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The Building Envelope
A building consists of the superstructure and the substructure. The superstructure is that part of the building structure that is above the top of the foundation walls. The substructure is the building's foundation system. The building envelope or building shell includes the elements that enclose the conditioned (heated and cooled) space and interface the indoor spaces with the outdoor environment. It includes the wall, roof, and opening assemblies (e.g., walls, windows, doors, roof, floor, and foundation), insulation, air barriers, vapor retarders, weather stripping, and caulking. The building envelope provides shelter from severe weather. It also plays a major role in sustaining the indoor environment and , as a result, influences the heating, ventilating and air conditioning (HVAC) systems in a building.
Historically, structural integrity and durability were the prime characteristics of a well-designed building envelope.
Today, the design and construction of a building envelope must incorporate other characteristics that contribute to high performance, such as airtightness, thermal insulation, heating and cooling load reduction, and amount of embodied energy.
Optimal design of the building envelope increases construction and design costs. However, the additional cost for a high performance envelope must be paid for through cost avoidance achieved by a need for smaller heating, cooling, ventilating, and lighting equipment and in reduction of operating and maintenance costs.
One important factor that contributes to overall performance of a building is the energy it uses, with one aspect being the energy consumed in constructing it. Embodied energy is the sum of all energy used to extract, process, refine, fabricate, trans port, and install a product or material. It includes all energy consumed from removing natural resources (e.g., mining iron ore, cutting timber) through material or product installation. it's a concept that attempts to measure the energy expense of an item. Both direct and indirect energy consumed are included in determining a material's embodied energy. For example, consider a brick: embodied energy of the brick includes the energy used to extract clay from the pit, transport the raw material to the manufacturing plant, refine and mix the clay, extrude and cut the brick, fire the brick in a kiln (oven), stack the brick on a crate, transport and deliver it to the building site, and eventually lay the brick in a wall. It also includes the indirect energy required, including energy required to fabricate the equipment and materials needed to manufacture a brick (e.g., trucks, kilns, and mining equipment).
Computation of embodied energy for a specific material is an inexact science because it's affected by many factors.
The manufacturing technology used to refine a material or fabricate a product can be different with different manufacturers. Factors such as distance of the building site from manufacturer and the distance trades people must travel to and from the site during construction are also all part of the embodied energy calculation. Furthermore, embodied energy can be estimated for the material or product before it's installed, after it's installed, or after it's installed and including maintenance, repair, demolition, and disposal of the building structure. As a result, the published values of embodied energy are not universally fixed.
A relationship exists among the embodied energy of materials, the complexity of processing the material, and pollution. Generally, the fewer and simpler the steps involved in a material's extraction, production, transportation, and installation, the lower its embodied energy. As a result, the embodied energy of a material is often reflected in its cost. Release of pollutants is also linked with embodied energy. For example, on average, about 1000 lb (450 kg) of CO2 are produced per million Btu of embodied energy.
The unit measure for embodied energy is typically the Btu/lb (MJ/kg or GJ/kg). TBL. 1 provides embodied energy values for common materials for comparison purposes. Generally, plastics, virgin (not recycled) metals, concrete, and bitumen/ asphalt products are high in embodied energy. Engineered wood products [e.g., medium-density fiberboard (MDF), oriented strand board (OSB), and glued-laminated timber products] and recycled metals and glass have a moderate amount of embodied energy. Earthen (e.g., brick, adobe), and sawn timber products, if they are harvested and produced nearby, are low in embodied energy.
Embodied energy is a significant contributor to the life cycle cost of a building material, product, or system. Total embodied energy of a building ranges from about 400 000 to 500 000 Btu/ft^2 (4.5 to 5.5 GJ/m2) of finished floor area depending on floor size and type, cladding material, and number of stories. Embodied energy can be the equivalent of several years of operational energy consumption (e.g., from lighting, heating, cooling), ranging from approximately 10 years (for typical houses) to over 30 years (for commercial buildings). As buildings become more energy efficient in their operation, total embodied energy can approach half the lifetime energy consumption.
Tbl. 1 range of reported values of embodied energy for common building materials. These values are for comparison only. Embodied energy is not a universally fixed value; it varies from one location to another.
Glass Cement Plaster Bricks Timber (sawn softwood)
Tbl. 2 energy profile of a typical residence.
% Energy Profile of a Typical Residence vs. Space heating 3 Space cooling 1 Water heating 1 Lighting Refrigeration Cooking Electronics Computers Other
Tbl. 3 cooling load profiles of a typical residence and office building.
