Energy efficiency considerations in window replacement projects

by Katie Daniel | June 1, 2017 9:44 am

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by Craig A. Hargrove, CDT, RRC, AIA, LEED AP
At some point, windows reach the end of their useful life. The question is, how does a building owner know whether it is advisable to defer a window project another year or two, or to call in a design professional right away? At what point is window replacement inevitable?

The first thing to consider is the comfort of building users. Most complaints about office environments involve thermal comfort, often originating at deteriorated windows. Leaks at window openings are obvious indicators of compromised performance, but more subtle problems, such as difficult operation or drafts, may also become troublesome. (While safety is the primary consideration in any building assembly, systemic failure of the structural integrity of properly designed, installed, and maintained windows is usually indicative of a severe state of advanced deterioration. Most window assemblies will generally reach a condition prior to this where a host of other factors—many of which are discussed in this article—will point to the obvious need for replacement.)

To keep occupants comfortable, a certain amount of energy must be added to or removed from the building interior by the heating and air-conditioning systems. This thermal load can become much higher in buildings with inefficient windows, as HVAC systems run overtime to meet the demand of excess heat transfer through the fenestration.

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The cycling of temperatures and the migration of moisture from one day or season to the next can manifest at windows in the form of:

• shrunken gaskets and seals;
• warped, faded, or displaced frames; and
• etched or fogged glass.

As glazing becomes scratched, distorted, or clouded, building occupants may complain of compromised views, along with bothersome glare that can affect daylighting schemes.

Another factor to consider is maintenance and cost control. As windows age, it may become increasingly difficult and cost-prohibitive to find replacement parts. Keeping up with the needs of older windows is more demanding, as maintenance personnel spend hours responding to user complaints. (While windows have generally become more thermally efficient, increased maintenance efforts over time are generally the result of deterioration and failure of window components [e.g. gaskets, seals, and hardware] rather than shortcomings in the original design.) Protecting building infrastructure from environmental infiltration and chasing after damage can become a strain, and the cost of ongoing problems adds up.

As with any capital improvement, aesthetic appeal is also a driving force. New windows can add equity to a commercial property, and also present a way to dramatically improve the building’s appearance.

Energy efficiency is not often a precipitating factor in the decision to replace aging windows. However, once user comfort, maintenance demands, and aesthetics conspire to make window replacement unavoidable, a window project offers the opportunity to improve energy efficiency and reduce operating costs. Whether owners pay for these expenses themselves or pass them along to tenants, energy savings can be a compelling consideration when designing new windows.

New window systems must comply with the requirements of the local code governing that jurisdiction. Any code that has adopted the International Energy Conservation Code (IECC) as 
a foundation also, by extension, uses a version of American Society of Heating, Refrigerating, and 
Air-conditioning Engineers (ASHRAE) 90.1-2013, Energy Standard for Buildings Except Low-rise Residential Buildings, as a reference standard. However, IECC identifies its own path to compliance, and adopting municipalities often further modify those requirements to meet their own, local needs.

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Basic window design
A truly energy-efficient window starts with good design. As defined by the American Architectural Manufacturers Association (AAMA) in the North American Fenestration Standard (NAFS) (NAFS can also be referred to as American Architectural Manufacturers Association/Window and Door Manufacturers Association/Canadian Standards Association [AAMA/WDMA/CSA] 101/I.S.2/A440-08.), window types are standardized according to performance grades, distinguished by design pressures:

• R class (15 psf [i.e. 720 Pa]), which is typically used in one- and two-family dwellings;
• LC class (25 psf [i.e. 1200 Pa]), which is usually for low- and mid-rise residential buildings;
• CW class (30 psf [i.e. 1440 Pa]), which is for low- and mid-rise buildings with higher loading requirements and heavier use; and
• AW class (40 psf [i.e. 1920 Pa]), which is used in high- and mid-rise buildings to meet increased loading requirements and limits on deflection.

Window class selection depends on application and expected use, with higher performance grades capable of withstanding greater operating force, deflection, and structural loading.

Knowing the applicable building code is critical to window specification. Requirements for structural stability typically cover frame, glass, anchorage, 
and substrate attachment. An architect or engineer should identify the existing substrate’s condition and determine whether it has decayed or been damaged over time. A window’s structural integrity is only as good as its attachment to the substrate—if the substrate itself is unsound, the window could become unstable.

Building codes also frequently stipulate requirements for air and water infiltration testing of new window assemblies. Even where the code does not mandate testing, it is a good idea to review test results from the manufacturer and conduct laboratory and field performance tests. AAMA and ASTM International provide guidelines for test methods that should be followed as the industry standard.

