The building envelope is a critical component of any facility
since it both protects the building occupants and plays a major
role in regulating the indoor environment. Consisting of the building's
roof, walls, windows, and doors, the envelope controls the flow
of energy between the interior and exterior of the building. The
building envelope can be considered the selective pathway for a
building to work with the climate-responding to heating, cooling,
ventilating, and natural lighting needs.
This section includes:
For a new project, opportunities relating to the building envelope
begin during the predesign phase of the facility. An optimal design
of the building envelope may provide significant reductions in heating
and cooling loads-which in turn can allow downsizing of mechanical
equipment. When the right strategies are integrated through good
design, the extra cost for a high-performance envelope may be paid
for through savings achieved by installing smaller HVAC equipment.
With existing facilities, facility managers have much less opportunity
to change most envelope components. Reducing outside air infiltration
into the building by improving building envelope tightness is usually
quite feasible. During reroofing, extra insulation can typically
be added with little difficulty. Windows and insulation can be upgraded
during more significant building improvements and renovations.
The building envelope, or "skin," consists of structural materials
and finishes that enclose space, separating inside from outside.
This includes walls, windows, doors, roofs, and floor surfaces.
The envelope must balance requirements for ventilation and daylight
while providing thermal and moisture protection appropriate to the
climatic conditions of the site. Envelope design is a major factor
in determining the amount of energy a building will use in its operation.
Also, the overall environmental life-cycle impacts and energy costs
associated with the production and transportation of different envelope
materials vary greatly.
In keeping with the whole building approach, the entire design
team must integrate design of the envelope with other design elements
including material selection; daylighting and other passive solar
design strategies; heating, ventilating, and air-conditioning (HVAC)
and electrical strategies; and project performance goals. One of
the most important factors affecting envelope design is climate.
Hot/dry, hot/moist, temperate, or cold climates will suggest different
design strategies. Specific designs and materials can take advantage
of or provide solutions for the given climate.
A second important factor in envelope design is what occurs inside
the building. If the activity and equipment inside the building
generate a significant amount of heat, the thermal loads may be
primarily internal (from people and equipment) rather than external
(from the sun). This affects the rate at which a building gains
or loses heat. Building volume and siting also have significant
impacts upon the efficiency and requirements of the building envelope.
Careful study is required to arrive at a building footprint and
orientation that work with the building envelope to maximize energy
benefit.
Openings are located in the envelope to provide physical access
to a building, create views to the outside, admit daylight and/or
solar energy for heating, and supply natural ventilation. The form,
size, and location of the openings vary depending upon the role
they play in the building envelope. Window glazing can be used to
affect heating and cooling requirements and occupant comfort by
controlling the type and amount of light that passes through windows.
Decisions about construction details also play a crucial role
in design of the building envelope. Building materials conduct heat
at different rates. Components of the envelope such as foundation
walls, sills, studs, joists, and connectors, among others, can create
paths for the transfer of thermal energy, known as thermal bridges,
that conduct heat across the wall assembly. Wise detailing decisions,
including choice and placement of insulation material, are essential
to assure thermal efficiency.
Windows
Glazing systems have a huge impact on energy consumption, and glazing
modifications often present an excellent opportunity for energy
improvements in a building. Appropriate glazing choices vary greatly,
depending on the location of the facility, the uses of the building,
and (in some cases) even the glazing's placement on the building.
In hot climates, the primary strategy is to control heat gain by
keeping solar energy from entering the interior space while allowing
reasonable visible light transmittance for views and daylighting.
Solar screens that intercept solar radiation, or films that prevent
infrared and ultraviolet transmission while allowing good visibility,
are useful retrofits for hot climates. In colder climates, the focus
shifts from keeping solar energy out of the space to reducing heat
loss to the outdoors and (in some cases) allowing desirable solar
radiation to enter. Windows with two or three glazing layers that
utilize low-emissivity coatings will minimize conductive energy
transmission. Filling the spaces between the glazing layers with
an inert low-conductivity gas, such as argon, will further reduce
heat flow. Much heat is also lost through a window's frame. For
optimal energy performance, specify a low-conductivity frame material,
such as wood or vinyl. If metal frames are used, make sure the frame
has thermal breaks. In addition to reducing heat loss, a good window
frame will help prevent condensation—even high-performance
glazings may result in condensation problems if those glazings are
mounted in inappropriate frames or window sashes.
