Selecting replacement windows is a complex task, but careful planning can bring multifaceted payoffs

— By James Piper, Contributing Editor

Window replacement projects offer facility executives the opportunity to make significant improvements in their facilities — improvements that will lead to higher occupant comfort, lower energy use and better looking buildings. But window replacement projects cannot be entered into lightly. They are long-term, high cost investments for facilities. Decisions made today concerning replacement window features and performance will have to be lived with for 30 years or more.

Unfortunately, many facility executives enter into replacement projects too lightly. Some simply replace existing windows in kind. Others focus on one element of the replacement project, such as energy savings, and fail to consider other factors that are equally important. Successful window replacement projects require careful planning if facility executives are to gain the best return on their investment. Facility executives must consider the economics of window replacement, particularly the economic issues that go beyond first costs. They must fully evaluate energy efficiency issues, matching features to facility needs. And they must carefully consider aesthetic issues, for windows are a dominant component in defining how a building looks.

Window Replacement Economics
Windows are a potential drain on operating budgets. They require periodic maintenance. They can be sources of large energy losses. They can make spaces within the building uncomfortable enough to drive away tenants. Most window designs are fairly efficient when new, but, as they age and deteriorate, their maintenance and energy costs increase. The problem is that the rate of deterioration for windows is usually so slow that it often goes unnoticed. The best defense against this slow deterioration is a program of regularly scheduled annual inspections. Inspections can identify small problems well before they develop into larger ones that will require significant repairs or total replacement.

Annual inspections should examine the physical condition of the windows. Are there any signs of rot or rust? Are all gaps between window frames and the walls properly sealed? Are sashes properly gasketed or weather stripped?

Annual inspections should also examine the operation of each window. Are operable windows easy to open and close? Does the window properly close and lock? Does window hardware function properly?

Window inspection programs will help to identify maintenance needs. Those needs will depend on the materials used in the construction of the window. For wooden windows, the single largest maintenance expense is painting. On the average, wooden windows must be repainted every four to five years. It is this expense that has led to the widespread use of aluminum- or vinyl-clad wood windows.

Painting is also a significant expense for steel windows. Depending on the environment, steel windows may require repainting every five to 10 years to avoid corrosion. Problems with corrosion have led to the replacement of steel windows with ones made from aluminum or vinyl.

Another maintenance expense common to all windows is caulking. Caulking is used to seal the gap between the window and the wall in which it is installed. If the caulking deteriorates, water can enter the space, resulting in damage to the window itself and to wall materials. Air entering through areas with damaged or missing caulking increases energy costs. If surfaces are properly prepared and a high quality caulking is used, it should not be necessary to recaulk windows for at least 10 years.

Energy Efficiency
The most widely recognized impact that windows have on facility operating budgets is their energy cost. There are three major ways in which windows contribute to building energy costs: conduction losses and gains through the glazing and frames, solar heat gain through the glazing and air infiltration through gaps between window components.

All new windows offer a tight fit between components to limit the rate of infiltration. The other two types of heat loss or gain can be controlled by selecting specific features to be included in the replacement windows, including the number of glazings, the solar transmissivity of the glazings and the thermal characteristics of the window’s frame.

While each of these features can reduce energy loss through windows, not all will be cost-effective in all applications. Factors such as climate, percentage of glass-to-wall area and building use will determine which features are most cost-effective. In general, the more energy efficient the window is, the higher its cost. What’s more, some energy efficient features will alter the window appearance, thus changing the look of the entire building — another factor facility executives have to carefully evaluate in making decisions about the energy efficiency of windows.

The thermal efficiency of a window is one of the most important energy considerations when evaluating replacement window options. The thermal heat gain or loss through a window is determined primarily by the number of glazings. The thermal efficiency of the window is rated in terms of R-value: the higher the R-value, the greater the window’s energy efficiency. Single glazed windows have an R-value of approximately 0.88. For dual glazed windows, the average R-value is 1.68. For triple glazed windows, the value increases to approximately 3.0.