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The code may mandate glass type for a given application. The three most common types of commercially available glazing are:

• annealed—raw glass that has not been heat-treated, which may be limited by code due to its susceptibility to thermal shock and mechanical stress, as well as its tendency to break into large, sharp pieces;
• heat-strengthened—glass that undergoes controlled heating and cooling to improve strength and fracture resistance, which is roughly twice as strong as annealed glass but still breaks into large, dangerous shards; and
• fully tempered—glass chemically or thermally treated to improve strength and shatter resistance, which breaks into tiny pieces less likely to 
cause injury.

Aside from structural and safety considerations, window options may be limited by energy code requirements, which are becoming increasingly stringent even for existing buildings. As of this writing, IECC is in use or adopted in 47 states, the District of Columbia, the U.S. Virgin Islands, New York City, and Puerto Rico. With each successive edition of the model code, performance criteria will likely continue to become more rigorous.

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What is an energy-efficient window?
For windows, two qualities broadly define energy efficiency: solar heat gain coefficient (SHGC) and thermal transmittance (U-factor). The first of these is defined as “the ratio of the solar heat gain entering the space through the fenestration area to the incident solar radiation” by ASHRAE 90.1-2013. Essentially, SHGC is a measure of how much of the sun’s heat transmits through the windows into the building interior.

In the past, maximum reduction of SHGC beyond that dictated by code was not considered optimal for buildings in cold climates because solar energy could help heat the building during the winter. However, due to inherent inefficiencies in building enclosures, the industry has largely revised its thinking on this issue, and recommendations now favor a reduction in SHGC across climate zones. There is, of course, always a trade-off—as SHGC is reduced, so is visible light transmission (VLT) or glass transparency.

The other major determinant for energy efficiency in windows, U-factor, is defined by ASHRAE 90.1-2013 as “heat transmission in unit time through unit area of material or construction… induced by a unit temperature difference between the environments on each side.”

A measure of a material or assembly’s propensity to transmit energy, U-factor is the inverse of R-value, which measures ability to resist energy transfer. Window manufacturers’ data should provide whole-assembly U-factor values, including both frame and glass, rather than the center-of-glass U-factor values that tend to make the window seem more efficient than it is.

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In terms of the energy code, defining what constitutes an energy-efficient window often demands calculations based on fenestration area and the performance of other building envelope elements. Both IECC and ASHRAE 90.1 provide two compliance paths:

• the Prescriptive Building Envelope Option, for buildings in which vertical fenestration is no more than 30 percent of gross wall area; and
• the Building Envelope Trade-off Option.

The number required for the prescriptive path can be increased to 40 percent when there are daylighting controls (i.e. methods to automatically regulate artificial lighting within daylight zones, defined by ASHRAE 90.1-2013 as “the floor area substantially illuminated by daylight”).

The Prescriptive path assumes windows are less energy-efficient than opaque wall assemblies, and it provides maximum values for U-factor and SHGC. Most current codes also permit adjustments in the SHGC and U-factor for a given set of conditions. For instance, the use of dynamic glazing (discussed later in this article) and projections on the building (such as eaves and cornices), in combination with window orientation, allows for an increase in the maximum allowable SHGC. For certain fenestration categories (i.e. fixed, operable, doors, and skylights), an area-weighted average that estimates the efficiency of a whole building section can be employed to calculate the maximum U-factor.

The Building Envelope Trade-off path is intended to demonstrate buildings with more than 40 percent vertical fenestration can function as efficiently as those with less window area. It offsets thermal transfer across the fenestration with efficiencies in wall and roof assemblies. However, for a window replacement project, the Building Envelope Trade-off route may not be an option, because it can be difficult (or even impossible) to identify efficiencies elsewhere in the building that could compensate for excess window area.

