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close this bookDesign Handbook on Passive Solar Heating and Natural Cooling (HABITAT, 1990, 162 p.)
close this folderVII. Detail design
View the documentA. General
View the documentB. Solar access, shading and window protection
View the documentC. Control of conductive heat flow
View the documentD. Evaluation of internal heat loads
View the documentE. Cross-ventilation and air flow
View the documentF. Glass-mass relationship
View the documentG. Air infiltration

B. Solar access, shading and window protection

This section deals with two aspects of detail design. Access to solar radiation and protection of buildings and their interiors from solar radiation. They have been grouped together because they deal primarily with the geometry of the sun's movement.

1. Solar access

There are two fairly well documented ways of protecting solar access: solar envelopes and shadow masks used in conjunction with a solar access butterfly.

The solar envelope concept has been studied extensively by Knowles at the University of Southern California. He defines it as "the volumetric limits of building that will not shadow surroundings at specified times. (usually between 9 a.m. and 3 p.m.)

The aim of the envelope is to protect solar access for the future.


Figure 71. Solar envelopes

Ralph Knowles' envelope is designed for whole-site solar access, which involves increased allotment size and therefore decreased density. which has development cost implications.

All solar access protection techniques require some trade-off between density and solar access. The Knowles' envelope is generated using the altitude angles of the sun, at nominated times. to establish imaginary planes sloping inwards from their base on the boundaries of the site. as illustrated in figure 72.


Figure 72. Knowles' envelope

Differing levels of solar access can be achieved by raising or lowering the height from which sloping planes are generated (the base plan height).

2. Shadow masks and the solar access butterfly

In order to consider topography and house orientation simultaneously. shadow masks and the access butterfly can be used. because the solar envelope is a form established in space and as such is difficult to visualize and to use in large-scale planning. The solar envelope is more appropriate in existing built-up situations.

The shadow mask principle takes into account factors such as gradient and orientation of slope and building orientation more easily than the solar envelope principle. The solar access butterfly is a method which enables vegetation to be located to the north of a house without shading the north wall in winter.

Using the shadow mask and butterfly principles, large housing estates can be designed in plan to ensure solar access to each dwelling. It is established for each dwelling for the position in which it is to be located. The composite mask that is formed is that of the shadow cast by a given building with a given orientation on a known slope and slope orientation. The shadow is a composite of the shadows cast at 9 a.m.. 12 noon and 3 p.m. at the winter solstice (22 June). It is during this time period that overshadowing of the north wall is unfavourable. If no overshadowing occurs at this time of the year then active systems, such as rooftop hot-water heaters, would not be shaded at any time of the year.

The composite shadow mask is established by:

(a) Presenting the building. tree or other object as a series of poles;
(b) Finding the shadow length and direction cast by those poles;
(c) Connecting the pole shadows into a composite shadow mask.


Figure 73. Presenting the object as a series of poles

(a) Presenting the object as a series of poles

The pole height is the distance between the gutterline, ridgeline or top of the object. and natural ground level. and can be measured from the elevation drawings of the building.


Figure 74 Measuring pole height from elevation

(b) Finding the shadow length and direction cast by each pole

This is done using table 10 to get shadow length factors established for the winter solstice. It gives the shadow length of a 1 unit length pole for various slopes of different gradients and orientations. The a.m. and p.m. values give the shadow length for approximately 9 a.m. and 3 p.m. and correspond to the 45 degree azimuth for Sydney (34 degrees South). (The 45 degree azimuth is adopted for ease of application and means that the a.m. shadow falls on a line 45 degrees west of north and the p.m. shadow falls on a line 45 degrees east of north) as shown in figure 75.


Figure 75. Shadows falling on a 45 degree line


Table 10. Midwinter shadow length factors for Sydney (Sydney altitude = 17°) (Azimuth = 45 ° (approximately 9 a.m. and 3 p.m.))


Figure 76. Shadow lengths

The length of shadows for a given slope are multiplied by the height of the pole to give the length of the shadow for that pole on that slope (see figure 76).

(c) Connecting the pole shadows into a composite shadow mask

The shadows for each pole are connected to give the composite shadow mask for the building, as shown figure 77.


