Getting Windows Right: A Guideby David Altenhofen
GreenBiz.com, August 2006
The corner office is a hallmark of success. People love natural light and expansive views; windows and curtain walls are among the most expressive design tools, and an executive on the rise is sure to see this as a perk. However, building owners and facility executives know that windows can mean trouble. They leak air: running down leaks in new buildings can be torturous, and maintaining or repairing older buildings' leaks may be expensive. Those large fields of glass also can create drafts and cold spots, opening the way for higher long-term operating and maintenance costs.
Getting windows right takes a lot more than drawing a pretty picture of a building and hoping for the best.
Know Your Window Systems
Window systems come in three major framing and glazing types: windows, storefront, and curtain wall. Windows are individual units, fixed or operable, set within a wall. Windows are available in wood, aluminum, steel, PVC, and combinations of these materials. Standard windows, generally glazed at the factory, come in a great many sizes and shapes; with custom units, the sky is the limit.
Storefront systems are used for larger areas of glazing than standard windows; they typically span from the floor to structure in the ceiling above. Storefront systems typically include entrance doors and vestibules. Glass in storefront systems is generally field-installed.
Curtain wall is typically used to glaze large areas of buildings. You've seen then suspended outside of the building structure, spanning floor levels. Curtain wall generally is glazed from scaffolding erected on the outside of the building.
Both storefront and curtain wall are made from aluminum extrusions, but can be found in wood and steel. Unitized curtain wall is a special type where modules -- typically one-mullion-bay wide by one floor high -- are assembled and glazed offsite, then hung on the exterior of the building. This system costs more to engineer and tool, is cost effective with repetition. That's why most large projects are done in this manner.
A window's type determines how it will behave, principally in terms of resistance to air and water. Most manufacturers subject their products to standard tests. Generally, curtain walls have the highest resistance to air and water penetration, while storefronts have a slightly lower resistance. Windows vary considerably in performance, from barely adequate under normal conditions to extremely tight during a hurricane.
Architects, facility managers, and builders know that manufacturers occasionally advertise performance based on a very small window. The results might not fit bigger jobs. And the tested values published for many systems, as well as the minimum values suggested in off-the-shelf master specifications, frequently are inadequate for the project. The pressure is on the architect to specify the best performance levels, without incurring unnecessary costs.
Light, Heat, Cooling Counts
When considering a window or window system's performance, start at the glass, which comes in a nearly endless array of types and combinations. Glass can be clear, tinted, reflective, patterned, printed, painted or invisibly coated to control solar radiation (low-e). Among these types, glass can be normal float glass, heat strengthened or fully tempered; it can be assembled in various combinations to create laminated glass with a variety of colored or patterned interlayers.
All of these types can be assembled to create insulating glazing units where two panes of glass sandwich sealed air to improve thermal performance. Commercial glazing in most of the United States is insulated glazing units.
Window systems have a substantial effect on heating and cooling in a building. The large expanse of windows often found in commercial buildings generally increases the cooling load (due primarily to solar radiation); the smaller, operable windows typical of residential buildings generally increase the heating load (due to thermal transmission and infiltration). Lowering light transmittance of glass with tinting, reflection, low-e coatings or shading can reduce cooling loads. Improving U-value with insulating glass and thermally broken frames can reduce heating loads.
The final selection of glass frequently balances conflicting desires: one, to maximize daylight, which means clear glass; the other, to minimize heat gain, usually with tinted or reflective glass. Clear or slightly tinted glass knocked reflective and dark bronze glass from its perch. Low-e coatings can provide good thermal performance and still allow for clear glass. If you need to minimize heat gain, you might need darker tints.
How will the occupant use the product? Offices where people sit all day must have better thermal performance than a lobby people pass through quickly. The most typical problem with windows is that large areas of cold glass can make people uncomfortable, no matter the air temperature. It is a law of physics that a warm body will radiate heat to a nearby cold surface. The cold glass will also set up drafts, adding to the discomfort. These problems can be easily overcome by washing the glass with a warm curtain of air, either from heating units at the bottom of the windows or from continuous air diffusers at the top.
Daylighting: Following Europe's Lead
Daylight is important to human well-being. Not only do studies show that daylighting improves productivity, they also predict increases in Seasonal Affective Disorder, a form of depression, which strikes in winter when daylight is shortest. Artificial daylight carefully formulated to supplant natural daylight can bolster productivity and improve worker health and safety. Try an automated lighting control system linked to the level of daylight available.
European window systems are on balance more sophisticated than their American counterparts. In Europe, workers have stronger unions and more socialized governments, and they insist on building comfortable, efficient workspaces. Code in some European countries actually proscribes access to natural light and operable windows for ventilation. Europeans also accept a broader definition of comfort, finding it acceptable to wear sweaters indoors during the winter and being slightly warm during the hottest summer afternoons.
European designers and manufacturers have responded with highly efficient operable windows for both residential and most commercial buildings. Small or narrow floor plates allow all occupants to be close to windows, and atriums and winter gardens mediate between interior and exterior environments. Technically sophisticated double curtain walls -- incorporating operable windows, ventilation, daylighting, shading, increased thermal performance and maintenance access -- allow office buildings to be nearly 100% glass yet consume little energy.
These European solutions are possible because of clients' willingness to invest in social benefits and incur high front-end costs with long paybacks. European construction offers some lessons to be learned on U.S. projects with clients that have similar goals and values.
Design and Document
Architectural design is an iterative process of developing schemes, analyses and compromises. Scheme selection is based on the highest satisfaction of the most important criteria with acceptable satisfaction of all criteria. Designers must pay appropriate attention to the interaction of all building systems, even at early conceptual design.
For example, if an owner were concerned about natural daylighting, an optimal concept would incorporate proper siting, light shelves, shading and glare control, lighting control systems, and appropriately reduced HVAC and electrical systems. It is not sufficient to simply show large areas of windows. It would be imprudent to simply remove light shelves and shading devices as part of a value engineering effort, as you would degrade the mechanical and electrical systems. As the building design gets more detailed, all system changes affect the entire project.
In Architects We Trust
With all of these considerations in mind, the architect must prepare a set of construction documents indicating every requirement to allow for accurate bidding and construction. A qualified architect is the appropriate individual to describe how the various systems come together as a total, watertight building enclosure. The construction documents must also include a specification carefully tailored to meet the unique requirements of the project. Do not rely on unedited master specifications - they're unacceptable.
Get to Work on a Unique Solution
With the exception of chain stores, virtually every project essentially becomes a prototype with a unique solution. Testing is critical in the successful installation and long-term performance of windows; large and complex projects require construction of mockups at offsite laboratories. These mockups are tested rigorously for conformance to the architect's performance criteria. On typical projects, onsite quality control saves time, energy, money, and headaches down the road.
learn more on topics covered in the film
see the video
read the script
learn the songs