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:
- Walls and Roofs
- Climate Considerations
- Doors, Windows, and Openings
- Thermal Efficiency
- Moisture Buildup
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.