Fundamentals of Building Science: Windows

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Windows can have a significant effect on performance of the building envelope (e.g., heat transfer, natural ventilation, passive solar heating and cooling), and , as a result, are addressed here. Windows are constructed of the glazing (glass) and frame. Glazings are translucent or transparent materials (i.e., glass and some plastics that allow light to pass through the building envelope). The type of glazing used in a window or skylight can have a dramatic effect on energy performance.

Window frames, sash, and mullions are available in a variety of materials including aluminum, wood, vinyl, and fiberglass.

Frames may be composed of one material or may be a combination of different materials (i.e., wood clad with vinyl or aluminum clad wood). Window frame and sash assemblies comprise only 10 to 25% of the window area, but can account for up to half of the window heat loss and can be the principal location for the formation of condensation. The thermal weak point is the edge of the glass and the window frame. To improve performance, manufacturers use thermal breaks in metal frames, increase the use of wood and clad wood sash and frames, and increase the use of frame materials with lower thermal conductivity, such as vinyl.



Performance Measures

The National Fenestration Rating Council (NFRC), a non profit, public/private organization created by the window, door, and skylight industry, provides consistent ratings on window, door, and skylight products. Builders and consumers use the NFRC Energy Performance Label to reliably compare one product with another, and make informed decisions about the windows, doors, and skylights they buy. An example of this label is shown in Ill. 3.13. The energy performance label lists the manufacturer, describes the product, provides a source for additional information, and includes ratings for one or more energy performance characteristics. NFRC rates all products in two standard residential and commercial sizes (i.e., "Res" and "Non-Res"). A description of the principal measures follows.



Overall Coefficient of Heat Transfer (U) and Thermal Resistance (R)

The overall coefficient of heat transfer (U) is a measure of how easily heat travels through an assembly of materials: the lower the U-factor, the lower the rate of heat transfer through the glazing and the more efficient the glazing. The U-factor has units of Btu/hr ft2 °F (W/m °C). Thermal insulating ability is also measured by the thermal resistance (R): a higher R-value indicates a better insulating performance. The R-value has units of hr ft2 °F/Btu (m °C/W). The R-value is the inverse of the U-factor (e.g., R _ 1>U and U _ 1>R; a U-factor of 0.5 Btu/hr ft2 °F is the same as an R-value of 2.0 hr ft2 °F/Btu). These values are discussed in Section 2.

The overall or whole-window U-factor of a window or skylight depends on the type of glazing, frame materials and size, glazing coatings, and type of gas between the panes. The overall U-factor should be used because energy-efficient glazings can be compromised with poor frame designs. R-values for common glazing materials range from 0.9 to 3.0 (U-factors from 1.1 to 0.3), but some highly energy-efficient exceptions also exist. Experimental super window glazings have a center of-glass R-value of 8 to 10 (U-factor of about 0.1), but have an overall window R-value of only about 4 to 5 (U-factor of about 0.25 to 0.2), because of edge and frame losses.

Ill. 13 The NFRC Energy Performance Label that includes ratings of energy performance characteristics on a window.

Solar Heat Gain Coefficient

The solar heat gain coefficient (SHGC) is the fraction of solar heat that is transmitted through the glazing and ultimately becomes heat. This includes both directly transmitted and absorbed solar radiation. The lower the SHGC, the less solar heat is transmitted through the glazing, and the greater its shading ability.

Solar transmission through windows and skylights can provide free heating during the heating season, but it can cause an external-load-dominated building to overheat during the cooling season and an internal-load-dominated building to require cooling during much of the year. Solar-induced cooling needs are generally greater than heating benefits in most regions of the United States. In some climates, solar transmission through windows and skylights may account for 30% or more of the cooling requirements in a residence.

In general, south-facing windows in buildings designed for passive solar heating should have windows with a high SHGC to allow in beneficial solar heat gain in the winter. East- and west facing windows that receive undesirable direct sunlight in mornings and afternoons should have lower SHGC assemblies.

Visible Transmittance

Visible transmittance (VT) is the percentage of visible light (light in the 380-720 nm range) that is transmitted through the glazing.

When daylight in a space is desirable, glazing is a logical choice.

However, low VT glazing such as bronze, gray, or reflective-film windows are more logical for office buildings or where reducing interior glare is desirable. A typical clear, single-pane window has a VT of about 0.88, meaning it transmits 88% of the visible light.

Types of Window Glazings

Common and innovative types of glazings available for use in windows and skylights include:

Single-Pane Glazings

Single-pane glazings are a single layer (pane) of glazing. They transmit about 88% of the solar light that strikes them. In the cooling season they are a significant source of heat gain and are also often a source of glare. Single-pane glazings should only be used where space heating and cooling are not needed in the building, such as a warehouse in a mild climate.