Figure 77. Connection of pole shadows Into a composite mask

Where a large number of houses are being planned, a computer program has been developed which will draw the composite shadow mask for a given building of any orientation on any slope, to a scale specified.

3. The shadow mask program

A complete listing of the program to run on a HP-85 desk top computer is contained in appendix 2 of Energy Efficient Site Planning Handbook by Kay, Ballinger, Hora and Harris. The program develops the shadow mask for a house of given dimensions on a slope of known gradient and orientation.

The shadow mask shows the maximum shadow that a building will cast and the limits of this shadow determine how close buildings may be placed together on any given slope.

The solar access butterfly can be used once the buildings have been located to determine where vegetation or other structures may be placed north of the house where shading may or may not be desirable. For example, shading of the north wall (bun not the rooftop hotwater collector) may be desirable in summer. Therefore deciduous trees may be placed at appropriate distances from the north wall according to their height. Similarly, evergreen trees must be placed to prevent shading in winter. This is the reverse of the shadow mask principle. The shadow mask and butterfly may overlap.

The butterfly specifies zones where obstructions may not be higher than a nominated height for that distance from the north wall.


Figure 78. Computer drawn shadow mask


Figure 79. Solar access butterfly-section

4. Solar access butterfly

The solar access butterfly saves having to draw a shadow mask for each tree or other object located to the north of the house. The butterfly is made up of a series of height restriction lines at specified distances from the north wall of the house, depending on topography. For example, no object 3-metres high can be placed closer to the north wall than the 3 metre height line (unless it is a deciduous tree). The east and west extremities of the butterfly fall along the 9 a.m. and 3 p.m. winter solar azimuth angles drawn from the north fae of the house.

For example, in figures 79 and 80 all objects 3-m high can be placed on the 3-m height line, those of 5-m high on the 5-m height line etc. These height lines can be established knowing the shadow-length of that object on the relevant slope using the shadow length factor table (see table 10). The shadow length is based on the altitude angle of the sun at different azimuths. Deciduous trees can be placed closer to the north facade than the height line indicates if the branches are not too dense to cause shading in winter. Care must be taken, however, to prevent shading of the rooftop collector by the deciduous trees in summer.

The shadow mask principle can also be used to determine how trees shade solar collectors. The shadow of the tree can be drawn using the bottom of the solar collector as the base height. Slope and orientation are still considered.


Figure 80. Shading of solar collectors

5. Site planning for solar access

Protecting the solar access of houses has some important implications for overall site design.

In order to achieve good orientation of passive solar houses. house design needs to relate to the block on which It is located. Broadly speaking. there are four types of house blocks which require different house types.

(a) Entry from north side (N)


Figure 81. North side entry

In this house type:

(a) Living areas and mayor bedrooms face the street;
(b) North-facing outdoor space will need to be screened from the street for privacy.

(b) Entry on south side (S)


Figure 82. South side entry

In this house type:

(a) Living areas and major bedrooms face the private yard;
(b) North-facing yard is screened from the street by the house:
(c) Service areas (kitchen. bathrooms) face the street;
(d) Living area is extended to the street for public entry.
(e) Entry on east side (E)


Figure 83. East side entry

In this house type:

(a) Side of the house faces the street:

(b) North-facing outdoor space will need to be screened to the side and north of the house. This also creates a sense of entry:

(c) Lots may need to be wider to prevent shading from a building to the north (this depends on slope).

(d) Entry on west side


Figure 84. West side entry

In this house type conditions are the same as for east-side houses.

(e) Entry from other directions

For other blocks. where the road runs. for example. north-east southwest, one of the above house types can be used.

In most situations, existing setback regulations can be adapted to meet the solar access criteria. A standard setback from the street (building line) is the easiest means of protecting solar access. provided the slope is constant. This, however. creates a monotonous streetscape-and does not allow houses to be sited on the lot according to how outdoor spaces are to be used.

For a detailed discussion of this area reference should be made to Energy Efficient Site Planning Handbook.

Trees, shrubs and other plants can have a beneficial effect on the microclimate as well as on the energy requirements of a house. Trees can block unfavourable winter winds and hence reduce heat loss, and funnel cooling breezes and hence reduce the cooling energy load. They also block unwanted summer solar radiation and improve the microclimate. Deciduous trees and vines can be used to allow winter sun penetration and block summer sun. Methods of landscaping for energy efficiency are detailed in the guidelines.