QUANTIFYING WINDOW ASSEMBLY PERFORMANCE

[7] [8] [9] [10]                   Window assemblies should be assessed for structural integrity, air and water infiltration, and potential condensation issues, as well as for energy efficiency. Often, much of this information is available from the manufacturer. However, before relying on published material, it is important to confirm: the data provided pertains to the exact assembly under consideration; and the test specimen incorporated the same glass to be installed in the project. Sometimes, conditions are project-specific and cannot be anticipated in testing performed by the manufacturer. For instance, potential condensation issues that might result from the installation of a replacement window in an existing opening may need to be evaluated through thermal modeling performed by a building enclosure specialist, using software programs such as THERM. Developed by Lawrence Berkley National Laboratory (LBNL), THERM allows design professionals to model two-dimensional heat-transfer effects in building components and evaluate an assembly’s energy efficiency. Although limited in their ability to assess complex real-world conditions, such as thermal massing, THERM and other computer models help anticipate problems with thermal bridging, condensation, moisture damage, and structural integrity. If the variables are too numerous, or there is a need to quantify performance within extremely specific parameters, physical testing of a window assembly—and ideally of a sample of the wall into which it will be installed—can be performed in lieu of computer modeling. For energy performance, tests are typically performed at a testing facility using a hot box, which is an apparatus that aims to replicate conditions typical of what is seen in the field. ASTM C1363-11, Standard Test Method for Thermal Performance of Building Materials and Envelope Assemblies by Means of a Hot-box Apparatus, is the recognized reference standard for such tests. During installation, windows should be tested for water penetration, as per ASTM E1105, Standard Test Method for Field Determination of Water Penetration of Installed Exterior Windows, Skylights, Doors, and Curtain Walls by Uniform or Cyclic Static Air Pressure Difference. By establishing a pressure differential across the building envelope, this test method encourages water from a calibrated spray grid at the exterior to migrate into the building. Window assemblies and the surrounding substrate can then be evaluated for watertightness.

Glazing strategies for energy-efficient windows
The overarching goal of window design is to optimize visible light transmittance, along with exposure to natural light and exterior views for building users. To this end, glazing strategies are frequently employed to:

• maximize energy efficiency;
• take full advantage of natural light and exterior views;
• reduce glare;
• decrease U-factor and SHGC; and
• limit the need for artificial lighting.

Use of window shades and other opaque blinds or screens is not optimal. This is because not only do such window treatments obscure daylight and limit views, but they also tend to be incorrectly operated by building occupants. Shades automatically programmed with sun sensors are a viable option, but they are often costly, consume at least some energy, and—as an active rather than a passive system—require periodic maintenance.

Instead, the industry has advanced a number of technologies that improve efficiency while preserving the natural light and vistas afforded by large areas of glass. By balancing desired levels of visible light with heat gain control, the design team can recommend window assemblies that meet energy-efficiency standards and improve occupant comfort.

Dual glazing
Perhaps the most prevalent glazing strategy for energy efficiency, dual glazing consists of two panes of glass assembled into one integral unit by use of spacers and a perimeter seal. The space between panes is often filled with an inert gas (usually argon) to form an insulating glazing unit (IGU). Dual glazing is often used in conjunction with other strategies, such as tinting, low-emissivity (low-e) coatings, or fritting.

Triple glazing
Similar to dual-glazed IGUs, but with three panes of glass instead of two, triple glazing has not been widely used in the United States due to cost. However, progressively stringent energy codes have increased the prevalence of triple glazing in recent years, which should have the added effect of bringing down manufacturing costs.

Low-e coatings
Low-emissivity coatings are factory-applied treatments to reduce the ultraviolet (UV) and infrared (IR) light that passes through glass, limiting heat gain while preserving VLT. (As with anything installed on the surface of glass, low-emissivity coatings will have some effect on VLT; however, the effect is arguably less than with other methods of reducing SHGC such as tinting or fritting. Hard coatings, which are typically used on single-pane glazing on an exposed surface, have a greater overall effect than soft coats, but again, less than other methods. The surface to which a soft coat is applied has a greater effect on performance than VLT.)

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Window tinting
Like low-e coatings, tinting cuts down on solar heat gain and glare, but it may reduce VLT by blocking part of the visible spectrum. Highly reflective coatings on tinted glass can limit VLT to less than 10 percent, compared with more than 
90 percent transmission for uncoated clear glass.

Fritted glass
By introducing ceramics or other materials into glazing, fritting creates a pattern (screen) that reduces glare and SHGC. Color and location is essential in maximizing results. New generations of frit glazing are experimenting with pattern organization on dual panes of glass that can vary the assembly’s translucency to maximize efficiency.

Photochromic glazing
Also called ‘dynamic glass,’ photochromic glazing can reduce glare and solar heat gain, as well as the need for window treatments, lighting, and shading devices, through the introduction of thin films that react to solar loads. Photochromic glazing generally falls under two major categories: thermochromic and electrochromic.

Thermochromic glass uses a laminate comprising organic compounds to react passively when exposed to solar loads. 
In the presence of such loads, the glass goes from clear to tinted and back again. Electrochromic glass, also known as ‘smart glass,’ employs an applied electric current to affect an inorganic coating to alter glass translucency or opacity.

When the code is not enough
If strict energy conservation codes mean compliant 
window assemblies are already energy-efficient, why 
would anyone bother to surpass code requirements? The answer lies in additional perks that provide value beyond improved sustainability.

