Energy efficiency considerations in window replacement projects

Custom details may be necessary wherever the roof, window, and wall systems interface, as shown above.

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.

High-performance replacement windows resolve thermal inefficiencies and leaks the retrofitted storm windows at this historic building failed to adequately address.

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.

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