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20 April 2015

Shading: It Is Not All Bad

While the primary inter-related factors of site, glazing, thermal mass and insulation are instrumental in maximising the passive thermal performance of a building, the significance of shading follows close behind.

Shading essentially only influences and controls the solar radiation gains to the interior of the building, and hence the 'initial' amount of available 'passive heat' which is then retained or lost by the thermal mass and insulation characteristics of the building. More shading gives less solar radiation entering the exposed rooms – a positive in summer and a negative in winter – but can upset the 'sweet-spot' proportions of the other three factors if not considered at the same time. Some forms of shading can be movable, so as to reduce the negative effects of fixed types.

In decades past, when the energy for heating and cooling came mostly from utility suppliers, shading and screening was usually associated with the control of glare, privacy and aesthetics etc, but now and in the future, much more careful thought will need to be given to the positioning and effect of shading on the particular aspect of overall passive thermal performance of a house. As buildings become more energy efficient, the proportional effect of shading will increase, requiring more considered and calculated design detailing – especially as shading often is or has to be a constructed fixture. To be effective the geometry, quality, and quantity should be considered right from the beginning of the design process, and not treated as an afterthought add-on. At the developed design stage, even minor changes to the interior/exterior space planning, elevational form, and aesthetics can dissolve any necessary shading into the overall appearance and function, rather than standing out as a last-minute correction.

Shading for a building comes in a variety of categories over which the building owner will have differing degrees of ability to manipulate and control:
Firstly, there is the environmental aspect of the surrounding physical geography and flora, the climate throughout the year, and the present and future neighbouring built structures.
Secondly, the influence of the particular characteristics of the property which are in control of the owner – will existing structures and vegetation be retained or demolished; are there particular outlooks or view-shafts to be retained; are there neighbour privacy issues in directions where it would be preferable to have full sun access; are there client design brief requirements for shading along orientations which are ideally suitable for achieving necessary solar gains.
Thirdly, the developed design plan and elevations, and detailing of the construction and materials, can facilitate or hinder the shading design, and the need for it.

As with the inter-related factors of glazing, thermal mass, and insulation, the maximising of the influence of shading, (by having or not having), on the passive thermal performance of a building is greatly dependent upon the designer's experience plus thoughtful design – there are no 'rules-of-thumb'.

While constructed shading devices, and the influence of environment elements, are normally only associated with the solar gains through the glazing, there is also an effect on the adjacent walls. The exterior wall of a room facing the sun will be adding to the passive solar gain, (albeit at a slower rate than the windows), but those portions in shade can be losing the gain at the same time, especially in winter.

There is another important, separate and technical aspect to shading which must not be forgotten. This is the technical aspect of the effect of differential heating/cooling across the surface of glass, due to the moving shadow cast by the shading. The glass itself can be treated as a shading type. When detailing windows and glazed panels, and selecting the types of glass to use, thought must be given to reducing the potential for cracking and breakage. Standard, laminated and toughened glass all act differently, and tinting and e-glass coating can add complexity, especially for double glazed units.

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