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The local governing authority may offer benefits to those who exceed baseline requirements for energy performance. In New York City, zoning laws provide a deduction from gross square footage for buildings that have wall and fenestration assemblies exceeding energy code requirements. Such incentives illustrate the type of recompense that might be offered by state or local governments for energy-efficient design.

Another motivating factor in the decision to go beyond code requirements might be reduced operating expenses. Although the cost of a window replacement project is 
unlikely to be offset by energy savings in fewer than 20 years, incremental increases in the efficiency of a new assembly may pay for themselves in five years or less. A low-emissivity coating that reduces SHGC, or warm spacers in frames that lower U-factor, can improve efficiency enough to recuperate the extra up-front cost in a relatively short time.

Planned upgrades to an HVAC system also present an opportunity to realize cost savings from improved window efficiency. The building envelope and mechanical system are in a symbiotic relationship—as one becomes more efficient, the other need not work as hard. In theory, improving the energy efficiency of windows permits a reduction in the size of the mechanical package. However, calibrating window performance and HVAC output demands detailed analysis.

Return on investment (ROI) benefits of high-performance window upgrades include reduced maintenance costs, 
added equity, and the possibility of increased rental rates. Energy codes based on IECC and ASHRAE 90.1 provide good benchmarks for window performance, which have come a long way in a short period. Before undertaking a window replacement project, however, it is worthwhile to evaluate whether exceeding code requirements might be a good investment.

Realizing design goals
Although other factors (including user comfort, appearance, and maintainability) often initially take precedence when window replacement is under consideration, energy efficiency soon becomes part of the conversation. Energy savings alone will not pay for the cost of a typical window replacement 
project in a reasonable amount of time. However, ancillary factors make energy efficiency an important part of such projects.

Beyond energy considerations, a window replacement project offers the opportunity to optimize the user experience through a reconsideration of daylighting and views of the exterior. By setting design goals at the outset of a window replacement, owners may be able to realize multiple objectives, from improved interior comfort to reduced operating costs, with one well-designed project.

DAYLIGHTING AND VIEWS

[13] A window replacement project may present an opportunity to improve user comfort by incorporating daylighting schemes. By introducing appropriate levels of natural light into a space, daylighting can reduce the need for artificial illumination, lowering electricity expenses and providing the health benefits of full-spectrum lighting. Daylighting may be quantified in several ways, one of which is through the glazing factor—the ratio of exterior to interior illumination, expressed as a percentage. The architect or engineer may perform calculations to determine whether a minimum two-percent glazing factor is achieved for all daylighted spaces. Another method for determining daylighting requirements is to demonstrate through computer simulation at least 269 lux (25 foot-candles) of daylight is available for illumination. Daylighting schemes usually need redirection or glare-control devices to maintain energy efficiency and user comfort. For windows receiving direct sunlight, interior shading may be required to manage glare and limit heat gain. Automatic photocell controls for light screens, blinds, or curtains can be programmed to adjust shading depending on incident light levels. Advanced glazing technologies, including electrochromic and photochromic ‘smart glass,’ can adapt light transmission levels in response to electric controls or sunlight. Although it may add to project costs, increasing the window opening size to amplify natural light and expand views to the exterior may be considered as part of a window replacement project. Windows that are 0.8 m (2 ½ ft) at their base to 2.3 m (7 ½ ft) at their head above a finished floor (AFF) are considered optimal, as they are most effective at distributing daylight deep into spaces. When designing daylighting schemes, it is important to consider not only the window dimensions and glazing, but also the channeling of light within and between rooms. Interior glazing allows borrowed light from exterior windows to reach inside spaces; low partitions and open-plan office layouts are other options for distributing natural light across large areas. While daylighting schemes may add to the up-front cost of a project, providing better-quality natural lighting can pay dividends in improved user experience. From a return-on-investment (ROI) standpoint, a pleasing daylighting design can also add value to the building.

Craig A. Hargrove, CDT, RRC, AIA, LEED AP, is senior vice president and director of architecture at Hoffmann Architects Inc., a firm specializing in the rehabilitation of building exteriors. As manager of the New York City office, he leads project teams in developing building enclosure solutions for existing buildings and in consultation for new construction. A CSI-certified Construction Document Technologist, Hargrove received his architecture degree from New York Institute of Technology. With a focus on high-performance building envelope design, he is a LEED AP, RCI Registered Roofing Consultant, and member of the National Institute of Building Sciences (NIBS), New York Landmarks Conservancy, New York chapter of the U.S. Green Building Council (USGBC), and Building Owners and Managers Association’s (BOMA’s) Codes and Regulations Committee. Hargrove may be reached at c.hargrove@hoffarch.com[14].

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