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.

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. 

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

Measurements: 

• 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.

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.