6. Shade, shading devices and window treatments

In general terms the purpose of shading is to protect the building and its occupants from the heating effect of solar radiation and in specific cases from visual glare. Solar radiation absorbed by the building fabric will cause a rise in temperature which may be undesirable when the air temperature is within or above the accepted comfort zone. In such circumstances direct sun on the occupants (radiant energy) may also cause discomfort or at least contribute to it. Direct sunlight reflected off surfaces in the field of view will probably cause glare discomfort. The function of shading then is to control or eliminate these conditions whilst the function of windows is to allow air flow through openings, maintaining views and admitting adequate daylight.

Shading can be categorized as follows:

(a) External projections, i.e., overhangs and projecting blade walls;

(b) Systems integral with the window frame or attached to the building face, i.e., Louvre and screens;

(c) Specially treated glasses, i.e heat absorbing and reflective glass;

(d) Internal treatments either opaque or semi-opaque, i.e., curtains and blinds.

The first two can be addressed generally as geometric shading.

7. Geometric shading devices

Shading can be used on buildings to protect either the windows or the walls or both from solar radiation. The aesthetic value of shading systems and the shadows which result is also important in the overall building design as it allows the designer to define volumes and model the building surface. The dynamic qualities of shading provides the opportunity for the designer to present an ever changing image.


Figure 85. Examples of geometric shading forms

For the convenience of detail design, external shading for buildings can be classified as being vertical, horizontal or a combination of the two. The range of possible designs is limited only by imagination and appropriate materials.


Figure 86. Horizontal and vertical shadow angles

The colour and heat storage capacity such devices will also contribute to the control of the environment adjacent to the windows. In theory shading devices of heavy materials will store heat during the day and help maintain a warmer environment near the windows when the air temperature drops. Whilst this may be desirable in cool winter areas it may not be desirable in hot arid areas. Light-coloured surfaces may cause more radiant energy to be reflected into the openings; with this will come daylight and perhaps unwanted glare.

The calculation of the cut-off lines for any shading device is important in the detailed design and so one must first determine the horizontal shadow angle (HSA) and the vertical shadow angle (VSA). This information is available from many sources including available computer programs. Using the shadow mask provided with most sun position charts it is a simple matter to read off the appropriate values.

In most locations in Australia, except for the cool-temperate areas, it is desirable to be able to exclude summer sun at all times. During the cooler periods of summer a little early morning sun may be acceptable and to some people even desirable. Consider carefully the use of the building being designed and especially the spaces associated with the shading under consideration. For example, some early morning sun may be desirable in bedrooms and bathrooms provided it is excluded for the rest of the day. Such sun in hot weather would be most undesirable in living areas which will be used late into the evening.

Buildings facing true north are easiest to shade as the sun path across them is symmetrical and seasonally manageable with simple horizontal projections. Such projections do not generally hamper views and the psychological connection with the outside. If the projection distance is correct there is effective blocking of sun in summer and minimum obstruction in winter.

The further the fae faces towards the east or the west, the more difficult it will be to provide fixed seasonally-effective shading. The workable solutions require a combination of horizontal and vertical elements. If the fae faces north-east then, unlike a north-facing facade, it is extremely difficult to obtain an optimum solution with a fixed shading system.


Figure 87. Vertical blade shading for east or west facades

The east-facing facade requires a vertical style of shade to block low altitude radiation. The sun around the middle of the day is high in the sky and so easier to block. Unfortunately appropriate vertical shading for east- and west-facing fact aces tends to defeat the purpose of the window as the view out is disrupted and restricted by such devices. Adjustable devices are therefore more attractive except that maintenance becomes an important issue.


Figure 88. Two types of sun charts available for use in Sydney

Suitable adjustable shading devices for non-north-facing shading devices include canvas or aluminium lath roll-down awnings, retractable timber or metal external-quality venetian blinds, and metal roller-shutter type units especially designed for external window use. Where fixed awnings are acceptable the most satisfactory solution is to use vertical blades turned towards the north and capped at the top as shown in figure 87.

