Windows and Doors
- By Thomas G. Dolan
- October 1st, 2010
Windows and doors are such basic components of any building that it would be easy to assume that the choices are so standardized there wouldn't be much need for discussion or debate, especially in terms of questions such as energy efficiency and LEED certification. But the situation is a bit more complicated than might first appear.
For instance, many manufacturers tend to use their own methods to calculate thermal efficiency of doors, or extrapolate known data from the properties of the material used to come up with a rating. "The most important advice I can give is to make sure whatever door you are considering on buying has results from an independent laboratory test," says Daniel Depta, manager of Marketing for Special-Lite, Inc., a door manufacturer located in Decatur, Mich.
"Independent lab tests guarantee that a manufacturer cannot do the calculations and come up with the performance he or she expects. It's easier for manufacturers to do their own calculations. But what we found, in our experience, is that the value we came up with can be way off the true performance of the door itself. Independent testing is expensive and time-consuming, but it has to be done."
The glass monumental door, typically made with aluminum tubes on the outside and the rest glass is, for obvious reasons, often favored by architects. For the glass doors provide high visibility, which is good for both aesthetics and security. But glass doors, Depta maintains, do not have good thermal properties.
Problems with other standard type doors, such as wood or hollow sheetmetal, is that they require repainting or refinishing which, Depta says, "adds to the life-cycle costs and can have the unintended consequences of releasing hazardous material into the environment."
Terry Zeimetz, AIA, Commercial Marketing manager for Pella Corporation, says his company produces exterior doors, but also windows. He agrees that glass doors are not as good at stopping air infiltration as some other doors, but this can be modified to some degree by the nature of the fittings as well as the thickness of the glass.
"Generally, in schools, there is a far greater amount of glass surface area in windows than in doors, so it's more important to get windows right in terms of conductivity," Zeimetz says. "And windows are not as subject to the abuse that doors are in schools."
So much starts with the material choice, says Zeimetz. Glass is all basically the same, but there's a big difference in terms of durability between double or triple glazing. And the type of frame you put the glass into is also critical. Certain frames, such as metal or aluminum, have the highest conductor and poorest energy rating. But wood, aluminum-clad wood, fiberglass or vinyl frames all have good insulating properties. All frames can be used with any type of glass. But the type of frame changes the energy efficiency of the window.
Another important dynamic, Zeimetz points out, is the emissivity coating, which has to do with the amount of solar radiation allowed to pass through the glass. "The smart coating rejects bad solar energy but lets in visible light, so you don't need as much artificial light, and also rejects the heat gain," says Zeimetz. "But there are all sorts of variations that can be applied — high, middle or low in terms of the amount of light allowed in, and you may actually want to allow more heat to come in."
"Providing natural lighting and a view to the outside for students is very important in school construction," says Michael Turner, vice president of marketing for YKK AP America, Inc. "A lot of the studies I've read talk about how natural lighting and a view to the outside helps enhance the student's ability to learn."
Some planners, he explains, try to balance energy performance of windows with a low heat transfer with an opaque wall. Typically there is the R factor, which specifies how much insulating capability the walls has. But he balances this with the U factor, which is calculated in a similarly standardized way by the National Fenestration Rating Council (NFRC), which specifically targets energy performance of windows and doors so consumers can make wise choices in purchasing residential or commercial products.
Turner recommends selecting a window and door that will provide as much visible area as possible. "For schools, you need to make sure you go beyond residential or light commercial to specify an architectural grade," says Turner. "And make sure you choose hardware that is durable enough to withstand the school-related operating cycles of opening and closing."
Turner explains that different climates and different seasons necessitate different ways of controlling the sunlight. "A trend in this respect is toward shading devices," Turner says. "You can use a device on the exterior to shade the direct sunlight without degrading the thermal performance of the window itself." Turner adds that a light shelf helps reflect light into an occupied space so that students sitting on the other side of the room from the window can still receive the benefits of the natural light.
Edward Wansing, technical program support coordinator for the Collaborative for High Performance Schools (CHPS) in San Francisco, says that a century ago, the preoccupations and complexities that go into thinking about windows and doors did not exist. Back then there was no air conditioning, windows and doors were the means of getting natural ventilation and, although it was not though of such at that time, natural daylighting. But that changed in the 1970s and 80s due to the oil embargoes and resulting energy crunch. The trend was to minimize windows and keep them shut, both to keep air conditioning in during hot months and to keep heat in during the cold months. The materials designed to keep the natural world out were often filled with harmful chemicals. So, with the lack of fresh air, and even air flow, along with increased artificial light and toxic chemicals, a very sorry phenomenon developed — the so-called "sick building," which more accurately described the condition of the people who had to work or reside inside.
So, it's out of this man-made trap that a number of different efforts were made to break down these barriers and let the natural world back in. The first U.S. Green Building Council convened around the turn of this century. Although the resulting LEED certification program was aimed at buildings, it didn't specifically target schools. So, many who were on the first committee, and who wanted a LEED program specifically targeted for schools, created CHPS.
The main advice Wansing gives is "not to just look at your windows and doors for the best energy mode. Trying to get the lowest solar heat gain may not always be the best approach. We advocate an integrated design approach, balancing indoor air quality, environmental quality, natural day lighting and ventilation. Natural ventilation, if the climate will allow it, along with those other factors, will lead to better test scores, less absenteeism and better operating efficiency."
Wansing explains that the amount of heat that is kept out of a building, or kept in, should be balanced with a number of different factors. During cold winters, you will want to let heat in, but during the summer keep it out. Also look at the orientation of the building to the sun. Overhangs or shading devices can be designed so the sun won't hit the glass in the summer, but allow it to provide its warmth during the winter.
"Also understand where the sun will be at any particular time during the day, and how the temperature inside the building will change," Wansing says. "Even though it may be 50 degrees outside, if there are 30 kids in a classroom with computers, it may be very warm inside."
Wansing also strongly recommends that schools follow the CHPS guidelines for interlocking systems between the mechanical and natural worlds. For instance, if the windows are open, the air conditioning system automatically is switched off.
The sick buildings of a few decades back, that allowed the creation of mold and so many other health hazards resulting from the shutting off of natural lighting and ventilation, were mistakenly designed out of the perceived need to cut energy costs.
"Most people know the green movement is out there, but many people still believe the natural way costs more," Wansing says. "But it doesn't have to cost more. In fact, if you do the right integrated design approach, you will save long-term operating costs while you are creating a very healthy environment conducive to learning."