systems within a building. Fenestration components include glazing
material, either glass or plastic; framing, mullions, muntins, dividers,
and opaque door slabs; external shading devices; internal shading
devices; and integral (between-glass) shading systems. In this chapter,
fenestration and fenestration systems refer to the basic assemblies
and components of exterior window, skylight, and door systems
within the building envelope. Fenestration can serve as a physical
and/or visual connection to the outdoors, as well as a means to admit
solar radiation. The solar radiation provides natural lighting, referred
to as daylighting, and heat gain to a space. Fenestration can be fixed
or operable; operable units can allow natural ventilation to a space and
egress in low-rise buildings.
Fenestration affects building energy use through four basic
mechanisms: thermal heat transfer, solar heat gain, air leakage, and
daylighting. The energy effects of fenestration can be minimized by
(1) using daylight to offset lighting requirements, (2) using appropriate
glazings and shading strategies to control solar heat gain to
supplement heating through passive solar gain and minimize cooling
requirements, (3) using appropriate glazing to minimize conductive
heat loss, and (4) specifying low-air-leakage fenestration
products. In addition, natural ventilation strategies can reduce
energy use for cooling and fresh air requirements.
Today, designers, builders, energy codes, and energy-efficiency
incentive programs [such as ENERGY STAR (www.energystar.gov)
and the LEED Green Building Council (www.usgbc.org)] are asking
more and more from fenestration systems. Window, skylight, and
door manufacturers are responding with new and improved products
to meet those demands. With the advent of computer simulation software,
designing to improve thermal performance of fenestration
products has become much easier. Through participation in rating
and certification programs (such as those of the National Fenestration
Rating Council) that require the use of this software, fenestration
manufacturers can take credit for these improvements through certified
ratings that are credible to designers, builders, and code officials.
A designer should consider architectural requirements, thermal performance,
economic criteria, and human comfort when selecting
fenestration. Typically, a wide range of fenestration products are
available that meet the specifications for a project. Refining the specifications
to improve energy performance and enhance a living or
work space can lower energy costs, increase productivity, and
improve thermal and visual comfort. Carmody et al. (2000, 2004),
the Efficient Windows Collaborative (www.efficientwindows.org),
and CEA (1995) provide guidance for carrying out these requirements.
FENESTRATION COMPONENTS
Fenestration consists of glazing, framing, and, in some cases,
shading devices and insect screens.
Glazing
The glazing unit may have single or multiple glazing. The most
common glazing material is glass, although plastic is also used.
The glass or plastic may be clear, tinted, coated, laminated, patterned,
or obscured. Clear glass transmits more than 75% of the
incident solar radiation and more than 85% of the visible light.
Tinted glass is available in many colors, all of which differ in the
amount of solar radiation and visible light they transmit and
absorb. Coatings on glass affect the transmission of solar radiation,
and visible light may affect absorptance of room temperature
radiation. Some coatings are highly reflective (such as mirrors),
whereas others are designed to have a very low reflectance. Some
coatings result in a visible light transmittance that is as much as
twice the solar heat gain coefficient (desirable for good daylighting
while minimizing cooling loads). Laminated glass is made of two
panes of glass adhered together. The interlayer between the two
panes is typically plastic and may be clear, tinted, or coated. Patterned
glass is a durable ceramic frit applied to a glass surface in a
decorative pattern. Obscured glass is translucent and is typically
used in privacy applications.
Insulating Glazing Units
Insulating glazing units (IGUs) are hermetically sealed, multiple-
pane assemblies consisting of two or more glazing layers held
and bonded at their perimeter by a spacer bar typically containing a The preparation of this chapter is assigned to TC 4.5, Fenestration.
Copyright © 2005, ASHRAE
31.2 2005 ASHRAE Handbook—Fundamentals (SI)
desiccant material. The desiccated spacer is surrounded on at least
two sides by a sealant that adheres the glass to the spacer. Figure 1
shows the construction of a typical IGU.
Glazing. Common types of glass used in IGUs are clear, tinted,
and low emissivity (low-e). Because of its energy efficiency, daylighting,
and comfort benefits, low-e coated glass is now used in
more than 30% of all fenestration products installed in the United
States. Tinted and reflective glazing can also be used to reduce solar
heat gain through fenestration products. Low-e coatings can also be
applied to thin plastic films for use in IGUs.