Fenestration can be a source of discomfort when solar gain and
glare interfere with work station visibility or increase contrast
and visual discomfort for occupants. Daylighting benefits will be
negated if glare forces occupants to close blinds and turn on electric
lights, for example, to perform visual tasks optimally.
Facility managers should choose appropriate window technology
that is cost-effective for the climate conditions. Computer modeling,
using a tool such as DOE-2
or Energy-10,
will help determine which glazing system is most appropriate for
a particular climate. In coastal California, for example, single
glazing may be all that can be economically justified, while in
both hotter and colder climates, more sophisticated glazings are
likely to be much more effective.
Walls and Roofs
For buildings dominated by cooling loads, it makes sense to provide
exterior finishes with high reflectivity or wall-shading devices
that reduce solar gain. Reflective roofing products help reduce
cooling loads because the roof is exposed to the sun for the entire
operating day. Specify roofing products that carry the ENERGY
STAR® roof label-for low-slope
roofing products, these have an initial reflectivity of at least
65%. ENERGY STAR roof products are widely available with single-ply
roofing, as well as various other roofing systems. Wall shading
can reduce solar heat gain significantly—use roof overhangs,
window shades, awnings, a canopy of mature trees, or other vegetative
plantings, such as trellises with deciduous vines. To reduce cooling
loads, wall shading on the east and west is most important, though
especially for buildings with year-round cooling loads, south walls
will benefit from shading as well. In new construction, providing
architectural features that shade walls and glazings should be considered.
In existing buildings, vegetative shading options are generally
more feasible.
Insulation
With new buildings, adding more wall insulation than normal can
be done for a relatively low-cost premium. Also consider thermal
bridging, which can significantly degrade the rated performance
of cavity-fill insulation that is used with steel framing. With
steel framing, consider adding a layer of rigid insulation.
Boosting wall insulation levels in existing buildings is difficult
without expensive building modifications. One option for existing
buildings is adding an exterior insulation and finish system (EIFS)
on the outside of the current building skin. With EIFS, use only
systems that include a drainage layer to accommodate small leaks
that may occur over time—avoid barrier-type systems.
Roof insulation can typically be increased relatively easily during
reroofing. At the time of reroofing, consider switching to a protected-membrane
roofing system, which will allow reuse of the rigid insulation during
future reroofing—thus greatly cutting down on landfill disposal.
While we think of insulation as a strategy for cold climates,
it makes sense in cooling climates as well. The addition of insulation
can significantly reduce air conditioning costs and should be considered
during any major renovation project. Roofs and attics should receive
priority attention for insulation retrofits because of the ease
and relative low cost.
Climate Considerations
Assess the local climate (using typical meteorological-year data)
to determine appropriate envelope materials and building designs.
The following considerations should be taken into account, depending
on the climate type.
Hot/Dry Climates
Use materials with high thermal mass. Buildings in hot/dry climates
with significant diurnal temperature swings have traditionally employed
thick walls constructed from envelope materials with high mass,
such as adobe and masonry. Openings on the north and west facades
are limited, and large southern openings are detailed to exclude
direct sun in the summer and admit it in winter.
A building material with high thermal mass and adequate thickness
will lessen and delay the impact of temperature variations from
the outside wall on the wall's interior. The material's high thermal
capacity allows heat to penetrate slowly through the wall or roof.
Because the temperature in hot/dry climates tends to fall considerably
after sunset, the result is a thermal flywheel effect—the building
interior is cooler than the exterior during the day and warmer than
the exterior at night.
Hot/Moist Climates
Use materials with low thermal capacity. In hot/moist climates,
where nighttime temperatures do not drop considerably below daytime
highs, light materials with little thermal capacity are preferred.
