Definition of a Net-Zero Energy Home
A Net-Zero Energy Home (NZEH) is “capable of producing, at minimum, an annual output of renewable energy that is equal to the total amount of its annual consumed/purchased energy from energy utilities” and emits zero net carbon (1). This concept is becoming increasingly popular as people are becoming more aware of the effects of buildings on the environment. However, to build a net-zero home, in-depth design considerations to minimize “the energy requirements for space heating, cooling and water heating” are required (2). This will result in the least amount of artificial lighting, heating, and air conditioning to be used to achieve human comfort level (2). Designing for Building Orientation
The best building orientation for making efficient use of solar energy is south. Thus, running the building’s long axis from east to west and facing within 30 degrees of due south is strongly recommended (Figure 1). This allows the house to receive at least 90 percent of the optimal winter solar heat gain. The building’s south orientation should also be clear from obstacles to allow unblocked sunlight to enter the house (3). Use and Placement of Windows
Windows let in sunlight but trap long-wave radiation, making the indoor temperature rise; however, in the absence of sunlight, windows let out considerable amount of heated air due to their high conductivity. To minimize this effect, selecting windows with special coatings are recommended. Window sizes have to be determined carefully because of these unique properties, to balance heat loss and heat gain: Net window area should be at least five percent of net floor area with each room or space having one or more windows. Glare can often become problematic especially through south-facing windows but this can be prevented by using low-emissivity coated windows. Sloped or horizontal windows such as skylights must be used with caution because they can become major areas of uncontrollable heat loss, overheating, and condensation (3). Controlling Airtightness
Holes, cracks, floors, walls, ceilings, roofs, and outlets are all susceptible locations of air leakage. Air leakage equals energy leakage because as heated air leaks out of the building, the cooler air outside tends to get sucked into the building. Therefore, tight sealants around all joints and openings are required.
Proper Insulating Techniques
An NZEH should also be well insulated around the building envelope to minimize heat transfer. This is achieved by using proper installation of insulation that meets the required R-value (Figure 2). This will not only minimize the energy loss but also reduce the need for supplementary heating (3). Providing Ventilation by Mechanical or Natural Systems
Ventilation can either be mechanically or naturally provided. Before energy conservation became an issue to building occupants and the construction industry, buildings were not as airtight as they are today and natural ventilation was sufficient. Building occupants could open and close windows for fresh air and continuous ventilation was always present through the building’s cracks and openings. In today’s airtight buildings, natural ventilation is unreliable because buildings have fewer openings and cracks for natural air flow and the weather is often too cold or rainy for occupants to leave windows open for maintaining adequate relative humidity and fresh air circulation (4). One of the mechanical ventilation systems is the exhaust-only system, which exhausts air out of the building through an exhaust fan (Figure 3). This can be cost effective and functional provided that the building is airtight enough to run this system. If the building has cracks that act as an air path, the air that gets exhausted out can get sucked back into the building, essentially defeating the purpose of the system. Also, in humid climates, the exhaust-only system tends to cause condensation problems in...
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