There are two types of low-e coating. High-solar-gain coatings
primarily reduce heat conduction through the glazing system, and
are intended for cold climates. Low-solar-gain coatings, for hot climates,
reduce solar heat gain by blocking admission of the infrared
portion of the solar spectrum. There are two ways of achieving lowsolar-
gain low-e performance: with a special multilayer solarinfrared-
reflecting coating, or with a solar-infrared-absorbing outer
glass. To protect the inner glazing and building interior from absorbed
heat from this outer glass, a cold-climate-type low-e coating
is also used to reduce conduction of heat from the outer pane to the
inner one. Some manufacturers construct IGUs with one or more
suspended, low-e coated plastic films between the glass panes and
with a spacer that has better insulating properties and a dual sealant
that improves the seal around the gas spaces.
Spacer. The spacer separates the panes of glass and provides the
surface for primary and secondary sealant adhesion. Several types
of spacers are used in IGU construction today. Each type provides
different heat transfer properties, depending on the spacer material
and geometry.
Heat transfer at the edge of the IGU is greater than at the center,
because of greater heat flow through the spacer system. Spacer systems
have been developed to minimize heat flow at the edge of the
IGU. These spacer systems are referred to as warm-edge spacers.
In IGU construction, warm edge spacer designs reduce edge heat
transfer by substituting materials that have lower thermal conductivity
than aluminum (e.g., stainless steel, galvanized steel, tin
plated steel, polymers, or foamed silicone). Traditional spacers are
often made of aluminum.
Fusing or bending the corners of the spacer minimizes moisture
and hydrocarbon vapor transmission into the air space through the
corners. Desiccants such as molecular sieves or silica gel are also
used to absorb moisture that was initially trapped in the IGU during
assembly or gradually diffuses through the seals after construction.
Sealant(s). Several different sealant configurations are used in
modern IGU construction. In all sealant configurations, the primary
seal minimizes moisture and hydrocarbon transmission. In dual-seal
construction, the secondary seal provides structural integrity
between the lites of the IGU. A secondary seal ensures long-term
adhesion and greater resistance to solvents, oils, and short-term
water immersion. In typical dual-seal construction, the primary
seal is made of compressed polyisobutylene (PIB), and the secondary
seal is made of silicone, polysulfide, or polyurethane. Singleseal
construction depends on a single sealant to provide adhesion of
the glass to the spacer as well as minimizing moisture and hydrocarbon
transmission. Single-seal construction is generally more
cost-efficient than dual-seal systems. A third type of sealant used in
IGU construction takes advantage of advanced cross-linking polymers
that provide both low moisture transmission and equivalent
structural properties to dual-seal systems. These sealants are typically
referred to as dual-seal-equivalent (DSE) materials.
Desiccants. Typical desiccants include molecular sieves, silica
gel, or a matrix of both materials. Desiccants are used to absorb
moisture that was initially trapped in the IGU during assembly or
that gradually diffused through the seals after construction.
Gas Fill. The hermetically sealed space between glass panes in
an IGU is most often filled with air. In some cases, argon and krypton
gas are used instead, to further reduce the energy transfer.
Framing
The three main categories of window framing materials are
wood, metal, and polymers. Wood has good structural integrity and
insulating value but low resistance to weather, moisture, warpage,
and organic degradation (from mold and insects). Metal is durable
and has excellent structural characteristics, but it has very poor
thermal performance. The metal of choice in windows is almost
exclusively aluminum, because of its ease of manufacture, low cost,
and low mass, but aluminum has a thermal conductivity roughly
1000 times that of wood or polymers. The poor thermal performance
of metal-frame windows can be improved with a thermal
break (a nonmetal component that separates the metal frame
exposed to the outside from the surfaces exposed to the inside).
Polymer frames are made of extruded vinyl or poltruded fiberglass
(glass-reinforced polyester). Their thermal and structural performance
is similar to that of wood, although vinyl frames for large
windows must be reinforced.
Manufacturers sometimes combine these materials as clad units
(e.g., vinyl-clad aluminum, aluminum-clad wood, vinyl-clad wood)
to increase durability, improve thermal performance, or improve
aesthetics. In addition, curtain wall systems for commercial buildings
may be structurally glazed, and the exterior “framing” is simply
rubber gaskets or silicone.
Residential windows can be categorized by operator type, as
shown in Figure 2. Traditionally there are several basic window
types: casements; fixed picture windows; horizontal and vertical
sliders; pivoting; awning; or projecting windows; dual-acting windows;
and special applications such as skylights and greenhouse or
garden window inserts. The glazing system can be mounted either
directly in the frame (a direct-glazed or direct-set window, which is
not operable) or in a sash that moves in the frame (for an operating
window).
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