In some hot/moist climates, materials such as masonry, which functions
as a desiccant, are common. Roofs and walls should be protected
by plant materials or overhangs. Large openings protected from the
summer sun should be located primarily on the north and south sides
of the envelope to catch breezes or encourage stack ventilation.
Temperate Climates
Select materials based on location and the heating/cooling strategy
to be used. Determine the thermal capacity of materials for buildings
in temperate climates based upon the specific locale and the heating/cooling
strategy employed. Walls should be well insulated. Openings in the
skin should be shaded during hot times of the year and unshaded
during cool months. This can be accomplished by roof overhangs sized
to respond to solar geometries at the site or by the use of awnings.
Cold Climates
Design wind-tight and well-insulated building envelopes. The thermal
capacity of materials used in colder climates will depend upon the
use of the building and the heating strategy employed. A building
that is conventionally heated and occupied intermittently should
not be constructed with high mass materials because they will lengthen
the time required to reheat the space to a comfortable temperature.
A solar heating strategy will necessitate the incorporation of massive
materials, if not in the envelope, in other building elements. Where
solar gain is not used for heating, the floor plan should be as
compact as possible to minimize the area of building skin.
Doors, Windows, and Openings
Size and position doors, windows, and vents in the envelope based
on careful consideration of daylighting, heating, and ventilating
strategies.
The form, size, and location of openings may vary depending on
how they affect the building envelope. A window that provides a
view need not open, yet a window intended for ventilation must do
so. High windows for daylighting are preferable because, if properly
designed, they bring light deeper into the interior and eliminate
glare.
Vestibules at building entrances should be designed to avoid the
loss of cooled or heated air to the exterior. The negative impact
of door openings upon heating or cooling loads can be reduced with
airlocks. Members of the design team should coordinate their efforts
to integrate optimal design features. For passive solar design,
this includes the professionals responsible for the interactive
disciplines of building envelope, daylighting, orientation, architectural
design, massing, HVAC, and electrical systems.
Shade openings in the envelope during hot weather to reduce the
penetration of direct sunlight to the interior of the building.
Use overhangs or deciduous plant materials on southern orientations
to shade exterior walls during warmer seasons. Be aware, however,
that deciduous plants can cut solar gains in the winter by 20 percent.
Shade window openings or use light shelves at work areas at any
time of year to minimize thermal discomfort from direct radiation
and visual discomfort from glare.
In all but the mildest climates, select double- or triple-paned
windows with as high an "R" value as possible and proper shading
coefficients within the project's financial guidelines.
The "R" value is a measure of the resistance to heat flow across
a wall or window assembly (with higher values representing a lower
energy loss). Shading coefficient is a ratio used to simplify comparisons
among different types of heat reducing glass. The shading coefficient
of clear double-strength glass is 1.0. Glass with a shading coefficient
of 0.5 transmits one-half of the solar energy that would be transmitted
by clear double-strength glass. One with a shading coefficient of
0.75 transmits three-quarters.
Select the proper glazing for windows, where appropriate. Glazing
uses metallic layers of coating or tints to either absorb or reflect
specific wavelengths in the solar spectrum. In this manner, desirable
wavelengths in the visible spectrum that provide daylight are allowed
to pass through the window while other wavelengths, such as near-infrared
(which provides heat) and ultraviolet (which can damage fabric),
are reflected. Thus, excess heat and damaging ultraviolet light
can be reduced while still retaining the benefits of natural lighting.
More advanced windows use glazings that are altered with changing
conditions, such as windows with tinting that increases under direct
sunlight and decreases as light levels are reduced. Research is
being conducted on windows that can be adjusted by the building
occupant to allow more or less heat into a building space.
Thermal Efficiency
Determine the building function and amount of equipment that will
be used. The type of activity and the amount of equipment in a building
affect the level of internal heat generated. This is important because
the rate at which a building gains or loses heat through it skin
is proportional to the difference in air temperature between inside
and outside. A large commercial building with significant internal
heat loads would be less influenced by heat exchanges at the skin
than a residence with far fewer internal sources of heat generation.
