Energy Usage

Energy Usage and Thermal Analysis Of a 400m2 Upmarket Residence in Johannesburg, South Africa.


The objective of this project was to analyse the performance of a residence with three Expanded Polystyrene (EPS) products/systems installed in comparison with a conventional unimproved design of the same residence. The energy usage of the base-case building, with all applications typical of a residential building were developed using the VisuaIDOE® software. These were compared with the usage of the building with the three EPS products/systems improvement. The comparative costs of the base case and EPS products/systems were prvided by industry and were used to develop Life Cycle Cost comparisons.

Internal temperature improvements resulting from the three EPS systems were demonstrated using the NewQuick® or Building Toolbox® software. This analysis also provided the ‘percentage persons comfortable index’ (PPC). A comparison was drawn between the base case model (which was developed in detail), and the improved base case model which was modified with the three EPS products/systems. The base-case residential design and a description of the improved system are detailed in Annexure A. The effective thermal performance of the 195mm EPS ‘rib and block’ suspended floor slab system (255mm overall slab depth) was calculated using the ASHRAE Zone Method. This method uses a series-parallel calculation around a conductive zone in order to account for transverse heat flows though insulation systems with highly conductive intrusions. Transverse heat flows do tend to be under-estimated by conventional calculation. A conventional simplified Fourier’s Law method was applied for calculating the thermal transmittance of the 40mm thick EPS insulated cavity walling system. IS013370: Thermal Performance in buildings – heat transfer via the ground – calculation methods were applied for calculating the thermal transmission of the insulated slab (11JFS) on ground applications for input to the energy model below.

A VisuaIDOE® based computer modelling of the base-case, and three EPS systems, was performed. This built up the impacts of all building energy uses and accounts for their inter-relationship. For example, much of lighting energy ends up as heat and impacts on heating or cooling loads in buildings. The occupants of a building influence energy usage with their metabolic activity. Usage of appliances was also taken into consideration. The model was run for Johannesburg only.

The energy cost and capital cost impact of each system was developed. This data was used in a Life Cycle Cost (LCC) calculation. LCC method is accepted for energy cost evaluations as it provides a long term view which brings into account projected energy costs rather than short term costs. The energy cost escalation used in this report was 2.5% over the rate of inflation over 30 years. A discount rate of 7.0% was used. Predicted temperatures within the buildings and the PPC for the hottest hour, were obtained for comparison between the base-case and the improved system. These and the internal air & radiant temperatures were obtained via a NewQuick® model.

Base - Case Model Development

Buildings of different occupancies have widely differing energy usage patterns and the influence of energy saving systems varies greatly for climatic regions. Building size, orientation, lay-out and material designs and equipment choices also influence energy usage. In order to demonstrate the heating and cooling energy usage reductions resulting from the incorporation of the EPS improvements a 400m² modern flat roofed residential design with typically large window areas, based in a highveld climate region (using Johannesburg weather data), was proposed as the base case for the VisualDOE model. It was assumed that heating was via under-floor electrical resistive heating and that cooling was via low efficiency split units. Other energy appliance load intensity assumptions are detailed in the building description of Annexure A.


Thermal transmittance of systems

The Effective Thermal Transmittance of the EPS ‘rib and block’ suspended floor slab system (255mm overall slab depth) was calculated using the ASHRAE Zone method. A result of 0.48W/m2K was reported. This result was six times the thermal resistance of the standard 255mm concrete slab which has a thermal transmittance of 3.2 W/m2K.

The Thermal Transmittance of the EPS walling system with 40mm of expanded polystyrene within a double brick wall cavity was calculated using the simplified Fourier’s Law. A result of 0.72 W/m2K was achieved versus 2.63 W/m2K for the un-insulated wall.

The Thermal Transmittance of the EPS flooring system with 40mm of expanded polystyrene (within a double brick wall cavity) was calculated using the simplified Fourier’s Law. A result of 0.63 W/m2K was achieved versus 2.04 W/m2K for the un-insulated floor.

Energy Usage and heating and cooling load reduction

The VisualDOE model results are set out below:

The calculations included the base case unimproved building, the improved building with all proposed EPS systems incorporated and then three models which unload each of the three systems individually in order to assess the thermal efficiency of each product/system on its own. The final model in the series showed the performance of a building with IBR steel roof and plasterboard ceiling.

The sections of the VisualDOE report cover:

  • Electrical usage by major application
  • Energy cost – impact on Life Cycle Cost
  • Energy usage by month (showing seasonality)
  • Monthly electrical power demand


EPS Energy Table2
EPS Energy Table3

Effect on air temperatures in an unoccupied house

The Building Toolbox/NewQuick software showed a decrease of maximum temperature of 7.1 ºC versus a concrete slab, and 11.0 ºC versus an IBR (Steel) roof and ceiling, compared to the combined three systems in place in an improved building. The increase in minimum temperature was of the same order. With an unimproved slab or with an IBR roof and ceiling the PPC was 0%. The PPC was 100% for the improved building at the hottest hour.


The reduction in energy usage as result of the combined EPS product/systems was 31.4 kWh/m2 per annum. This is the expected reduction in combined heating and cooling energy load over a year expressed per unit area of the building. The reduction was attributed to the additional EPS insulation. The individual effect of each measure was: Wall Insulation 10.6 kWh/m².a Floor Insulation 18.9 kWh/rn².a Roof Insulation 15.8 kWh/m².a When considered that the proposed SANS 204 Part 2 Deemed-to-satisfy requirement for energy usage in residential building in Johannesburg may be as low as 126 kWh/rn².a, it is evident that this represents a major area in which energy usage reduction can be achieved. The reduction of peak electrical demand is over 40% for the cold winter months in Johannesburg. The reduction in heating demand can in some instances be complete. If windows are north facing and the building is not too deep, and if south facing windows are significantly reduced, then the heating can be eliminated completely even for a temperate climate such as Johannesburg, but certainly in total for milder climates such as for Pretoria and Cape Town. The annual energy usage reduction with all EPS measures applied was just under 33% of those impacts which can be influenced by the shell of the building, for the region. In relation to a base-case of a building with an un-insulated ceiling and IBR (Mild steel) roof the reduction is 44%.