Effectiveness of external shading for VELUX roof windows in the climatic conditions of the Czech Republic

The air permeability of the structures is assumed to be similar to that of an average building from 2005¹, corresponding to a value of 2 ACH at 50 Pa (the value required by passive house concepts is 0.6 ACH at 50 Pa).
The used windows: two GGL M08 windows (780 x 1398 mm) on the southern roof plane. Both windows can be opened for natural ventilation. Detailed information about the windows is provided in Table 2.

It is assumed that the attic room is inhabited by one person. The attic resident is away from home on weekdays from 8 AM to 5 PM, otherwise the room is fully occupied.
Heating is set to a temperature of 21 °C, and there is no air conditioning in the building. In the shading option, the external awning or external roller shutter is controlled according to the indoor temperature using a remote control set to 24 °C.
The internal gain from devices is assumed to be constant at 3 W/m².

Ventilation Settings
Mechanical ventilation is installed in the house for basic air circulation at a rate of 0.5 ACH. Ventilation is active throughout the year.
The roof windows serve to limit overheating and are automatically opened when the indoor temperature reaches 27 °C. Both roof windows are controlled simultaneously. It is assumed that the doors to other parts of the building are always closed.

Description of Individual Cases
Three cases were considered: one without shading the roof windows, one with shading using VELUX external awnings, and one with shading using VELUX external roller shutters. In all cases, operable VELUX roof windows were used.

Simulation Results
Figure 2 shows the indoor temperature in all three cases as well as the outdoor temperature during the warm period from May to August. The results show that the temperature indoors with unshaded roof windows exceeds 30 °C on several days during this period. In the cases with shading using external awnings and external roller shutters, significantly lower temperatures were achieved. The largest temperature difference is 5.5 °C (VELUX external awnings) and 7.0 °C (VELUX external roller shutters).


Figure 2
No awning – No shading
With awning – With awning
With roller shutter – With external roller shutter
Outdoor temperature – Outdoor temperature

Figure 3 shows in more detail the period from August 5 to August 8. It also illustrates category 3 according to the active house specification version 2.0.


Figure 3
No awning – No shading
With awning – With awning
With roller shutter – With external roller shutter
Outdoor temperature – Outdoor temperature
Comfort limit – Comfort value

Temperature comparisons show that the use of external shading can achieve significantly lower temperatures. Figure 2 shows the temperature in the hottest period of the year from May to August. The largest temperature difference is about 6 °C and remains constant for a considerable part of the specified period.


Figure 4: Comparison of resulting indoor temperature
With awning – With awning
With roller shutter – With external roller shutter

Simulation Software
The calculations were performed using VELUX Energy and Indoor Climate Visualizer 2.1, build 47. More information about the software can be found at http://eic.velux.com or in the software manual (available online on the same website). The calculations and simulations were carried out by Thorbjørn Færing Asmussen and Anna Karina Hammelev Hansen from the VELUX A/S Center for Daylight, Energy, and Indoor Environment.


The VELUX product range includes a wide selection of roof windows, skylights, and solutions for flat roofs. VELUX also offers a full range of shading and decorative accessories – from roller blinds, venetian blinds to external awnings and external roller shutters, including a complete system of mounting and control elements for manual or remote, electric, or solar control.