Building Envelope
Before upgrading energy consuming equipment, building owners should reduce the load served by this equipment. Lighting, internal heat producing equipment and the building envelope should be improved. The geographic location will determine to a large extent the strategies employed.
Following are the basic strategies. More exotic measures will be covered separately.
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, as these have an initial reflectivity of at least 65 percent. 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 south walls will benefit from shading as well, especially for buildings with year-round cooling loads. In new construction, providing architectural features that shade walls and glazings should be considered. In existing buildings, vegetative shading options are generally more feasible.
Roofing
Roofs are fundamental in protecting buildings from moisture infiltration and other outside weather conditions. Commercial buildings typically use low-slope or flat roofs, and the choice of roofing material affects issues regarding moisture, standing water, durability and appearance.
Built-up roofing is a popular choice of roofing used on commercial, industrial and institutional buildings. BUR is used on flat or low-sloped roofs and consists of multiple layers of bitumen and ply sheets. Components of a BUR system include the roof deck, a vapor retarder, insulation, membrane and surfacing material.
Other commercial building roofing materials include metal (primarily steel or aluminum) and concrete. Solar shingles and roofing made from recycled materials are energy-efficient options.
Learn More:
- DOE Federal Energy Management Program. “Greening Federal Facilities” 2nd Edition – Low Slope Roof section. (PDF 322 KB) Download Adobe Reader
- ENERGY STAR Guide to Roofing Materials
- Cool Roof Calculator
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 losses 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 hydro chlorofluorocarbons (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.
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 re-roofing. At the time of re-roofing, consider switching to a protected-membrane roofing system, which will allow reuse of the rigid insulation during future re-roofing—thus greatly cutting down on landfill disposal.
While insulation is 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.
Moisture Buildup within the Envelope
A key consideration for building envelopes is to keep out water vapor, specifically the control of moisture migration as a result of both vapor diffusion and air transport. Methods used to keep water vapor out, however, can also trap it inside the building, particularly if there are moisture issues during the building phase. Since water vapor typically moves from the warm side to the cool side of a building, climate and season are important factors and different climates require different strategies. One such strategy is known as a vapor barrier, often a vapor tight sheet of plastic or metal foil, placed as near to the warm side of the wall construction as possible. Detailed information about vapor barriers and other strategies to reduce moisture buildup can be found in the Building Science Corporation’s report, Insulations, Sheathings, and Vapor Diffusion Retarders.
Weather-strip all doors and place sealing gaskets and latches on all operable windows. Careful detailing, weather-stripping, 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 weather-stripping and seals periodically to ensure 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.
Envelope Openings for Doors and Windows
Doors, windows and vents in the envelope need to be carefully positioned based on consideration of daylighting, and 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, heating, ventilating, and air conditioning systems, 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 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.
Recent developments in window technology have yielded electrochromic windows, which are windows that can be darkened or lightened electronically. A small voltage applied to the windows will cause them to darken; reversing the voltage causes them to lighten. This capability allows for the automatic control of the amount of light and heat that passes through the windows, thereby presenting an opportunity for the windows to be used as energy-saving devices. To learn more about electrochromic windows, visit the National Renewable Energy Lab (NREL)’s website.
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.
Climate Considerations
The specific climate zone where a building is located has an impact on envelope building materials and the building’s design. 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.
The above article is from the DOE, EERE website.
