Assessment of daylight quality in simple rooms
Impact of three window configurations on daylight conditions, Phase 2
September 2006
As a direct continuation of the pilot project ”Impact of three window configurations on daylight conditions, Phase 1”, which the Danish Building Research Institute (SBI) carried out in 2003 at the request of VELUX, the second and last part of the study ”Assessment of daylight quality in simple rooms – Impact of three window configurations on daylight conditions, Phase 2” is now published.
Phase 2 represents an extensive number of simulations performed for the three rooms under overcast, intermediate and sunny sky conditions for each window - representing all orientations and all seasons. As a tool for the analyses, the Radiance Lighting Simulation System was used.
The approach in the study was to compare daylight conditions with different window types by investigating a number of lighting parameters. And even if there is no universal definition of light quality, the results gave clear indications of which room would be the brightest, under which conditions glare might be a problem and which type of window would yield the greatest luminous variation (or visual interest).
Overall, phase 2 fully supports the main conclusion from Phase 1 in that the roof window yielded a daylight factor that was about twice as high as that of the vertical window and more than three times as high as that of the dormer window and the roof window provided a larger variation in lighting. This result is important because the daylight factor was measured under an overcast sky, a condition which prevails in Denmark more than 65% of the time.
Phase 2 also shows several new aspects in relation to comparison of windows and the use of solar shading devices. The roof window provided less glare with more acceptable light transitions in the view towards the window – whereas the dormer window generally created a “tunnel” effect, yielding a more concentrated light beam and lower light levels in the room.
The study also considered the need of solar shading and the effects of awning blinds.The roof window resulted in more direct sunlight during summer, while during winter the roof window resulted in less direct sunlight than the vertical window. From an energy point of view, it is preferable to use a shading device during the summer (to avoid overheating) as opposed to the winter when free solar heat gains and sunshine are welcome. The study shows that the use of external screening – with an awning blind – is a very effective solution for solar shading and it reduces the risk of glare.
See further details in the enclosed Power Point presentation or in the complete report (enclosed as a pdf file).
The Research Team
Senior researcher Kjeld Johnsen headed the project, and the team included Karl Grau from the Health and Comfort Department of the Danish Building Research Institute. Marie-Claude Dubois, associate professor at the School of Architecture, Laval University, Quebec, Canada has performed the large number of radiance simulations together with students from the school.
A study with the objective of providing easy-to-use documentation
In VELUX architect binders, we have stated for many years that roof windows installed in 45° roof pitch offer up to 40 per cent more light than dormers. However, our internal calculations and tests indicate that the difference may be even bigger. Based on these assumptions, and because we are occasionally asked by specifiers to provide documentation that roof windows provide more daylight, we made an agreement with the Danish Building Research Institute in 2003 to perform a study with the objective of providing easy-to-use documentation on daylight conditions.
VELUX assisted in the definition and framework of the study, which was financed by the Danish Building Research Institute and VELUX.
The Danish Building Research Institute (SBI)
The Danish Building Research Institute (SBI) is a government research institute under the Ministry of Economic and Business Affairs in Denmark. You can read more details about the SBI organisation at
Link to the complete report: