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By James Piper
October 2004 -
Windows & Exterior Walls
Windows represent a major investment for facility executives. They carry a high first cost. They require regular cleaning and maintenance. They have a large influence on the comfort of building occupants. And their replacement is expensive and disruptive to building occupants.
Any project that involves upgrading or replacing windows must be entered into carefully, especially considering windows’ typical 30 to 40 year service life. Unfortunately, too many facility executives enter into window replacement and window film installation projects without fully understanding the long-term impact of the project. To them, window projects seem the perfect solution to years of complaints from building occupants, rapidly rising energy costs and increased security concerns.
These facility executives have bought into the concept that window projects are the magic bullet for building woes. Reality can hit hard, often only after millions of dollars have been invested in the window project. Unrealistic expectations will only lead to disappointment.
Before replacing windows or installing window film, facility executives must understand the realistic benefits and limitations of the project. Properly planned and executed, window projects can enhance a facility’s operations in many ways. The level of enhancement achieved, however, depends on the particulars of the application and the performance of the new or modified windows.
One of the most common reasons for window replacement and window film installation is energy savings. Windows have a large impact on the energy efficiency of a building. Thermal conduction through a window is typically 10 to 20 times as great as through exterior walls, increasing the load on the building’s heating and air conditioning systems. The infrared portion of sunlight also increases cooling loads. In buildings with large areas of glass, infrared energy can heat perimeter areas enough to make them uncomfortable, even during winter months.
New window designs using multiple glazings, low-e coatings, spectrally selective coatings or reflective glazings can reduce both the rate of thermal conduction through the window and the rate of infrared gain. Solar window film applied to existing windows will reduce thermal conduction slightly, but the primary energy benefit is limiting infrared heat gain. These reductions will reduce energy costs; the question is how much.
Too often, exaggerated or misleading claims are made as to the energy benefits of window replacement or reflective film installation. The worst of these claims lead facility executives to believe that installation of the new window product will reduce energy use by as much as 50 percent.
In reality, a well-planned and implemented window program will reduce the window’s contribution to the overall building load by 50 percent or more. Because the window’s contribution is only a part of the overall heating and air conditioning load for the building, the window project’s impact on the total energy cost will typically be 10 to 15 percent. For typical window replacement projects, this translates to a payback based solely on energy savings of 20 to 30 years. For window film installations, the payback can be as short as five years.
Because there are so many variables related to the specifics of the application, it is essential that facility executives have the energy impact of the project analyzed before committing to a replacement or upgrade project. Analyzing different options will also allow facility executives to select the windows that are best suited for their particular application. Failure to do so will result in unrealistic energy savings expectations.
Similar unrealistic claims have been made for solar control window films. The amount of savings that will be produced by the film depends on its level of infrared reflectivity, the exposure of the window and the climate where the building is located. Again, before committing to a window film project, have the energy impact analyzed for the facility. Do not project the savings based on some generalized application.
Another common mistake made when considering replacement windows is to simply replace the existing windows with ones of the same type and style. The motivation to replace in-kind can be strong. It minimizes the aesthetic impact on the building. It minimizes alterations that must be made to the building’s exterior and interior finishes. And in many cases, it minimizes the cost of the replacement project.
The main drawback of replacement in-kind is that it limits the options that are available with new window designs. Advances in window technology and features have made a range of options available. If these options aren’t considered, the installation might fail to deliver the levels of energy savings, comfort and security that are possible with today’s technology — a failure that is all the more serious given the long life span of replacement windows.
All windows, regardless of type or style, require maintenance. Painting, cleaning, caulking and replacing components are maintenance expenses that add to the life-cycle costs. How much maintenance is required will depend on the features of the window. That maintenance can’t be overlooked when evaluating options. Maintenance costs over the service life of windows can exceed the installed cost of the units.
Evaluate the maintenance requirements of each option. Do the windows require painting? Can the windows be cleaned from the inside? How difficult is it to replace broken glazings? How available are replacement glazings?
Maintenance is also important for window film. Early generation films could easily be damaged or develop bubbles. Today’s films offer better adhesion and resist surface damage. But the films still have a service life that is less than that of the window to which they are applied. This means that it will be necessary to replace the film sooner than the windows.
Windows in many facilities remain the weak security link. Even if the windows are not operable, it is still relatively easy to gain access by cutting or breaking a window.
Reducing those risks doesn’t mean having to board up all window openings. But it is important to select secure replacement windows. Inoperable window sashes or ones with a limited range of operation can be installed. Window glazings can be installed that resist breakage. Barriers can be installed on the outside or inside of the windows.
Preventing unauthorized people from gaining access through windows is not the only security concern facility executives need to consider. Equally important is the threat posed by someone wanting to damage the building or to injure those inside. While there are several window options that can be used to help improve building security, identifying what the threat is to the facility is an important first step. Without a clear understanding of the threat, designers might be protecting against events that pose no real risk, while overlooking those that are more likely to take place.
For example, even if the building has a very low probability of being a target for an attack, it may be located close to one that is a likely target. In that case, the windows should be designed to protect occupants from the effects of an attack on nearby buildings. Once the real threats are understood, products can be selected to protect against them at the desired level.
One window security product that has received a lot of attention in recent years is window film. With the raised level of concern for protecting occupants from flying pieces of glass, manufacturers have been widely promoting window films. When applied to the interior surfaces of existing windows, the films are said to be able to hold the broken pieces of glass in place in the event of a shock to the window glass from an explosion, storm or earthquake, or even from something thrown at the window. As a result, facility executives may feel that applying a window film will protect building occupants.
No window film will make a window blast-proof. Window films will not make windows shatter-proof, hurricane-proof, bullet-proof, vandal-proof or burglar-proof. All the films can do is to make the glass in the window more shatter-resistant. It is this shatter resistance that reduces the risk of injury both inside and outside of the building from flying shards of broken glass. Given enough force, the window glass will still break. Simply applying any film to a window will not necessarily increase protection for building occupants.
There is a wide variety of window films on the market. Some are specifically designed to enhance safety, while others are designed to improve energy efficiency. To gain the level of protection required, the product must be matched to the task that it is supposed to perform. If security is an issue, select a window film product that is designed specifically to provide protection.
Most solar control window films are based on a single layer of thin film, measuring from 0.0005 to 0.009 inches thick. While they are fully adhered to the window glass and can hold broken pieces of glass in place, they simply do not have the strength to resist tearing under the pressures exerted by blasts or thrown objects.
In contrast, security and safety window films are much thicker, are constructed using multiple layers of materials, and use much stronger adhesives. Some have as many as 30 layers of material to help absorb the energy of an impact.
In selecting a security film, review the manufacturer’s data on the film’s adhesion strength and its tear strength. Make certain that the film is designed for security and safety, not just solar control. Solar control capabilities can be part of the film’s construction, but what is important for security applications is the film’s ability to hold the pieces of glass in place.
James Piper is a writer and consultant who has more than 25 years of experience in facilities management. He is a contributing editor to Building Operating Management.