Selected solar charts for use in Australia are provided in the annexes at the end of this book. More extensive information is provided by Phillips and also by Hassall.

In some situations where it is appropriate, a suitably designed shading system can also double as a security screen, thus helping to offset the expenditure against other purposes. The commercially available roller-shutter units fulfil this role as would a suitably designed louvre system.

When designing or selecting a shading system, it is important to consider the sources of energy to be blocked and the projected purpose of the rooms or internal spaces being shaded. As discussed in an earlier section, the sun's energy reaches the window mostly by direct radiation which is easily blocked. In addition to this there may also be heat and glare (from visible wavelength energy) from solar energy reflected off surrounding surfaces including the ground outside the window, especially if it is light in colour. Where the windows being treated face in a northerly direction. it is relatively easy to shield the ground and the windows by extending the horizontal overhang in some form. This extension should be designed to be removed or otherwise made ineffective in winter (deciduous vegetation, adjustable louvres or removable fabric would achieve this goal).

The studies conducted by researchers in Europe where sun intensities are not as high as in Australia indicate that even grass surfaces in direct sunlight all day reach as much as 10C higher than shade air temperature. If the surfaces were paved with either concrete or bitumen then they could easily rise to 20C above shade air temperature.

The fixed shade element should be large enough to provide the protection needed to surfaces below whilst allowing maximum penetration of the sun in midwinter. The choice of materials and the exact dimensions must be appropriate to the building design. Table 11 gives some suitable permanent and summer-only projection factors for north-facing facades. Such values can be applied without modification to faes facing up to 15 either side of true north. They can be applied to facades a little outside this range where there are tall external obstructions to the east or west as appropriate, such as trees or neighbouring buildings. Facades outside this range should be dealt with individually by calculating the shading effect of the treatment proposed. As the larger shade elements covering external surfaces will also provide more suitable outdoor living spaces. it is important to consider whether such shade protection should also give rain protection to the covered outdoor space.

8. Climate-specific recommendations (a) Cool-temperate

Shading must be designed to allow maximum winter sun penetration and partial sun penetration at other times except during hot summer spells. To achieve this there must be flexibility in the design. as certain times of the year both options may be required depending on the specific weather conditions at the time. As a guide the shading should totally exclude the sun in the warm to hot months (beginning of October to mid-March). The overheated days outside that time will have a reduced effect on the interior as the shading is still partially effective.


Table 11. Values of solar heal transmission (infra-red) and visible light for various glass types

(b) Hot-arid

Shading is more clear-cut in this climate zone. It is vital to maintain maximum shade from late spring through summer to early autumn. As the winter Is usually colder than In the cooltemperate areas (due to longer periods of clear skies and cold driving winds) it is most desirable to maximize winter sun penetration. The use of removable shade extensions is possibly more appropriate as the change from one season to another is more clear-cut. In this climate zone it is beneficial to shade the walls of the building, especially where the outer skin has a heat-storage capacity. such as brick or concrete (external walls must be insulated).


Figure 89. A breakdown of how solar energy passes through 6-mm heat-absorbing glass.


Figure 90. A breakdown of how solar energy passes through 6-mm heat- reflecting glass.

(c) Warm-humid

This climate zone is characterized by wet warm periods with extremes of humidity levels and the lack of a cold season. Extensive shading to exclude the sun all year round without impediment to ventilation and air movement is most important. As these areas tend to be in the low latitudes the sun's path is usually high (refer to sun charts for latitudes below 20) and so the shading of the east and west faes requires the most attention. Extensively shaded areas can also be beneficial as outdoor living space in these zones.

9. Design of glass and window systems (a) Treated glass

An alternative to external shading is the use of tinted or reflective glass that reduces the amount of solar energy transmitted. The use of such an approach to window design is that the available daylight is also reduced. In cases where winter sun penetration is desirable, such glass is not appropriate. However on east- and west-facing facades, treated glass may be the best solution. Table 11 shows the transmission characteristics of a selected range of glass available in Australia.

The technical aspects of the transmission of solar energy through glazing has been described earlier in this book. Each glass type can therefore be described by its shading coefficient, but the difference between the transmitted solar heat and the transmitted visible energy (daylight) should be noted: both must be considered.

