Why building energy assessments matter

Heating, ventilation, and cooling (HVAC) of buildings can use a significant quantity of energy: American and Australian estimates place it in the region of 30 % of total consumption. It follows that reducing the heating and cooling needs of a building can have a significant impact on long-term energy consumption and energy costs.

The best time to reduce heating and cooling needs is during the building design phase. Once the building is completed, there are few options remaining – for example, adding efficient air-conditioning systems or retrofitting of shading and glazing. These are rather like sticking a band-aid on instead of preventing, or at least reducing, the problem before it arises.

Does adding energy efficient components improve building performance?
Building model showing layout and orientation

In South Africa solar loads are often high, and good designs will always address this. However, some people are not aware that blindly “throwing” better-performing glass or insulation at a building does not necessarily lead to improvement; in fact, in some circumstances it might actually make things worse. This has been illustrated below for the building model shown in Fig. 1.

The building construction is typical for the Western Cape: plastered brick cavity walls, a well-insulated ceiling and single clear glazing in the windows. If the owner wished to use external louvre shutters, or double or quadruple glazing to improve it from this baseline, what would the impact be on the annual building performance? Instinct says that progressively improving these components should lead to a reduction in energy consumption, but is this necessarily the case?

Does adding energy efficient components improve building performance?
Comparison of results: annual heating and cooling

The annual thermal energy that must be added or removed is shown in Fig. 2, assuming that air-conditioning is used to keep the indoor temperature between 19-25 °C. The use of fixed shutters (orange column) drastically reduces the cooling required relative to the base case (black column), but increases the heating many times; overall the combined (total) heating and cooling energy is less than for the base case. The best case in terms of total energy use is from controlled shutters (grey column) that are opened and closed depending on the indoor temperature. Using double glazing with solar control (yellow column) instead of shutters gives reasonable performance with respect to heating and reduces the cooling compared to the base case. But upgrading from double to quadruple glazing (dark blue column) leads to worse performance than the double glazing even though the thermal insulation is far better! This is because quadruple glazing traps more heat indoors in hot weather and increases the cooling required for this building.

So, don’t make the error of thinking that you can improve a building just by randomly using more expensive, better performing components: you need to ensure that they work together with the building as a whole to achieve the best case overall. Balance is critical.

Green Plan

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