The building envelope must respond to a range of climatic conditions including extremes of temperatures, solar radiation, precipitation and wind. The envelope may be required to act as a buffer, filter or concentrator at different times. Appropriate selection and manipulation of surface materials can control thermal and solar transmission.
Design strategies for the building envelope include:
The balance between heating, cooling and day-lighting is a critical consideration for the choice of orientation and size of opening. Building type and Building Regulations also influence this choice. However, the use of additional devices such as overhangs, shutters, blinds and louvers allow some scope to correct or limit the unfavourable orientations for large glazed areas. The sizing of north facing openings is less affected by seasonal variations and may be determined largely by day lighting and cross ventilation requirements. North facing openings can provide an almost uniform daylight source. Effective cross ventilation typically requires openings distributed across opposing facades, with minimal internal barriers to impede airflow. The proper treatment of south and west facing windows is therefore very important to prevent unnecessary heat gain. For single sided ventilation the shape of opening becomes important, horizontal formats being more economical in simulating internal air velocities. The design of openings should be undertaken in conjunction with the overall solar strategy.
The building envelope design strategy must encompass winter and summer conditions so that, for example, excessive solar heat gain can be avoided in summer while adequate daylight is available throughout the year, thus avoiding the need for artificial lighting during the day, consequently reducing cooling loads.
Blocking the solar radiation from reaching the building, particularly the glazed, but also the other opaque surfaces (including the roof) and reflecting the solar radiation is fundamental to the prevention of heat gain. While shading systems must provide good solar protection in summer, they should not reduce solar gain in winter, impede natural lighting or obstruct cross ventilation. Well-designed shading systems can actually enhance natural day lighting and ventilation. Shading systems can be either fixed or movable and placed internally, externally or between double glazed panels. Vegetation can also be used to provide shading.
The type of glazing used can also affect the solar heat gain of the building. Glazing may be either clear or may have special coatings or treatments to enhance itís reflective or heat absorbing properties. Electrochromic glass allows the radiation transmission properties to be altered by varying an electric current that is passed through the glass panel. Other new types of high performance glass called low-e glass are also now available which have low emission values compared to normal glass. The use of sun films can also reduce the penetration of solar radiation.
Fixed shading systems include structural elements such as balconies and projecting fins or shelves and non-structural elements such as canopies, blinds, louvers and screens. The orientation and shape of the opening to be shaded, relative to the position of the sun at different times of the day and year is critical to the design of fixed shading systems. Each orientation will need to be examined separately, taking account of direct and diffuse and reflected components of solar radiation throughout the day and year. Typically horizontal shading is used for south facades while vertical fins or louvers are more efficient for east and west facades. Fixed shading systems are generally used externally as when used internally heat build-up between the system and glazing can reduce the effectiveness of the system by as much as 30%.
Movable shading is use either internally or externally. Control can be either manual or power assisted and may be automated to respond to changing conditions such as current radiation levels and daylighting or thermal requirements.
Awnings can reduce heat gain by up to 65% in summer on south facades and up to 80% on east or west facades. The geometry of awnings is similar to that of horizontal overhangs but efficiency will also depend on how opaque the material is to both direct and diffused radiation and the presence of dust which might change the absorption and radiation characteristics of the awning. Normally, an air gap should be provided between the awning and glazing for air circulation. The efficiency of awnings may also deteriorate with age and weather damage.
Venetian blinds can permit simultaneous ventilation and shading which is controllable and may allow daylight to be reflected, to the ceiling, for example. With the exception of reflective blinds, curtains and blinds fitted internally are less satisfactory as they provide shade only after radiation has passed through the glazing. The use of curtains and internal blinds may often conflict with the daylighting or ventilation needs.
Vegetation can be used effectively for shading of the building. A major advantage of natural shading using vegetation is that plants constantly rearrange and reposition their leaves for maximum solar exposure and therefore maximise shading, while artificial shading is generally inflexible. A curtain of vines or creepers in the external walls will reduce heat penetration and help maintain cooler temperatures within the rooms. Shady trees will control the light and heat reflected off the roads and pavements onto the walls and roof of the structure if it is within the shadow range. Terrace gardens can further reduce heat transmission through the roof. As the roof is responsible for 50% of the heat load this can achieve temperature drops of 3 degrees C to 5 degrees C.
Thermal insulation may combine two physical processes; reducing the thermal transmittance of the envelope and maximising long wave radiation. Usually, only the first is taken into consideration, but both these processes can be incorporated in the concept of radiant barriers. The development of higher quality foil products and research showing the most efficient way to install these, have resulted in major energy savings in hot regions, for example, a low emissivity material like aluminium foil, next to an air gap will impede radiation, thus reducing the temperature of the inner layer and also radiant room temperature. At night, the foil blocks radiant heat exchange, reducing night cooling. When properly installed, radiant barriers can reduce cooling loads by as much as 10%.
Air infiltration represents a major cooling load in hot climates. By sealing areas or introducing air infiltration barriers where different building materials join together, summer heat gain and excessive use of mechanical equipment for cooling can be dramatically reduced.