Both external and internal shades control heat gain. In general, external shades are more effective than internal shades because they block the solar radiation before it enters the building. When using an internal shade, such as blinds or a curtain, the short-wave radiation passes through the glass and hits the shade. Depending on the colour and relectivity of the shade, some percentage will be relected straight back out the window, but the rest will be absorbed by the shade itself, effectively heating it up.
The energy from the hot internal shade is then given off as long-wave radiation, half into the room space and half towards the window. Unfortunately, due to the greenhouse effect, long-wave radiation is trapped between the glass and the internal shade, heating the air within this space.
This heated air will rise, exit at the top and draw in cooler air from below, creating a form of convection cycle that continually draws cool air from the bottom of the space, heats it up and pushes it out into the room.
However, if the right type of internal shade is used in this tropical climate zone, it can outperform external shading devices. To understand this, it is useful to revisit the distribution of direct and diffuse solar radiation in the Malaysian climate.
The Malaysian Test Reference Year solar radiation data (Chapter 2) showed that the average daily diffuse radiation is higher than the direct radiation. Over the entire year, on a horizontal surface, the sum of diffuse radiation is 44% higher than the sum of direct radiation. The total solar
heat gain received by a window is the sum of both direct and diffuse radiation.
On any vertical surface, without any external shading devices, only 50%
of diffuse solar radiation is captured by the window because it is only exposed to half the sky dome. If the same window is added with external shading devices, the percentage of diffuse solar radiation captured by the window is dependent on its view factor of the sky and ground relected diffuse radiation. Due to the fact that typical external shading
devices are designed to prevent heat gain from direct solar radiation while maintaining a good view out of the building, the reduction of diffuse radiation will not be as signiicant.
Moreover, it is fairly easy to design external shading devices that will provide full (or nearly full) protection from direct radiation without affecting the view out of the building.
However, it is almost impossible to design external shading devices to provide full (or nearly full) protection from diffuse radiation without signiicantly affecting the view out.
CHART 6.4 | AvERAGE DAILY RADIATION DATA fOR SUBANG TEST REfERENCE YEAR
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12:00:00AM 3:00:00AM 6:00:00AM 9:00:00AM 12:00:00PM 3:00:00PM 6:00:00PM 9:00:00PM 12:00:00AM
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Building Energy Eiciency Technical Guideline for Passive Design | 123 CHAPTER 6 External & Internal Shades
The SHGC of internal shades are available from some blind/curtain suppliers in Malaysia. However, the SHGC provided by these suppliers are normally applicable when the blind/curtain is combined with single clear glazing. It is very important to note that the SHGC of internal shades is dependent on the type of glazing it is combined with.
One common method to obtain exact SHGC values of internal blinds with the glazing used is through the use of the calorimeter measurement methodology. This method is said to be both time consuming and economically internal shading devices is provided in Table 6.4. A more comprehensive version is found in the ASHRAE handbook.
From Table 6.4, it can be summarised that dark coloured internal shades have higher SHGC values than lighter coloured internal shades, indicating that the solar heat gain of dark blinds/curtains are higher than lighter coloured ones. In addition, the same internal shade that provides a low SHGC internal blind value of 0.25 when it is used in combination with a single clear glazing (SHGC glazing of 0.81) has a different SHGC value of 0.64, when it is used in combination with a Bronze Low-e Double Glazing (SHGC glazing of 0.26). This indicates that the effectiveness of internal blinds is dependent on the type of glazing used.
Users of Table 6.4 should be aware of this and not use the 1st SHGC value found for type the internal shades used.
Again, it is also possible to use the factors found in Table 5.2.1 to estimate the peak cooling load and energy reduction of the fenestration unit based on the total reduction of the SHGC.
