cold climates
In cold conditions, the main objective is to reduce the flow of heat out of the building. The components of the building envelope—windows, doors, roofs, floors/foundations, walls, and air infiltration barriers—are all important sources of heat loss;[8][9] In a well-insulated home, windows then become a major source of heat transfer.[10] The resistance to conductive heat loss for single glazing&action=edit&redlink=1 "Glazing (window) (not yet drafted)") standard corresponds to a R&action=edit&redlink=1 "R-value (insulation) (not yet drafted)") of approximately 0.17 m⋅K⋅W or more than twice that for typical double glazing (compared to 2–4 m⋅K⋅W for bulk glass wool[11]). Losses can be reduced by good air conditioning, mass insulation and minimizing the amount of non-insulating glazing (particularly non-sun facing). Interior thermal radiation can also be a disadvantage with spectral selective (low-E) glazing. Some insulating glazing systems can double or triple the R values.
Vacuum panels and airgel surface insulation are two technologies that can improve the energy performance and thermal insulation effectiveness of residential and commercial buildings in cold climate regions such as New England and Boston.[12] In the past, the price of thermal insulation materials that showed high insulating performance was very high.[12] With the development of the materials industry and the rise of scientific technologies, more and more insulation materials and technologies have emerged during the century, giving us several options for building insulation. Especially in cold climate areas, a large amount of thermal insulation is needed to deal with heat losses caused by cold climate (infiltration, ventilation and radiation). There are two technologies worth discussing:
VIPs stand out for their ultra-high thermal resistance,[13] their thermal resistance capacity is four to eight times that of conventional foam insulation materials, leading to a lower thickness of thermal insulation in the building shell compared to traditional materials. VIPs are generally composed of core panels and metal enclosures.[13] Common materials used to produce core panels are fumed and precipitated silica, open-cell polyurethane (PU), and different types of fiberglass. And the center panel is covered by the metal enclosure to create a vacuum environment, the metal enclosure can ensure that the center panel stays in the vacuum environment.[13] Although this material has high thermal performance, it still maintains a high price in the past twenty years.
Airgel was first discovered by Samuel Stephens Kistle in 1931.[14] It is a kind of gel in which the liquid part is replaced by gas, in fact, it is composed of 99% air.[14] This material has a relatively high R-value of around R-10 per inch, which is considerably higher compared to conventional plastic foam insulation materials. But difficulties in processing and low productivity limit the development of aerogels,[14] the cost price of this material still remains at a high level. Only two companies in the United States offer the commercial airgel product.
hot climates
In hot conditions, the largest source of thermal energy is solar radiation.[15] This can enter buildings directly through the windows or can heat the building shell to a higher than ambient temperature, increasing heat transfer through the building envelope.[16][17] The Solar Heat Gain Coefficient (SHGC)[18] (a measure of solar heat transmission) of standard single glazing can be around 78-85%. Solar gain can be reduced by adequate shading from the sun, light-colored roofs, spectrally selective (heat-reflecting) paints and coatings, and various types of insulation for the remainder of the envelope. Specially coated glazing can reduce the SHGC to around 10%. Radiant barriers are highly effective for attic spaces in hot climates.[19] In this application, they are much more effective in hot climates than in cold climates. For downward heat flow, convection is weak and radiation dominates the heat transfer through an air space. Radiant barriers must face an adequate air gap to be effective.
If refrigerated air conditioning is used in a hot and humid climate, then it is particularly important to seal the building envelope. Dehumidifying humid air infiltration can waste significant energy. On the other hand, some building designs rely on effective cross ventilation rather than refrigeration air conditioning to provide convection cooling from prevailing breezes.
In hot and dry climate regions like Egypt and Africa, thermal comfort in summer is the main issue, almost half of the energy consumption in urban areas is exhausted by air conditioning systems to meet people's demand for thermal comfort, many developing countries in hot and dry climate regions suffer from electricity shortage in summer due to the increasing use of cooling machines.[20][21] A new technology called Cool Roof has been introduced to improve this situation.[22] In the past, architects used thermal mass materials to improve thermal comfort, the heavy thermal insulation could cause the time lag effect which could slow down the heat transfer rate during the day and keep the indoor temperature in a certain range (Hot and dry climate regions usually have a large temperature difference between day and night).
Cool roof is a low-cost technology based on solar reflectance and thermal emission, which uses reflective materials and light colors to reflect solar radiation.[21][22] Solar reflectance and thermal emission are two key factors that determine the thermal performance of the roof, and can also improve the effectiveness of thermal insulation since about 30% of solar radiation is reflected back to the sky.[22] The shape of the roof is also taken into consideration, the curved roof can receive less solar energy. compared to conventional shapes.[21][23] Meanwhile, the drawback of this technology is obvious: high reflectivity can cause visual discomfort. On the other hand, the high reflectivity and thermal emission of the roof will increase the heating load of the building.