An observation was made by Ludwig (1970) whose Figure illustrates solar radiation for different H/W ratios (Figure 4.7). On the flat plan, most of the absorbed solar radiation is re-radiated to the sky as long-wave radiation. In a medium density of H/W= 1, most of the reflected solar radiation hits other buildings, as well as the ground, until it is absorbed near or at the ground. For higher densities of H/W= 4, most of the absorption takes place at a high level of the canyon, reducing the amount of radiation that reaches the ground.
Ch 4: Thermal Comfort and Buildings’ Thermal Performance
Fatima M Elaiab 73
Dealing with solar radiation: There are a range of techniques can be used to
prevent solar radiation from reaching building surfaces:
Shading Devices: devices such as wide roof overhangs, shading fins, thick vegetation or external shutters can be used to protect windows and wall surfaces. It is also often possible to shape the building such that some parts of it are self-shading.
Surface Colouring: if opaque building elements exposed to solar radiation are painted white or very light colours, much of the incident radiation can be reflected away from the surface.
Insulation: any surface that is exposed to high levels of solar radiation in summer should be well insulated to reduce the transfer of heat. The best location for this insulation is on the outside surface; however this may not always be practical. In climates with a high diurnal range in summer (hot days and cold nights), it may be preferable to store daytime heat for release later at night when the temperature falls. In this case, exposed surfaces should comprise a thick layer of heavyweight material with a high thermal capacitance and a thermal lag of around 8-10 hours.
Double Roof Systems: a double roof system, as shown in Figure 4.8, uses a ventilated air gap between an upper exposed roof and a lower protected roof. Much of the solar gain from the upper leaf is carried away by the air before it can pass to the lower leaf.
Figure 4.8: diagram illustrated a double roof system with an air gap between the main roof and the shaded roof
A study by Behzad (1991) investigating the effect of roof shading on the ceiling surface temperatures, in this study two types of roofs were modelled; un-insulated, and insulated having 50mm of insulation material in the form of the "warm roof" type and both roofs were with same colour and have a solar absorptivity of 0.5. External surface temperatures of roofs on the 21st of July when there is no obstruction to intercept direct solar radiation together with those when they are shaded were calculated. The study found that shading the roof can significantly reduce the surface temperature of the asphalt. For example, the maximum surface
Ch 4: Thermal Comfort and Buildings’ Thermal Performance
Fatima M Elaiab 74 temperature of the asphalt in a well shaded (90% shaded) un-insulated roof is 11.6°C lower than the corresponding value when it is not shaded. The corresponding difference for the insulated roof is 13.2°C as shown in Figure 4.9 (a). While the calculated cooling requirements of the reference house during a three days simulation period, from 20th to 21st July, are depicted in part (b) of Figure 4.9. The cooling requirements of the well shaded (90% shaded) uninsulated roof is practically the same as that of the insulated but unshaded roof.
Thermal Mass: this too applies only Behzad study in such climates with a high diurnal range (Iran climate). In this case, lots of thermal mass is used in the interior of the building to even out temperature fluctuations. To be effective when used in this way, the mass must be exposed internally and not covered over with carpets, cupboards or panelling.
There is some design flexibility here as, depending on the exact conditions at night, some delayed conduction gain may be desirable or not.
If so, the form of the building can be designed such that exposure is limited to certain surfaces at specific times to control the collection and release of conducted heat. If not, then shading, surface colour and insulation should be used on the outside surfaces to reject solar gains. It may also be desirable to use a night-purge ventilation system to cool the mass down at the end of each day (see the night-purge ventilation section).
Figure 4.9: a) the effect of roof shading on the roof surface
Temperatures .b) the effect of roof shading on the ceiling surface temperatures (21st of July) Source: behzad sodagar, 1991
Solar Control Glass: if external window shading is not practical, highly reflective glass can be used. This will not be as effective and may even be against local planning regulations in some places where bright solar reflections are a hazard. Heat absorbing glasses and internal blinds/curtains
Ch 4: Thermal Comfort and Buildings’ Thermal Performance
Fatima M Elaiab 75 are the least effective option as they allow the radiation to enter the space (either directly or as long-wave radiation from the heated glass) before shading the occupant and if user control is necessary, use blinds encased in a double glazed unit where excess heat gains are vented to the outside (Figure 4.10).
Figure 4.10: the effects on internal heat flow of external versus internal shades Source: www.wiki.naturalfrequency.com/wiki/solar_control