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Energy-efficient Windows

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While designing energy efficient buildings, improving thermal performance of the fenestration is an important consideration. Windows, though provide light and ventilation to interiors, they are also the least insulated elements and can negatively impact a building’s energy efficiency. Buildotech finds out about the latest thermally-sensitive window technologies and materials.

A building owner can improve the energy efficiency of existing fenestration by various methods. Adding, storm windows, caulking and weatherstripping around windows to reduce air leakage and installing window treatments or coverings like blinds, awnings and high-reflectivity film to reduce heat loss/gain are some of the options. However, these aren’t the most effective methods of maintaining energy efficiency of a building envelope. According to Energy Conservation Building Code (ECBC), the building envelope design must take into consideration both the external and internal heat loads. External loads include mainly solar heat gains through windows, heat losses across the envelope surfaces and unwanted air infiltration in the building. Proper location, sizing, and detailing of windows and shading form are an important part of the bio-climatic design as they help to keep the sun and wind out of the building or allow them when needed.

DESIGN CONSIDERATIONS

ORIENTATION: Buildings with windows facing predominately north use less energy than homes facing east, south or west. With high-performance windows and shading strategies, these differences can be considerably less.

WINDOW AREA: Energy use increases with window area using windows with clear and high-solar-gain glazing. With high-performance windows, energy use may not increase at all when using a larger window area.

SHADING CONDITION: For north facing buildings shading devices will have little impact. For south-facing buildings, overhangs can be effective to block the hot summer sun. Shading devices have less impact when using high-performance windows with low-solar-gain glazing.

Also, window’s energy efficiency is dependent upon all of its components (frame, glazing, sealants &, spacers). The type and quality of window frame affect a window‘s air infiltration & heat gain and play an important role in energy efficient fenestration design. Thus, newer technologies are being explored in windows manufacturing to improve their energy efficiency, such as materials and coatings with lower thermal conductivity, reduced width profiles, improved connection details and novel perimeter seal systems for better thermal insulation. In India, wooden and aluminum frames are being replaced by PVC (Poly vinyl chloride) and UPVC (unplasticized poly vinyl chloride) for thermal efficiency and reduced costs in the long run. The energy star qualified windows and skylights feature invisible glass coatings, vacuum-sealed spaces filled with inert gas between the panes, improved framing materials, better weather stripping, and warm edge spacers all of which reduce undesirable heat gain and loss.

Window Frames

Presently, there are three kinds of framing materials used, which are metal, wood and polymers. Wood has a good structural integrity and insulating values but low resistance to external weather conditions. Metal frames have poor thermal performance, but have excellent structural characteristics and durability. Aluminum is the most preferred metal for frames, but it is highly conductive. Vinyl window frames which are primarily made from polyvinyl chloride (PVC) offer many advantages. Available in wide range of style and shapes, PVC frames have high R– value (Resistance value) and low maintenance. According to, Rajeeb Dabash, Head Marketing, Tata Housing Development Company Ltd, “As 37% of interior energy loss is through fenestration, window frame profiles made of low thermal conducive material go a long way in reducing the loss of energy. For example, PVC frames designed with multi chambers and tight seals can reduce transfer of heat and noise penetration considerably”.

The biggest disadvantage of aluminum as an energy-efficient window frame material is its high thermal conductance. Even more than the problem of heat loss, the condensation problem has spurred development of better insulating aluminum frames. The solution lies in the “thermal break” by splitting the frame components into interior and exterior pieces and by using a less conductive material to join them. Current technology with standard thermal breaks has decreased aluminum frame heat loss rate from 2.0 to about 1.0 Btu/hr-sq. ft-°F.

Insulated vinyl frames are the new high performance frames used with high performance glazing. The insulating value of vinyl frames is improved by creating smaller cells within the frame replacing large hollow chambers that allow unwanted heat transfer. Another option is to fill the large hollow cavities of the frame with insulating material. Another, polymer-based approach is to use extruded engineered thermoplastics, a family of plastics used extensively in automobiles and appliances. Like fiberglass, they have some structural and other advantages over vinyl.

Till now, the standard solution for insulating glass units (IGUs) has been the use of metal spacers and sealants. In the single-seal system, an organic sealant, typically a butyl material is applied behind the spacer and serves to hold the unit together and prevent moisture intrusion. Whereas, in a double-seal system, butyl material, seals the spacer to the glass to prevent moisture migration and gas loss and a secondary backing sealant, often silicone, provides structural strength. However, these seals are normally not adequate to contain special low-conductance gases.

