, and materials, while reducing building impact on human health and the environment, through better siting, design, construction, operation, maintenance, and removal-throughout the building’s lifecycle.
According to the U.S. Green Building Council, Washington, architects, real-estate professionals, facility managers, engineers, interior designers, landscape architects, construction managers, lenders, and government officials use LEED certification to help transform the built environment to sustainability. State and local governments across the country are adopting LEED for public-owned and public-funded buildings; there are LEED initiatives in federal agencies, including the Depts. of Defense, Agriculture, Energy, and State; and LEED projects are in progress in 41 countries, including Canada, Brazil, Mexico, and India.
Windows play a big role in green design. They contribute to indoor environmental quality, optimized energy performance, and recycled content for LEED project points. Energy-efficient products help with the overall building energy assessment and thermal comfort for the building occupants.
Windows are a primary source of energy exchange in a commercial building. According to the U.S. Dept. of Energy (DOE), Washington, windows are the weak link in the building envelope, and as much as 30% of the energy loss in buildings is through windows. One sq. ft. of window conducts approximately 10 times as much heat as 1 sq. ft. of wall. By increasing the energy efficiency of windows, a commercial building can reduce its requirement for air conditioning and heating, and because electricity is a major contributor of nitrogen oxide, help lower levels of nitrogen oxide emissions as well.
“Energy is an increasingly important issue for builders and developers these days. We have a great challenge with aluminum products, but we are continuing to find the technology to meet the standards. We have to maximize the performance of the components that are needed for new and renovated buildings,” said Mike Manteghi, director of research and development for window supplier TRACO, Cranberry Township, PA, and a board member of the National Fenestration Rating Council (NFRC), Greenbelt, MD.
The DOE is pushing to ultimately achieve zero-energy commercial buildings by the year 2025, according to Manteghi. “As a result, we are looking at the U-value of the overall product and solar-heat gain coefficient[SHGC]. We are also seeing a trend toward high-performance glass and the use of more glass in the future to brighten buildings,” Manteghi added.
There are two energy-code criteria that must be met-SHGC, and the U-value of the thermal property. High-performance windows with low U-factors result in inside glass surface temperatures much closer to the room air temperature. Windows with warm-edge technologies and non-metal frames are also less likely to have condensation on the frame or at the glass edge.
Using windows for daylighting offers several benefits to the building and its occupants. Daylighting can reduce electricity consumption from the interior lights when used in conjunction with automatic lighting controls. This saves energy costs when the natural daylight is sufficient and the electric lights are dimmed. Natural daylight also provides a healthier building environment by providing a positive effect on the occupants and potentially making the environment more productive. The following issues continue to be important in window design when considering daylighting:
- color and glazing
- reflective glass
- light-to-solar ratio
- window-to-wall ratio.
The desirable color qualities of daylight are best transmitted by neutrally colored tints that alter the color spectrum to the smallest extent.
To the greatest extent possible, designers should avoid the use of reflective glass or low-e coatings with a highly reflective component. These reduce the quality of the view and the mirrored effect is unpleasant to occupants after dark.
The use of high-performance and selective low-e glazing reduces the visual light transmission (VLT) proportionately less than reflective coatings or tints. Dividing the VLT by the SHGC is a good rating of the performance of the glass. If the result is less than 1.0, then the glass is a poor choice for visual quality and daylighting. If the result is higher than 1.55, it is a high-performance visual and daylighting option.
For view and a positive connection to the outdoors, people prefer a minimum 20% to 30% ratio of window area to wall area. Glazing the wall areas below “desk height” (0 to 30 in. above the floor) offers little or no benefit for daylighting an office or view for the occupants.
Glass as fortress?
Our awareness of terrorist acts directed at buildings has significantly increased. Several major events in the past two decades have led the federal government to mandate blast mitigation for all new construction and most major retrofits. The fenestration industry is focusing its efforts on producing glass products that not only protect our federal buildings, but also protect nearby buildings, people, and occupants. They are also working with the government to establish standards that cover security and protective glazing.
Bomb blasts are the worst-case scenario when it comes to terrorist attacks on buildings. Blasts require special fenestration solutions, as blast loads happen quickly. High-level targets include government buildings; federal, state, and local agencies; domestic and overseas operations of U.S. companies; military compounds; Washington in general; and buildings and homes near the threat areas.
According to Applied Research Associates Inc., Albuquerque, in the past, blast-resistant design was typically used only for facilities close to explosive materials, or those that were well-known targets of attack. We are living today in an environment of enhanced risk that requires protective designs and risk management for most public and private facilities.
