Achievable energy savings report

Energy-efficient thermal comfort

REHAU recently undertook a performance modelling exercise to determine how efficient their fenestration systems are in local conditions. Together with Solid Green Consulting, they share feedback on the energy savings achievable with REHAU uPVC windows and doors through performance modelling.

Setting targets 

The amount of power needed for lighting and comfort often makes up 40% to 60% of a building’s energy use. The study focussed on optimising the passive design of a building, to reduce this demand in favour of renewable energy. Achieving this balance is crucial to achieve net-zero energy in operation.

Solid Green Process Based Design graphic - 1123675

The following fenestration targets are considered:

  • Daylight: Natural light is optimised when performance metrics for three lighting variables are met, ensuring a well-lit area that does not create unwanted heat gains. These metrics are spatial daylight autonomy (sDA), annual sunlight exposure (aSE) and useful daylight illuminance (UDI).
  • Thermal comfort: Balancing light and heat gain is key to ensuring mechanical cooling is not required to maintain comfort. Controlling solar gains and reducing heat loss from the building will increase net-zero hours significantly.

Climate considerations

Understanding the climate affects passive and active design elements of a building. In the study, thermal comfort was determined for both summer and winter, in Johannesburg, Durban and Cape Town. 

For example: The comfort range for a building in Cape Town is 22-24°C and the average annual temperature is 16,5°C, therefore cooling is not required for the bulk of the year, provided that solar gains are controlled. However, active heating may be required to maintain comfort.

Reducing loads 

As shown in the climate data, controlling solar gains and reducing heat loss from the building will increase net-zero hours significantly for a building. Heat gains and losses occur from the ceiling, walls and windows, so insulation and fenestration are important interventions to reduce the loads. 

Solid Green Climate graphic - 1123680

Daily Temperatures - Cape Town

Passive design simulation model

Solid Green Passive Design Simulation - 1123676

The 3D model used for the study, was modelled as per architectural drawings, including correct orientation to show the effect of shading at various times of the day specific to the building’s location. 

Results: insulation versus fenestration

Climate: Winter 08:00am on 7 July, Cape Town


  • U-value: How quickly conductive heat gains enter the space.
  • Solar heat gain coefficient (SHGC): What portion of the sun’s energy enters the space
  • Visible light transmittance (VTI): How much natural light enters the space

Solid Green Impact of Insulation graphics - 1123678

The impact of insulation was compared between three options, where window VLT and SHGC values remain constant. Results showed that insulating the roof/ ceiling and walls reduces heat loss and increases internal air temperatures.

Solid Green Impact of window construction U value - 1123677

The impact of the window construction U-value, where the glass VLT and SHGC values were kept constant showed that improving the window construction U-value improves the building’s heat retention, resulting in increased air temperatures.

Results: energy reduction

Peak energy reduction when comparing REHAU uPVC double and single glazed windows, versus single glazed standard metal windows, showed excellent results for the energy consumption of the modelled building using REHAU building fenestration systems.

Using effective fenestrations systems will not only significantly reduce Peak energy usage of the building and increase the general thermal comfort, it will also further greatly assist in the reduction in annual energy consumed from heating and cooling in the housebuilding. The figure above shows the reduction of peak load shown as a percentage at the different construction U-values in each location modelled i.e. Cape Town, Johannesburg and Durban, compared to single glazed standard metal windows.

Solid Green Heating Energy Summary - 1123679

Issue: Achieving net-zero goals through building fenestration.
Solution: Process-based based design helps to achieve net-zero energy goals with REHAU uPVC fenestration systems.


Engineering progress

Enhancing lives

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