If the use of tinted or reflective glass is planned then the day-to-day use of the enclosed spaces must be considered. The quality of light is altered by such glass to the extent that interiors tend to be dull and lacking in daylight at all times. In winter and on dull days this may be considered undesirable in terms of user expectations. Whilst such glazing may reduce glare on bright days, it takes away much of the wonderful dynamic quality of daylight that is available to the designer for modelling interiors. The designer must use such glazing materials with great care. It should be noted that building codes in most parts of Australia limit the visible reflectance of such glass to approximately 20 per cent.

The treated glasses available can be grouped as follows:

(a) Heat-absorbing glass (usually appears tinted with colour throughout) comprising interference particles within the glass. Solar energy is intercepted and absorbed by the glass causing the temperature of the glass to rise. This effect can also be achieved with a range of tinted films or coatings applied to the glass surface (often used to reduce radiant energy transmission in motor car windows). The heat absorbed by the glass is reradiated to both the interior and exterior. This often causes discomfort to nearby occupants in hot climates as they sense a hot surface radiating heat toward them. The relative effectiveness can be seen in a comparison of the absorbed component compared with the reflected component. Figure 89 illustrates the distribution of energy as defined by ASHRAE for a typical sample of heat-absorbing glass.

Table 12. Thermal of various glazing types

Glazing


Heat transmittance U-value(W/m² .degC)

Solar heat gain coefficient (SHG)

Solar shading coefficient (SC)

Single 3-mm glass

e = 0.87.

5.9

0.88

1

Double-glazing
12's-mm air space

e = 0.84

3.2

0.79

0.90

Double-glazing
12.5-mm air space

e = 0.2

2.2

0.76

0.86

Low emittance

e = 0.4

2.6

0.76

0.86

Single-glazing(t = 0.77)

e = 0.6

2.8

0.77

0.87

Low emittance

e = 0.1

3.1

0.79

0.90

Single tinted glass

e = 0.2

3.5

0 79

0 90

Visible transmission 0.2

5.9

0.39

0.45


Single reflective glass

e = 0.6

4.8

0.33

0.37

Visible transmission 0.2

e = 0.2

3.5

0.24

0.27

(b) Heat-reflecting glass has a mirror like appearance due to an interference coating of finely divided metal particles deposited on one surface (usually the surface to the inside to reduce abrasion of the coating). Solar energy-striking this glass is mainly reflected rather than absorbed. It is widely used in air- conditioned commercial buildings to reduce solar gain. Unfortunately as with heat-absorbing glass it reduces transmitted daylight also. It is not appropriate in certain applications where the building occupants expect to see out at night (i.e., hotels and executive rental space in the top floors of high-rise commercial buildings). Whilst it appears reflective from the outside in the daytime, it reverses at night giving the outside observer an excellent view of the interior and the occupant a mirror-like window.

(c) Low-emissivity glass (low-E glass) has been developed in recent years to reduce the transmittance of heat energy (infra- red) whilst maintaining the radiant transmission of solar energy. Such glass is normally used in a double- or triple-glazed situation where a low emissivity coating is located on one surface inside the unit to reduce the radiant transmission across the air space from the warmer glass layer to the colder layer. This glass can be used to reduce either heat loss from inside a building or heat gain from outside (in hot climates).

(d) Electrochromic glass. New coatings are being developed that allow the building user to vary the transmission characteristics in much the same way as some people have spectacles/hat darken in direct sunlight (they are known as photochromic). The electrochromic glasses are controlled by the application of an electrical charge across the coating thus making them opaque to selected energy wavelengths. Such products are already at the laboratory stage and should be available for use in buildings in the next few years.

(b) Combination window systems

Multi-layered glazing systems with adjustable shading between the glass layers have been in use for many years. They have been widely used in east- and west-facing glazing of airconditioned commercial buildings. Whilst such systems work, they are dependent on the occupant or the floor maintenance staff for the day-to-day adjustment. If the occupants on the east side of a building forget to close their shading system before they go home in the evening, the air-conditioning system has to cope with the additional load of the early morning sun. This is a serious problem in many commercial buildings where the airconditioning system has been designed on the assumption that the shades will be in place. It is also generally found that maintenance is high on such shading systems with moving parts.

