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Section III- Envelope a.) Summary: It is very important to ensure the envelope is designed correctly before it is built. Changes to the insulation or windows are very difficult after the envelope has been established. Exceed code o Wall roof insulation levels Continuous insulation Windows o Percent of wall area o Low e-coating Reduced infiltration o Air sealing o Air barrier o Vestibules Envelope commissioning b.) Technical Information: Building envelope includes the exterior walls, roof, and exterior windows, all of which need to be optimized to improve the energy performance and thermal comfort of a building. Code advances have been ratcheting up the requirement for wall and roof insulation levels, including continuous insulation. While going beyond code is always encouraged, the costs need to be balanced with the benefits. Envelope 1|Page www.sedac.org The building envelope is the boundary between the inside and outside of the structure through which heat transfer could take place. To improve the building energy performance, the design of building envelope is the first line of defense. In terms of the climate design, envelope could be a “closed shell” or barrier to provide maximum separation between inside and outside particularly in harsh climates (opaque envelope that can be heavily insulated), or it could be an “open frame”, which is more appropriate for temperate climates where a higher degree of contact between interior and exterior is desirable (transparent envelope). Heat flow phenomenon could occur within a building envelope (opaque or transparent) or via air exchange. For an opaque building envelope, the heat could conductively transfer through the mass. Given the non-homogeneous wall layers, heat transfers along the path of least resistance—this phenomenon is generally referred to as thermal bridging. However, the wall construction could potentially possess a beneficial thermal property called thermal mass. Thermal mass is a combination of wall material density and specific heat. This material property could result in time lag with respect to heat transfer, which in turn causes the wall’s response to the thermal force to be transient. In hot climates, thermal mass could delay the heat transfer from outside to inside and in cold climates, it could perform otherwise and keep the heat inside, both of which could considerably improve the thermal comfort in buildings and save cooling and heating energy. For a transparent building envelope, heat transfer occurs through windows/skylights that generally have lower R-values than other envelope components. Despite this inherent limitation, the fenestrations could admit sunlight to the building for heating and daylighting, which could improve building energy and thermal comfort performance. Heat transfer also could take place through air exchanges to the outside, mainly in a form of infiltration or ventilation. Infiltration is an unintended flow of outside air into the building through leaks in envelope, normal opening and closing of doors, etc, and ventilation is a purposeful introduction of outside air for fresh air or conditioning purposes. Generally, the infiltration is to be minimized through envelope air-tightness codes and standards; however, natural ventilation could help save cooling costs in buildings where opening windows would be appropriate. Envelope 2|Page www.sedac.org Stack Effect: Stack ventilation is a vertical movement of air through buildings—a natural phenomenon caused by air vertical density differences. In other words, as the air gets hotter, it becomes lighter and rises, which in turns causes vertical air movement within a building. Stack ventilation keeps inside air temperature slightly above outside air temperature and could be enhanced by coupling with thermal chimney effect to increase air flow. Envelope 3|Page www.sedac.org c.) Case Study: Science and Technology Magnet School in Champaign, IL Our case study school is very well insulated in the walls and roof with 3-6” of continuous polyiso insulation. R-18 to R-24 walls o 3” continuous insulation (polyiso) R-37 roof insulation o 6” continuous insulation (polyiso) o 3” minimum at drains Windows U-0.27, SHGC-0.31 Vestibules reduce air infiltration from doors Designed with heat exchanger to make up for reduced air infiltration d.) Potential Issues: Case Study: Hospital (Built in 2005) Envelope 4|Page www.sedac.org The hospital was designed before continuous insulation was required by code. The image below is from a thermal camera in one of the walls, which illustrates the difference in temperature of the insulated cavity and the steel studs. The temperature goes from 64 degrees to 75 degrees. Patients expressed that they were uncomfortable in their rooms. This was partly due to the cold areas acting like a heat sink absorbing IR heat from the room. When examining a window in one of the rooms, it was discovered that parts of the frame were 42 degrees on a day when the temperature outside was in the upper 20’s. This lack of insulation could also lead to water condensation and mold, which could be especially dangerous inside a hospital. Envelope 5|Page www.sedac.org