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. 

Process-based design 

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. 

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.  

Passive design simulation model 

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.  

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. 

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

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.

For more information, contact REHAU Window Solutions:
Tel: +27 82 809 7203

Be in the know! Don’t get left behind!
Subscribe to our free magazine on
Sign up for our newsletter: or join other discussions like these on, and

Subscribe to our Community👇

Stay Inspired, Stay Educated, Stay Informed.

This is how people in the built environment do it!

By subscribing you agree to receive our promotional marketing materials. You may unsubscribe at any time. We keep your data private.