A wide range of shading devices have been tried inside window units from conventional venetian blinds to roll-down reflective films etc., either manually controlled by the occupant or automatically controlled by a computer system. In a study of the latter case in a city building it was found that there would have to be 80 electric motors per floor to drive the shading system, requiring a full-time maintenance person just to keep the motors running.

10. Interior window treatments

Interior window treatments are generally the least effective in terms of rejecting solar energy. Once the sun's heat has penetrated the glass, it is difficult to ensure that enough is reflected back out as approximately 13 per cent of the energy transmitted through conventional window glass is absorbed by the glass. Even if 100 per cent is reflected off the interior blind or curtain then a further 11 per cent is absorbed into the glass on the return journey out. Usually the best an interior linings can achieve in terms of reflectance is less than 90 per cent. i.e., more than 32 per cent is then trapped. A good exterior shade can stop 80-90 per cent and still provide a view out.


Interior light-coloured blind. More diffuse light enters the room and more heat Is reflected outside (say 70 per cent compared with 5 per cent for the dark blind). It is only half as good at stopping heat getting into the room compared with the dark blind. with a shading coefficient of 0.4


Interior dark blind. This stops the sun directly striking the Interior of the space. Being dark it absorbs heat and only reflects 5 per cent of the incoming heat to the outside. The absorbed heat still gets into the room via convection and radiation. The room will be dark and there is very little diffuse light. The shading coefficient is about 0.8


Aluminized light coloured blind. The surface looks dark (90 per cent of external light is reflected outside) but it is reflective to Internal artificial light.

Figure 91. Thermal transmittance for some commonly-used interior window treatment.

Where such treatments are required to exclude solar radiation they will also restrict ventilation. This may be undesirable. Any energy not reflected off the fabric will be absorbed, causing the surface temperature to rise and radiate energy into the room.

Interior window treatments can vary from the simple holland roller blind to a most sophisticated semi-transparent material with metallized reflective backings or curtains with padded linings to reduce conductive heat flow. The range of fabrics available with alumined coatings are often highly reflective on the window side. The fabric is perforated or loose woven to provide a fine mesh of holes for vision out. The total area of holes can be as little as 10 per cent if they are very small and sufficiently close together to give excellent vision. The costs are as varied as the types and styles. Table 13 illustrates the shading coefficients and figure 91 gives the thermal transmittances for a range of commonlyused interior window treatments.

The hot air trapped between the glass and the fabric will be transported to the room behind by convection unless adequate provisions are made to restrict such flow, as discussed later.

11. Shading coefficients

To evaluate the effectiveness of a particular approach to shading or to compare a range of solutions, it is necessary to have a common standard. One such measure is the property known as the shading coefficient.

Where the reduction of solar gain is desirable in summer but not in winter, some type of adjustable or removable shading should be chosen for north glazing. Where east or west windows are unavoidable then reflective glass may be the most suitable choice to reduce summer solar heat gain. The shading coefficient concept can be used to rate various types of window-shading techniques, the values being used in various thermal evaluation methods or simply to compare the worth of different window treatments in regard to reduction of solar gain.

Table 13. Typical values for coefficients of some window systems

Window system

Shading coefficient
(SC)

3-mm clear sheet glass unshaded

1.00

3-mm clear sheet glass with the following:


Inside dark roller shade completely drawn

0.80

Inside dark venentian blind fully drawn

0.75

Inside medium venentian blind fully drawn

0.65

Inside medium roller shade full drawn

0.62

Dark-coloured drapes fully drawn

0.58

Averooe tree casting shade

0.60-0.50

Inside white Venetian blind fully drawn

0.56

Inside white roller shade fully drawn

0.41

Light-coloured drapes fully drawn

0.40

Outside vertical fixed fins on east/west

0.31

Outside canvas awning

0.25

Overhang. continuous on north side

0.25

Dense tree casting shade

0.25-0.20

Outside venetian blind

0.15

Outside movable horizontal or vertical louvres

0.15-0.10

Heavy drapes with white linings and folds

0.35

Note: For reflective film on glass chechk the relevant data available from the manufacturer and calculate SC for particular reflective film selected.

Values of unfed and reflective glasses available are given in table 11.