The new method includes replacing metal with insulating silicone foam spacer that incorporates a desiccant and has a high-strength adhesive at its edges to bond to glass. The foam is backed with a secondary sealant. Both extruded vinyl and pultruded fiberglass spacers have also been used in place of metal designs. Additionally, as manufacturers switch from conventional double glazing to higher-performance glazing, warm edge spacers are being used increasingly. The edge spacer has a thermal effect that extends beyond the physical size of the spacer to a band about 2-½ inches wide. The contribution of this 2-½-inch-wide “edge of glass” to the total window U-factor depends on the size of the window.

Glazing

Glass technology has made tremendous strides in the past few years and has become very sophisticated. Designers can now specify different types of glazing for different windows, based on orientation, climate and building design. The first approach is to alter the glazing material itself by changing its chemical composition or physical characteristics (e.g. tinted glass). The second approach is to apply a coating to the glazing material surface. Reflective coatings and films reduce heat gain and glare while more recent, low-E coatings improve both heating and cooling season performance. The third approach is to assemble various layers of glazing and control the properties of the spaces between the layers. Veena Sachdev, architect at Building Materials & Technology Promotion Council (BMTPC) explains, “Heat-absorbing glass contains special tints that allow it to absorb as much as 45% of the incoming solar energy, reducing heat gain. Glazing with low-emissivity (low-e) surface coating, reflect 40% to 70% of the heat while allowing the full amount of light to pass through. Reflective glass coated with a reflective film is useful in controlling solar heat gain, reduces solar transmittance but also reduces the passage of light all year long. Double- or triple-pane windows have insulating air or gas-filled spaces between each pane. Each layer of glass and the air spaces resist heat flow.”

TYPES OF WINDOWS

Windows are an important element in a passive solar architecture and it is important to consider their energy performance in relation to the regional climate. How the windows operate, impacts the air leakage rate and eventually the internal thermal comfortCasement: Casement windows are hinged at the sides and generally have lower air leakage rates than other window designs because the sash closes by pressing against the frame.

Casement: windows project outward providing significantly better ventilation and virtually the entire casement window area can be opened.

Hinged: These types of windows include awning window that are hinged at the top and open outward and hopper window which are hinged at the bottom and open inward. They have lower air leakage rates than sliding windows because the sash closes by pressing against the frame.

Double hung & Sliding: In double-hung units, both sashes slide vertically and only the bottom sash slides upward in a single-hung window. In double sliding, both sashes slide horizontally and only one sash slides in a single-sliding window. These window units generally have higher air leakage rates and maximum ventilation area available is the one-half of the total glass area.

Tilt & Turn: Designed to provide effective ventilation without creating an uncomfortable draft, dual action tilt and turn windows allow warm, stale air to exit at the top and cool, fresh air to gently flow in from the sides. The multipoint locking Tilt + Turn window provides the superior air tightness to increase energy efficiency.

Bi-folds: Bi-fold is a popular and versatile window with a wide opening to maximize view and airflow. Available in both out swing and in swing configurations, these windows manage the indoor climate by using performance seals to prevent air leakage.

Measuring Performance

U-factor: The rate of heat loss is indicated in terms of the U-factor (U-value) of a window assembly. The lower the U-factor, the greater a window’s resistance to heat flow and the better it’s insulating properties. High-performance double-pane windows can have U-factors of 0.30 or lower, while some triple-pane windows can achieve U-factors as low as 0.15.

Solar Heat Gain Coefficient (SHGC): The SHGC is the fraction of incident solar radiation admitted through a window, both directly transmitted and absorbed and subsequently released inward. SHGC is expressed as a number between 0 and 1. The lower a window’s solar heat gain coefficient, the less solar heat it transmits. Whole window SHGC including the effects of the frame is lower than glass-only SHGC and is generally below 0.8.

Visible Transmittance: The visible transmittance (VT) is an optical property that indicates the fraction of visible light transmitted through the window. While VT theoretically varies between 0 and 1, most values among double- and triple-pane windows are between 0.30 and 0.70. Higher the VT more the transmission of light is transmitted. A high VT is desirable to maximize daylight.

Air Leakage: Heat loss and gain occur by infiltration through cracks in the window assembly. It is indicated by an air leakage rating (AL) expressed as the equivalent cubic feet of air passing through a square foot of window area. The lower the AL, the less air will pass through cracks in the window assembly. Select windows with an AL of 0.30 or less (units are cfm/sq ft).

Condensation Resistance (CR): How well a window resists the formation of condensation on the inside surface is expressed as Condensation Resistance. CR is expressed as a number between 1 and 100. The rating value is based on interior surface temperatures at 30%, 50%, and 70% indoor relative humidity for a given outside air temperature of 0° Fahrenheit under 15 mph wind conditions. The higher the number, the better a product is able to resist condensation.

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