Impact of Climate
Although a higher R-value will result in improved energy efficiency, it is not always cost-effective since increasing the number of glazings increases the cost for the window. Although the economics of an installation will vary based on the particulars of that application, several rules of thumb can be used to help select the most appropriate number of glazings. For warm and moderate climates, the best choice is two layers of glazing. Triple glazed windows are usually justified only in applications with more severe climates, typically those having 7,500 or more annual heating degree days. Similarly, single glazed windows are the most economical choice only in applications where there is very limited demand for both heating and cooling.

Another important factor to consider when evaluating replacement window options is how much of the sun’s energy is transmitted through the window to the occupied space. The solar transmissivity of a window is controlled through the use of tinted or coated glazings. New generations of solar control window glazings can block solar heat gain through the window while allowing a significant amount of the visible light to pass through. As a result, solar control glazings have the same appearance as clear glazings but offer the advantage of reducing cooling loads in building perimeter spaces, particularly those buildings with a large percentage of glass.

Solar control glazings are cost-effective in nearly every climate. The warmer the climate or the greater the percentage of glass, the greater the need to block the sun’s infrared rays. But even in a northern climate, solar control glazings rated to block 80 percent of the sun’s infrared rays are cost-effective in buildings with a large percentage of glass.

In addition to allowing heat energy to enter the space during the cooling season, windows allow heat energy to escape from the occupied space in the form of long-wave infrared radiation. Low-emissivity coatings were designed primarily to reduce this radiant heat loss through windows by reflecting long-wave infrared radiation emitted from surfaces within the space, making them well suited for use in climates where the annual heating load is greater than the annual cooling load. But new low-emissivity designs can also help reduce solar heat gain in cooling dominated climates. Like solar control glazings, low-emissivity coatings are nearly transparent to visible light and therefore will not significantly alter the appearance of the facility. An additional benefit of the coating is its ability to block approximately 75 percent of the sun’s ultraviolet light, reducing the fading of materials in the occupied space.

The thermal characteristics of the window frame also contribute to the energy efficiency of the window. Wood and vinyl frames are good insulators and offer energy efficiency. Aluminum is a good thermal conductor and therefore not an energy-efficient material. To improve its performance, manufacturers install a thermal break between the frame’s inner and outer surfaces. Made of a poor conductor, thermal breaks improve the thermal efficiency of aluminum frames, making their performance comparable to wood and vinyl frames.

Aesthetic Issues
Windows are replaced primarily to reduce energy and maintenance costs. As a result, aesthetic issues are often overlooked during the replacement selection process until it is too late. Windows play an important role in how the building looks. Even a relatively minor change in the appearance of the windows can result in significant changes in both the appearance and the character of the facility. Ignoring aesthetic issues during the selection process can result in unpleasant surprises.

One of the biggest changes in building appearance comes from the type of glazing used. While some glazings, such as low-emissivity ones, result in no significant change, others can dramatically alter it. Tinted glazing, particularly dark tints, greatly alter a building’s appearance by making it difficult or impossible to see into the building’s interior during daylight hours. Reflective glazings have a similar effect. Before one installs a reflective or heavily tinted glazing, a test window should be installed to evaluate how the building’s appearance will be altered.

In addition to the color of the glazing, other factors such as the type of materials, the style of the window construction and the color of the frame will affect how the final installation will appear. Again, before committing to a particular type of replacement window, it is best to install a test window to determine how much of a change in the building’s appearance will result from the replacement window program.

If the replacement windows are properly matched to the needs of the building, facility executives will improve the working environment for the building’s occupants, in addition to reducing maintenance and improving energy efficiency. Thermally efficient windows will eliminate overly cold or warm areas on the building’s perimeter. Tight-sealing window components will eliminate drafts. Solar control coatings will help to eliminate overheating of perimeter spaces from the sun, even during the heating season. And replacement windows can improve the appearance of the facility, from both sides of the windows. The result will be a more comfortable working environment for building occupants.

James Piper, PE, PhD, is a consultant and writer with 25 years of experience in the facilities field.

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