Insulated Glazings

Insulated glazings trap air or an inert gas such as argon or krypton between two or more panes (layers) of glass, creating a glazing product with improved resistance to heat transmission.

Some typical U-factor ranges for common glass assemblies are:

Double glazed:

0.15-0.33 Btu/hr _ ft2 _ °F (0.85-1.87 W/m _ °C) 0.43-0.57 Btu/hr _ ft2 _ °F (2.44-3.24 W/m _ °C)

Triple glazed:

Historically, insulating windows were produced with a space between two glass layers (panes) that was filled with dehydrated (dry) air or flushed with dry nitrogen immediately before sealing the glass layers. The insulating effect of the confined air space between the two panes of glazing made a significant improvement in heat transmission. The optimal spacing for an air filled glazing unit is between 1/2 and 3/4 in (12 to 19 mm). At this spacing, radiant and natural convection heat transfer between the glazing surfaces is the best possible. Additional confined air spaces created by adding panes improve this insulating effect.

Convective air currents develop very easily with air and these currents carry heat to the top of the confined air space and settle into cold pools at the bottom of the space. An improvement to the thermal performance of insulating glazing units can be made by use of a less conductive, more viscous slow-moving gas that minimizes the convection currents within the confined air space. This "heavier" gas reduces conduction so that the overall transfer of heat is reduced. Manufacturers typically use argon and krypton gas fills to improve thermal performance. Argon is inexpensive, nontoxic, inert, clear, and odorless. The optimal spacing for an argon-filled unit is about 1/2 in (11-13 mm). Krypton offers better thermal performance but is more expensive to produce. Krypton is particularly useful when the confined air space must be thinner than normally desired (e.g., 1/4 in, or 6 mm).

The optimum gap width for krypton is 3/8 in (9.5 mm). A mixture of krypton and argon gases can also be used as a compromise between thermal performance and cost.

Spacers, sometimes called edge-spacers, are used to separate and hold apart multiple panes of glass within insulating windows. Because of its excellent structural properties, window manufacturers began using aluminum spacers. However, aluminum is an excellent conductor of heat and the aluminum spacer used in most standard edge systems represented a significant thermal bridge at the edge of the insulating glass unit.

During cold weather, the thermal resistance around the edge of a window is lower than that in the center of the glass; thus, heat can escape, and condensation can occur along the edges. To ad dress the edge problem, window manufacturers have developed a series of innovative edge systems. Warm-edge spacers either incorporate a thermal break in the spacer assembly or are constructed from a low-conductivity material. If warm-edge technology is not used with low-e windows (see below), much of the benefit of these technologies is lost because heat is con ducted along the glass and across the edge spacer.

Tinted (Heat-Absorbing) Glazings

Tinted glazings, also known as heat-absorbing glass, block solar transmission by absorbing heat in the glass itself. Unfortunately, this causes the glass temperature to rise, increasing the radiation coming off the glazing into the indoor space. Thus, tinting by itself yields only a modest shading coefficient, with SHGCs in the range of 0.5 to 0.8. With glazings having SHGCs below about 0.5, indoor vegetation grows more slowly or could die from the reduced sunlight. The most common colors for tinted glass (e.g., bronze and gray) block visible light and solar heat in equal proportions, that is, they are not spectrally selective. Green- and blue-tinted glazings are more spectrally selective; they offer greater transmission of visible light and slightly reduced heat transfer compared with other colors of tinted glass. Black-tinted (gray) glass should be avoided because it absorbs much more visible energy than near-infrared radiation.

Reflective Glazings

Reflective glazings have a semitransparent metallic coating applied to the surface(s) of clear or tinted glass. They have better shading coefficients because they reflect rather than absorb infrared energy. However, most reflective glazings block daylight (visible light) more than solar heat. Reflective glass has worked well in hot climate applications, where a high level of solar control is necessary. However, reflective glass reduces cooling loads at the expense of daylight transmittance, so the reduction is offset somewhat by the heat created by the additional electric lighting required. Reflective coatings are available for single pane applications, while some coatings must be sealed inside multiple-glass units.

Low-e Coatings

Low-emittance (low-e) coatings are microscopically thin, virtually invisible, metal or metallic oxide layers deposited directly on one or more surfaces of glass or on plastic films between two or more glazings to suppress radiative heat flow and reduce the U-factor. Emittance (e) is the ratio of emitted radiation to that of a black body at the same temperature. It is a measure of the amount of radiant heat transfer between two surfaces with a given roughness at a different temperature. A low-e coating rated at 0.20 reduces infrared radiation heat transfer by 5 times. It is based upon the emissivity and physical configuration of the surfaces. (See Section 2.) The principal mechanism of heat transfer in an argon or krypton gas-filled insulating glass unit is thermal radiation from a warm pane of glass to a cooler pane. Coating a glass surface with a low-e material and facing that coating into the confined space between the glass panes impedes radiant heat transfer significantly, thus lowering the total heat flow through the window. When applied inside a multiple-pane glazing, a low-e coating will reflect heat back into the habitable space during the heating season. This same coating will reflect heat back outdoors during the cooling season, slightly reducing heat gain.