The General Services Administration, Washington, has created several national standards and criteria for blast resistance such as Window Glazing Analysis Response and Design, and U.S. General Services Administration Standard Test Method for Glazing and Glazing Systems Subject to Air Blast Loadings.
While windows are important architectural and functional components of a building, glass fragments caused by accidents, disasters, or events such as terrorist attacks, can seriously injure building occupants. To mitigate glass-fragment hazards, designers must consider a multitude of factors, including a building’s occupancy, function, and anticipated threats and risks to people and mission.
As a result, protective glazing-material design and selection may not be simply a matter of following building-code requirements. The solution may be a governing component of the building envelope, site, or interior design, around which other building systems must be designed and integrated.
Mitigating the blast
Today, blast-mitigating window systems must do the following:
- prevent deadly glass shards from flying into the room
- keep glass in the frame
- keep the frame from disengaging from the wall
- keep the surrounding wall intact.
Protective glazing is used to counter threats to buildings and occupants including bomb attacks, ballistic attacks, burglary or robbery incidents, forced entry, detention containment, and natural disasters such as seismic occurrences, hurricanes, and tornados. A selection of the most appropriate protective glazing systems must be made for each project to address the specific threat. The type of protective glazing material selected may also vary within a building depending on the location of the window or building, size of the glazed opening, occupant load, and criticality of functions and missions housed within the facility, as well as other considerations such as whether windows will be fixed or operable.
According to the website Whole Building Design Guide (WBDG), www.wbdg.org, in any bombing attack there are three basic types of effects that occupants may experience: the human body’s response to direct blast loadings; fragment and debris impacts; and loss of balance and subsequent impact of the person into his/her surroundings, due to the passing blast wave or violent movement of a structure.
The debris generated, or the collapse of structures produced during an explosive attack causes the majority of injuries and death in a bombing event. For example, more than 5,000 people were injured by flying glass and debris in the bombings of two American embassies in Africa in 1998. The types of injuries that occurred included deep lacerations and eye injuries.
When designing window systems to resist blast forces it is important that the glazing, framing, and anchorage be designed to withstand the required forces. Generally, the glazing should be the weak link. i.e., it is not desirable for the window system to prematurely fail and blow into occupied spaces due to failure of the frame or anchorage. This approach is commonly referred to as balanced design.
Hurricane research has proven that flying debris, rather than high winds, is the main cause of window failure during hurricanes. Once flying objects pierce windows and doors, hurricane-force winds can enter the structure. Internal pressure causes the roof to blow off, resulting in massive wind and water damage. As a result, new laws have been enacted to regulate and test windows and doors marketed as impact resistant. These products must be able to withstand initial flying debris impacts and then hold up against hurricane-force wind gusts. Once a product passes these stringent tests, it can be classified as impact-resistant.
Many methods of protecting a home or building against hurricanes exist, including plywood, window film, shutters, and impact-resistant windows. None can guarantee the viability of a building, but testing methods today help to evaluate success in expected conditions. The current “gold standard” in testing is the Miami-Dade County Hurricane Impact Test, which simulates storms with winds of 110 mph or greater.
Wind speeds of 110 mph or greater are possible, and prepared for, throughout Florida and all coastal regions of the United States, but that does not mean that every impact-resistant window will work in every area. Along with hurricane performance, energy efficiency must be considered.
In the near future, builders will be specifying impact-resistant windows from the coast of Texas to the coast of Maine. Not only are codes driving this migration toward impact-resistant windows, but insurance companies are providing incentives for added protection to residential and commercial structures. Many states have adopted impact codes. However, according to the Ducker 2006 U.S. Market Study Report (Troy, MI), only 51% of commercial buildings in Florida have some sort of storm protection and only about 1% of the commercial buildings along the Gulf and Atlantic coast, outside Florida, have some sort of storm protection.
Even more high-tech windows are just around the corner or already appearing as test designs in green commercial buildings. Photovoltaic windows take what used to be a passive component of the building envelope and make it an active component in which the window generates power for the building.
Control systems integrated into the building design automatically open and close windows to optimize heating and air-conditioning loads, saving energy costs or ventilating the building when carbon-dioxide levels reach a certain point to maintain indoor air quality. These systems can also monitor the inside and outside air temperature, building humidity and air quality, and automatically close windows depending on the weather.
Also being tested in today’s greenest buildings: automatic control of shading or daylighting for the building, as well as systems that automatically raise or lower blinds or shades depending upon the solar heat gain or glare at a person’s workstation. Dynamic glazing, which varies the tinting of window glass according to weather conditions, also can be integrated into the control system.