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The Building Envelope

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The Building Envelope The Building Envelope Written by Bill Younger EB1850e The building envelope is made up of the windows, doors, walls, foundation, WSU/CE Energy floor, ceiling and roof and is the barrier between the conditioned indoor Program Library environment and the outdoors. Under most circumstances, you will use less energy in your HVAC system to control heating, cooling, outside air, and 925 Plum St. SE, Bldg. #4 humidity levels in your building if the envelope works well as a barrier. P.O. Box 43165 Olympia, WA Even with a very good envelope, your building will lose heat in cold 98504-3165 weather and gain it when it's hot outside. Your basic objective is to Tele:(360) 956-2000 minimize unwanted heat gain in the summer and heat loss during the Fax: (360) 956-2217 winter. Spokane Office 1212 N. Washington #106 Spokane, WA 99201-2401 Tele: (509) 625-5319 Fax: (509) 625-5315 Envelope Heat Loss The heat required to maintain a desired indoor temperature at a specified mechanisms by which heat flows into and out of a building also called conduction or radiation. Conduction Conduction is the term applied to heat flow within a solid from a high- temperature lower-temperature region through the molecules in the material. Conduction requires that surfaces touch in heat to transfer. Because the different materials in an insulated assembly touch each other, conduction heat loss through solid components of the building envelope. For example, heat flows by conduction from areas to the cooler areas of concrete slabs, window glass, walls, ceilings, and other solid materials. The unit used for thermal transmittance (heat transfer) or conductance of a single building material or building often called the U-value. U-values are expressed in Btus per hour per square foot of area per degree temperature difference. Windows are commonly described by their U-values. Descriptions of building walls, floors, or ceilings, often use R-values instead of U-values. The U-value or conductance flows through a material and the R-value measures the resistance, or how slowly heat flows. The two terms are reciprocal. (R=1/U, U=1/R) Convection Convection is the process of transferring heat from one place to another by molecular movement through fluids such as water or air. Heat loss by convection commonly results from exfiltration or air leakage. Connective heat loss occurs when warm air is forced out, usually from the building (exfiltration), by cold incoming air, usually in the lower part (infiltration). The rate of transfer is increased when the wind blows against the building or when the temperature difference between the inside and outside increases. Radiation Radiation is the heat transfer by electromagnetic waves from a warmer to a cooler surface. The transfer of the sun's heat to the earth or the warmth of a campfire are examples of radiant heat transfer. Heating and Cooling Loads To determine the degree in which the thermal quality of the building envelope affects the energy consumption, it is important to evaluate the driving forces behind the heating and cooling loads in the building. Thermally Light Buildings A building whose heating and cooling requirements are proportional to the weather is considered a thermally light building. That is, when the outdoor temperature drops below the desired room temperature, heating is required and when the outdoor temperature goes above the desired room temperature, cooling is needed. In a thermally light building, the thermal performance of the envelope becomes a dominate factor in energy use and can usually be seen as seasonal fluctuations in utility consumption. High internal heat gains can cause the air conditioning system to operate even in cool weather. Thermally Heavy Buildings When factors other than weather determine the heating and cooling requirements, the building can be considered thermally heavy. The difference between thermally light and thermally heavy buildings is the amount of heat generated by people, lighting, and equipment within the building. Thermally heavy buildings typically have high internal heat gains and, to a certain extent, are considered to be self heating and more cooling dominated. This need to reject heat makes them less dependent on the thermal performance of the building envelope. Thermal Weight A simple "rule of thumb" for determining the thermal weight of a building is to look at heating and cooling needs at an outdoor temperature of 60 degrees Fahrenheit. If the building requires heat at this temperature, it can, too, considered thermally light, if cooling is needed, it is thermally heavy. Some buildings or areas within a building can be both thermally light and thermally heavy depending on their use. A meeting room, for example, can have significant heat gains from people, equipment, and lights when the room is occupied and not require any heating from the HVAC system on a cold day. The same meeting room, however, may require heat at the same outdoor temperature when the room is vacant. Thermal Mass Thermal mass saves energy by storing and releasing heat. For a building to take advantage of thermal mass, there must be a source of free or less expensive energy to charge the mass. The existence of thermal mass, such as concrete walls and floors, can have a substantial impact on the operation of HVAC system's and can be difficult to analyze. It can affect the HVAC systems ability to quickly respond to rapid changes in load caused by increased occupancy, equipment, or solar gains through windows. The effect of thermal mass on the building systems will vary by climate and type of building as well as the location of the mass within the structure. Thermal mass in exterior walls, for example, will slow down heat flow through the wall allowing a reduction in insulation requirements while maintaining performance levels similar to standard frame construction. High levels of mass located within the building tend to reduce the effectiveness of mass in the outside walls. Buildings that most benefit from thermal mass are typically those with substantial cooling loads. In this case, the thermal mass can be precooled at night using outside air for free cooling or less expensive offpeak electricity for mechanical cooling. This allows the mass to absorb heat the following day, reducing the need for operation of cooling systems during peak utility demand hours. Generally, thermal mass is part of the integral construction of the building and is not added for conservation reasons. Unfortunately, there are no easy rules to determine how thermal mass will affect different buildings. It is important to note its existence because it may help you understand behavior of the mechanical systems or reasons for some comfort complaints. Evaluating the Building Envelope When evaluating the building envelope, it is important to keep in mind the thermal weight of the building as well as the various types of heat loss and gain to determine the impact of the existing envelope on energy consumption. For example, If you have determined that the building has high internal heat gains and must reject heat a majority of the time, then perhaps the level of effort spent on envelope evaluation should be limited. Time spent measuring glazing areas and determining detailed wall R values may not be justified because of the limited potential for reducing energy consumption by making changes to these components. A small office building with minimal internal heat gains and substantial amounts of west facing glass on the other hand may greatly benefit from a more detailed analysis of the envelope and effects of solar heat gains on the summer cooling load. It is up to the individual energy auditor to determine the level of effort given to evaluation of envelope components and potential improvements. Windows Heat Loss Through Windows Windows can be one of the single largest sources of unwanted heat gain and loss in the thermal envelope. It is not unusual to find a glass area that comprises only 15 to 25 percent of the surface area of a building while contributing up to 75 percent of the heat loss. Windows typically lose heat Window infiltration can occur through conduction and air movement around the frames, and gain heat around the sash, frame or through solar radiation. glass. In addition to wasting energy, drafts from Window U values infiltration can lead to Window U-values are determined by testing the frame and glass as a comfort problems in whole unit. New developments in glass and frame technologies have surrounding areas. substantially improved the thermal performance of window units. Technologies such as "low E" glazing, thermally broken frames, and gas fillers between panes are becoming common in commercial building construction. Solar Heat Gain Windows are subject to solar heat gains which can have significant impacts on HVAC operation and occupant comfort. The amount of heat gain is dependent on orientation, season, time of day, glazing type, and shading by window coverings, overhangs, other buildings and vegetation. Solar gains through south facing glass will add heat to the building in the winter. East and west surfaces will gain the greatest amount of heat in the early morning and late afternoon hours during summer months. Winter heat gains may be desirable in thermally light buildings while any solar heat gains in a thermally heavy building will only contribute to the cooling load. East and west facing glass are primarily a problem during the summer. Low sun angles in the morning and late afternoon can result in substantial solar heat gains as well as unwanted glare. The problem of excess solar heat gains during the summer can be compounded by the build up of internal heat most buildings experience late in the day.
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