6mm Single Glazing SHGC of Draperies, Roller Shades and Insect Screens
ASHRAE ID Description VLT SHGC
glazing LSG Dark Closed Weave
1b Clear 88% 0.81 1.09 0.71 0.46 0.80 0.65 0.73
1d Bronze 54% 0.62 0.87 0.74 0.55 0.82 0.71 0.77
1f Green 76% 0.6 1.27 0.74 0.56 0.83 0.71 0.78
1h Grey 46% 0.59 0.78 0.74 0.56 0.83 0.72 0.78
Low-e Double Glazing, e=0.02 on surface 2 SHGC of Draperies, Roller Shades and Insect Screens
ASHRAE ID Description VLT SHGC
glazing LSG Dark Closed Weave
25b Clear 70% 0.37 1.89 0.89 0.72 0.93 0.82 0.86
25c Bronze 42% 0.26 1.62 0.90 0.76 0.94 0.85 0.88
25d Green 60% 0.31 1.94 0.90 0.76 0.94 0.84 0.88
25f Blue 45% 0.27 1.67 0.90 0.76 0.94 0.84 0.88
6mm Single Glazing SHGC of Roller Shades and Insect Screens
ASHRAE ID Description VLT SHGC
glazing LSG White
Opaque
1b Clear 88% 0.81 1.09 0.35 0.65 0.62 0.72 0.32 0.25
1d Bronze 54% 0.62 0.87 0.47 0.69 0.68 0.76 0.45 0.39
1f Green 76% 0.60 1.27 0.48 0.70 0.68 0.76 0.46 0.40
1h Grey 46% 0.59 0.78 0.49 0.70 0.69 0.76 0.47 0.41
Low-e Double Glazing, e=0.02 on surface 2 SHGC of Roller Shades and Insect Screens
ASHRAE ID Description VLT SHGC
glazing LSG White
Opaque
25b Clear 70% 0.37 1.89 0.66 0.86 0.83 0.89 0.61 0.57
25c Bronze 42% 0.26 1.62 0.71 0.88 0.86 0.90 0.68 0.64
25d Green 60% 0.31 1.94 0.70 0.88 0.85 0.90 0.67 0.62
25f Blue 45% 0.27 1.67 0.72 0.87 0.85 0.90 0.66 0.62
TABLE 6.4 | A SELECTION Of ASHRAE SHGC vALUES Of INTERNAL SHADES3
3 2009 ASHRAE Fundamentals, F15, Fenestration, Table 13.
SHGC OF INTERNAL SHADES
Building Energy Eiciency Technical Guideline for Passive Design | 125 CHAPTER 6 External & Internal Shades
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It is a common expectation that insulating the walls in a tropical climate reduces a signiicant amount of energy consumption in ofice buildings by reducing the peak cooling load in the building. While it is true that the insulation of walls in ofice buildings has a good potential of reducing the peak cooling load, its impact on building energy reduction is less signiicant due to the mild Malaysian climate.
The annual average day and night dry bulb temperature in Malaysia is 26.9°C (refer to Chapter 2) and without any signiicant differences in temperature proiles between June and December. During the night time to the early morning hours, the outdoor air temperature reaches an average low of 24°C.
Meanwhile, in modern ofice buildings today, there exists a signiicant amount of ofice equipment that are still consuming energy even after ofice hours. Even after a computer operating system is shut down but not switched off at the power point, it may still be consuming 5 to 15 watts of electricity in older computers or 1 to 2 watts in newer energy-rated computers. This energy consumption will become heat in the room. In addition,
there is equipment in ofice spaces that are usually not switched off at all, such as printers, fax machines, hot and cold water dispensers, refrigerators, some of the common-area lighting, security systems, etc. This night time parasitic energy consumption will produce heat within the building itself, causing the temperature inside the building to be warmer than the outdoor temperature during the night time and early morning hours. In a building without insulation, the heat built up during night time will be conducted out due to the temperature differences. However, in a building with insulation, this heat will be trapped inside the building. When the air-conditioning system is switched on again in the morning, this heat is removed at a cost to the energy consumption of the building.
This chapter provides a guideline on the optimum insulation to be provided for a typical ofice building in the Malaysian climate. In addition, it also provides an indication of the cooling energy required in a building where the equipment load is not well controlled during the night time as compared to the cooling energy reduction in a building where the equipment load is well controlled.