% Cooling Load Profile of Residence; % Cooling Load Profile of Office
Transmission Ventilation/infiltration Solar Occupants Lights Equipment Transmission Ventilation/infiltration Solar Occupants Lights Equipment
There are many ways to classify a building. For example, model codes classify a building based on occupancy (e.g., R for residential, A for assembly, E for educational, and so on) and construction type (e.g., Type I, Type II, Type III, and so forth).
When taking into account energy use, buildings can be classified by thermal load and thermal mass.
Heating/Cooling Load Classifications
Energy is consumed in buildings in many different ways. A heating energy profile of a typical American residence is shown in TBL. 2. Although this is based upon an aver age, space heating and cooling generally account for the largest fraction of energy consumed in most homes. As a result, transmission and infiltration (air leakage) through the building envelope play an important role in energy use in residences.
Cooling load profiles of a typical residence and a typical office building are shown in TBL. 3. Notice the differences in factors that contribute to the typical loads. In a residence, transmission, ventilation/infiltration, and solar gains dominate the loads. However, in the typical office building, heat from solar gains, occupants, lights, and equipment affect the cooling load much more and transmission and ventilation/infiltration affect the cooling load much less. As a result, the performance of the building envelope is less of a concern in office buildings in contrast to residential buildings.
From the perspective of energy use, buildings can be classified by how they use energy for space heating and cooling.
External-load-dominated buildings have the largest part of their energy consumption associated with the thermal characteristics of the building envelope; that is, heating and cooling loads are tied mainly to characteristics like thermal insulation and glazing type, orientation, and area. Such buildings are referred to as skin-load-dominated or envelope-load-dominated buildings. Climatic conditions also greatly affect the thermal performance of these buildings. Therefore, they can be referred to as climate-dominated buildings. In cold climates, the load is mostly heating, while in hot climates, the load is mostly cooling. In temperate climates, the load is heating with some cooling.
Single and multifamily residences are typically external-load dominated buildings.
Internal-load-dominated buildings have most of their energy consumption associated with large internal heat gains (e.g., from occupants, lighting, equipment); that is, heating and cooling loads are largely generated within the building itself. Office buildings, schools, hospitals, and factories are typically internal load-dominated buildings.
In building design, it's helpful to know how loads will dominate a building, because this will direct the focus of design strategies related to energy efficiency. The difference between classifications influences the significance of insulation standards and glazing area because of the desirability of solar heat gain. In exterior-load-dominated buildings in temperate to cold climates, strategies include high levels of insulation, minimizing infiltration (air leakage) through sealing the envelope, high performance windows (e.g., double or triple low-e glazing), and passive solar incorporated into the design that controls the amount of radiant heating loads in the winter and summer with window orientation and size and location and amount of heat absorbing thermal mass. In moderate and hot climates, solar control is crucial, because of the need to keep cooling loads under control.
Specific strategies to reduce energy use in internal-load dominated buildings include improvement of energy efficiency of equipment, appliances, and lighting and use of natural and climatic factors (e.g., daylighting to reduce electrical lighting, natural ventilation). Once internal loads are reduced, there is an increase in the importance of the thermal properties of the building envelope.
Building Mass Classifications
Buildings can also be classified by their mass content.
Lightweight construction uses light timber or light-gauge steel framing as the structural support system for nonstructural cladding (e.g., fiber cement, plywood, and steel). Heavyweight construction systems are usually masonry and include brick, concrete, concrete block, tiles, rammed earth, mud brick, and so on. See Ill. 1 and 2.
• Generally has higher embodied energy.
• Improves thermal comfort and reduces operational (heating and cooling) energy use, when used in con junction with passive design and good insulation.
• Is most appropriate in climates with high diurnal (day-night) temperature ranges and significant heating and cooling requirements.
• Requires more substantial foundation systems and causes greater site impact and disturbance.
• Should be avoided on remote sites where there is a high transport component.
• Generally has lower embodied energy.
• Can yield lower total life cycle energy use, particularly where the diurnal range is low.
• Responds rapidly to temperature changes and can pro vide significant benefits in warmer climates by cooling rapidly at night.
• Is preferred on remote sites with a high materials transportation component.
• Usually requires more heating and cooling energy in cold to warm climates (where solar access is achievable) when compared to heavyweight construction with similar levels of insulation and passive design.
• Can have low production impact (e.g., sustainably sourced timber) or high impact (unsustainably sourced timber or metal frame).
Ill. 1 Cast-in-place (in situ) concrete is a heavyweight construction system.
Ill. 2 Lightweight construction typically consists of light timber frame.