The design of shading devices can be quite complex. Whilst a wide range of computer-based methods to accurately shape shades for very specific purposes are introduced here, in their absence - and with a little understanding of the mechanics of solar position and the sun-path diagram - there are also manual methods that can be used.
Utilising Seasonal Variations
As you will know from your own personal experience, the most important characteristic of solar position is its seasonal variation. At the height of summer the sun rises much earlier and and sets much later and in completely different positions than in winter. Not only is it visible in the sky much longer, but has a much higher average altitude.
The aim of good shading design is to utilise these characteristic to maximum advantage - typically to exclude as much solar radiation as possible in Summer whilst letting as much through as possible during winter.
Some Preliminary Rules of Thumb
The design of any shading device, particularly horizontal shades, depends greatly on the exact path of the Sun through the sky. Thus, shading device strategies need to be tailored to the orientation of each window. Whilst some orientations are easy to shade, others are much more difficult as the Sun can be almost direct-on at certain times of the day. For a more detailed discussion of this, see the Shadow Angles topic.
The table below indicates the appropriate types of shading device for use on each orientation of a building. These are intended as guidelines only as there are many variations to these basic shading types and some significant room for innovation in the detailed design of shading.
|Equator-facing||Fixed Horizontal Device|
|East||Vertical Device/Louvres (moveable)|
|West||Vertical Device/Louvres (moveable)|
The design requirements for a shading device depend entirely on a building's use and local climatic conditions. In a multi-storey open-plan office building in a relatively warm climate, the occupancy and equipment gains may mean that heating is rarely required. In this situation, to avoid unnecessary loads, shading may be designed to completely protect windows all year-round.
In a domestic building or one that is occupied 24 hours, the release of stored heat during cold nights in winter may be important. In this case, the shading might be designed to fully protect windows during the summer months, but to expose them as much as possible to direct sun in winter so that the spaces within have a chance to absorb heat during the day. In climates where summers are also relatively cold, the requirement may even be to allow full solar access all year-round.
The requirement for solar protection and/or solar gain will be determined by the climate you are designing the building for. In very cold or very warm climates, shading decisions are really quite easy - you either want solar penetration or you don't. In moderate mid-latitude climates however, there will be times when solar radiation is of significant benefit as a heat source and times when it could be a problematic due to glare and overheating.
The first key design issue in these situations is therefore to determine the transition dates when temperatures go from being sufficiently cold to warrant supplementary heating to sufficiently warm to require protection, and vice versa. Also important are the relative heating and cooling stresses within the environment. If it gets really cold and dark in Winter, it may be that the amount of solar heating available is so small compared to the potential losses from having exposed glazed areas that full insulating shutters are used, with the windows opened up in summer to let in whatever sunshine is available.
Unfortunately each climate will be different so there really is no simple way of determining these solar radiation cut-off dates without a detailed analysis of the actual weather data for the site. To assist with this, you can use the WeatherTool and weather data available from Square One.
As a preliminary step, you may wish to use a manual method to calculate the size of shading devices you will likely need based on Shadow Angles. To design a horizontal shading device this way, use the following basic steps:
- Determine cut-off date:
This is the date before which the window is to be completely shaded and after which the window will be only partially shaded.
- Determine Start and End Times:
These represent the times of day between which full shading is required. Keep in mind that the closer to sunrise and sunset these times are, the exponentially larger the required shade.
- Look up Sun Position:
Use solar tables or a sun-path diagram to obtain the azimuth and altitude of the sun at each time on the cut-off date.
- Calculate the Shadow Angles:
Using the methods detailed in the Shadow Angles topic, calculate the HSA and VSA at each time.
- Calculate Required Depth and Width:
Once again, using the Shadow Angle methods, calculate the depth and width of the required shade on each side of the window.
The Effects of Shading
Being based on specific dates and times, the above methods provide a precisely shaped shade that will provide full protection over the date/time periods selected. However, this does not fully consider how much Sun will get in during Winter.
Unfortunately, a shading device does not suddenly stop working after a certain date (unless it is fully retractable). It will usually partially obscure the window year round, more-so in summer and less-so in winter.
In order to understand the full effect of a shading device, we really need to turn again to the sun-path diagram, Shading Masks and a full assessment of partial shading. It may be that, whilst we want 100% shading throughout most of summer, we could probably live with only 80-85% shading in early Autumn in order to gain a little extra solar gain in winter.
Part of this trade-off requires that the designer know what to cut back on or what to add in order to achieve the desired effect. The pattern of shading over the year from even a simple shade can be quite complex. High percentages of shading can occur much later than you expect unless you take significant care with your shading design.
To demonstrate this, an interactive diagram has been prepared below to show a range of different shading configurations and views of their shading masks. You can use the selector beneath it to see the effects of changing certain parameters of a simple shade.
For those interested, you can download and use the SolarTool to interactively manipulate a range of different shading devices in 3D and see their full shading effect displayed on a sun-path diagram.
Even at this stage we haven't considered the relative strength of the available solar radiation. It may be that, despite going to great efforts to allow solar gains in Winter, the cloudiness and turbidity is such that there is insufficient direct Sun to warrant it. This means looking further into the site weather data to compare direct and diffuse solar radiation values.
If these are sufficient, you should then consider using more advanced shading design methods based on the actual solar potential of the site.