Assessing the daylight transmittance of atria roofs in real buildings.

The displacement of artificial lighting with daylighting in buildings has been shown to reduce energy consumption and provide preferable spaces for building occupants. The atrium is one means of achieving daylighting objectives in medium to large scale buildings. The transmittance of daylight through atria roofs is the least understood area of atrium design, with a particular dearth of information concerning roof transmittance in real buildings. This study aims to further the current knowledge base with regard to roof transmittance through the measurement of transmittance in real buildings, and comparison to a newly proposed photoanalysis technique. This has the potential advantage of being able to assess the transmittance of existing roofs far more efficiently than physical illuminance measurement approaches. The daylight factor at points immediately beneath the roof was measured in two case study buildings, one of which was a simple a-frame, the other a space frame monopitched roof with significant over-shading from the urban context. Hemispherical photographs were taken in these buildings, and the proportion of visible sky seen at the corresponding viewpoints found using the program HemiView. The effects of changing the photograph viewpoint, quality, and using the classifying tool within HemiView were investigated. The buildings were then modelled on a computer and a comparison made with hemispherical images of the roofs derived through a rendering process. Illuminance simulations were then run using the program Radiance. The effects of the well and external obstructions were isolated, and the relationship between illuminance at the photocell point and average roof plane illuminance found. The consequence of parametrically adjusting the structure reflectance and glazing transmittance was explored, and simple relationships relating these two factors to average roof plane transmittance derived.Further roof types were modelled on a computer and analysed in the same manner. A means of relating the photographic technique to transmittance is presented, and a methodology described for application of the procedure to existing roofs. The process was demonstrated on 15 roofs in Sheffield. The thesis concludes with a summary and suggestions for future work.