Low-e coatings are transparent to visible light but not to infrared radiation. Some low-e glazings reduce solar gain, which can be undesirable in winter months. Different types of low-e coatings have been designed to allow for high solar gain, moderate solar gain, or low solar gain.

Spectrally Selective Glazings

Spectrally selective glazings have optical coatings that filter out much (about 40 to 70%) of the invisible solar heat gain that is normally transmitted through clear glass, yet allow the full amount of daylight to be transmitted. These coatings can be combined with tinted glazings to produce special customized selective coatings that can maximize or minimize solar gain to achieve the desired aesthetic effects and still balance seasonal heating and cooling requirements.

Suspended coated film (SCF) is a low-e system, which insulates better than typical low-e. Using the sputtering process, a wavelength-selective low-e coating is applied to thin plastic film. The film is then suspended between two plates of glass, which creates a unit that, as far as convection and conduction are concerned, is essentially a triple-pane unit.

Additionally, SCF provides a wavelength-selective coating on the suspended film. The coating blocks 99% of the ultraviolet wavelengths and varying amounts of infrared wavelengths, without blocking significant portions of visible light. The result is a glazing system with a low shading coefficient and high visible light transmission.

Super Glazings

Super glazings are innovations in glazing technology. They can achieve high thermal resistance (an R-value as high as 10) by blending multiple low-e coatings, low-conductance gas fills, barriers between panes that reduce convective circulation of the gas fill, and insulating frames and edge-spacers.

Under development are chromogenic (optical switching) glazings that will adapt to the frequent changes in the lighting and heating or cooling requirements of buildings.

Passive glazings will be capable of varying their light transmission characteristics according to changes in sunlight (photochromic) and their heat transmittance characteristics according to ambient temperature swings (thermochromic).

Active (electrochromic) glazings use a small electric current to alter their transmission properties.

Aerogels

An aerogel is a low-density, highly porous material similar to a translucent, super lightweight foam that is formed by extracting liquid from a gel-like substance. It contains extremely small air spaces or pores that are only a few hundred times larger than atoms. Aerogels are the lightest existing solid material, having a density of only about three times that of air. They are so lightweight and translucent that they are often referred to a "solid smoke." Aerogels have extraordinary thermal insulating properties and have near glass-like optical characteristics.

Silica aerogels are produced of pure silicon dioxide and sand, much like glass. Some silica aerogels are over 99.5% air, making them 1000 times less dense than glass. The best aero gel can provide 39 times more insulating ability than the best fiberglass insulation. One manufacturer ( Aspen) produces silica aerogels that have thermal insulation values between R-13 and 16 hr ft2 °F/Btu per inch (thermal conductivity values as low as 0.011W/m °C) at 100°F (38°C) and standard atmospheric pressure.

Aerogels have pore sizes ranging from 1 to 100 nm, and typically average between 3 and 5 nm. This is below the wavelength size of visible light. The pores act as particles that scatter white light and make the aerogel appear blue. Commercially available aerogels are slightly less transparent than window glass (about 90%) and have a slight bluish tinge.

Aerogels can be easily molded into different shapes (e.g., cylinders, cubes, plates of varying thickness), making them suitable for use as building materials. They have extraordinary thermal insulation values that are much higher than conventional thermal insulations used in buildings and appliances.

One distinctive potential application is as a composite with glass that could be used as a high-performance glazing product in windows. Present commercially available aerogels, with their slight hazy appearance and bluish tint, would be deemed unsatisfactory for most building occupants. Much research is being done in this area.

Whole-Window Performance

Different combinations of window or skylight frame style, frame material, and glazing change the actual thermal performance of a window unit. Commercially available insulating glazing systems can achieve a center-of-glass R-value of 10 (U-factor of 0.1) by sandwiching translucent glass fiber or foam insulation between plastic or composite glazings. There is a compromise between heat transmission and light trans mission. Window frames and spacers account for a large fraction of the heat transfer in advanced glazings. The result is a reduction in overall thermal performance for the window as a whole. For example, a wood-frame window with a center of-glass thermal resistance of 8 hr ft2 °F/Btu has a whole window thermal resistance of about 4.5 hr ft2 °F/Btu from the frame.

Air infiltration affects performance of the entire window unit. Manufacturers take steps to reduce air infiltration through the operable window sash by adding advanced weatherstripping that limits air leakage between the window sash and frame. During construction, flashing is installed around a window or door unit to reduce air and moisture infiltration. See Ill. 3.14.

Ill. 14 Self-adhered flashing installed to reduce air and moisture infiltration around window and door.

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