In general, build walls, roofs, and floors of adequate thermal
resistance to provide human comfort and energy efficiency. Roofs
especially are vulnerable to solar gain in summer and heat loss
in winter. Avoid insulating materials that require chlorofluorocarbons
(CFCs) or hydrochlorofluorocarbons (HCFCs) in their production,
as these are ozone-depleting compounds. Consider insulating materials
made from recycled materials such as cellulose or mineral wool,
if such items meet the project's performance and budgetary criteria.
If the framing system is of a highly conductive material, install
a layer of properly sized insulating sheathing to limit thermal
bridging.
Reflectivity
In regions with significant cooling loads, select exterior finish
materials with light colors and high reflectivity. Consider the
impact of decisions upon neighboring buildings. A highly reflective
envelope may result in a smaller cooling load, but glare from the
surface can significantly increase loads on and complaints from
adjacent building occupants.
Moisture Buildup within the Envelope
Under certain conditions, water vapor can condense within the building
envelope. When this occurs the materials that make up the wall can
become wet, lessening their performance and contributing to their
deterioration. To prevent this, place a vapor tight sheet of plastic
or metal foil, known as a vapor barrier, as near to the warm side
of the wall construction as possible. For example, in areas with
meaningful heating loads, the vapor barrier should go near the inside
of the wall assembly. This placement can lessen or eliminate the
problem of water-vapor condensation.
Weatherstrip all doors and place sealing gaskets and latches on
all operable windows. Careful detailing, weatherstripping, and sealing
of the envelope are required to eliminate sources of convective
losses. Convective losses occur from wind loads on exterior walls.
They also occur through openings around windows and doors and through
small openings in floor, wall, and roof assemblies. Occupants can
experience these convective paths as drafts. Older buildings can
prove to be a source of significant energy loss and added fuel and
pollution costs. Inspect weatherstripping and seals periodically
to ensure that they are air-tight.
Specify construction materials and details that reduce heat transfer.
Heat transfer across the building envelope occurs as either conductive,
radiant, or convective losses or gains. Building materials conduct
heat at different rates. Metals have a high rate of thermal conductance.
Masonry has a lower rate of conductance; the rate for wood is lower
still. This means that a wall framed with metal studs compared to
one framed with wood studs, where other components are the same,
would have a considerably greater tendency to transmit heat from
one side to the other. Insulating materials, either filled in between
framing members or applied to the envelope, resist heat flow through
the enclosing wall and ceiling assemblies.
Consider the following principles in construction detailing:
- To reduce thermal transfer from conduction, develop details
that eliminate or minimize thermal bridges.
- To reduce thermal transfer from convection, develop details
that minimize opportunities for air infiltration or exfiltration.
Plug, caulk, or putty all holes in sills, studs, and joists. Consider
sealants with low environmental impact that do not compromise
indoor air quality.
Incorporate solar controls on the building exterior to reduce
heat gain. Radiant gains can have a significant impact on heating
and cooling loads. A surface that is highly reflective of solar
radiation will gain much less heat than one that is adsorptive.
In general, light colors decrease solar gain while dark ones increase
it. This may be important in selecting roofing materials because
of the large amount of radiation to which they are exposed over
the course of a day; it may also play a role in selecting thermal
storage materials in passive solar buildings. Overhangs are effective
on south-facing facades while a combination of vertical fins and
overhangs are required on east and west exposures and, in warmer
areas during summer months, on north-facing facades.
Consider the use of earth berms to reduce heat transmission and
radiant loads on the building envelope. The use of earth berms or
sod roofs to bury part of a building will minimize solar gain and
wind-driven air infiltration. It will also lessen thermal transfer
caused by extremely high or low temperatures.
Building Grounds
Coordinate building strategy with landscaping decisions:
- Landscape and other elements such as overhangs are integral
to a building's performance.
- Decisions about the envelope need to be coordinated with existing
and new landscaping schemes on a year-round basis.
- Reduce paved areas to lessen heat buildup around the building
that will add to the load on the building envelope.
- Consider selection of a paving color with a high reflectance
to minimize heat gain.
- Glare factors should also be considered.
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