Abstract

Daylight redirecting blinds are a class of sun control device that are designed specifically to increase daylighting levels in buildings in addition to preventing unwanted solar gain and glare. Because they rely on many parameters such as complex geometry and may require automated controls to achieve their high illuminance performance, their angle-dependent optical characteristics cannot be represented or simulated accurately using the simple tools that are normally used at the beginning of the design process when rapid assessments are needed. Instead they require time- and resource-intensive simulation methods that are difficult to integrate into existing building design workflows at such an early stage of design. Therefore design guidance for these daylight redirecting blinds is needed in support of design decisions at the beginning of the schematic design phase – to assist in answering questions such as: How deep can a floor plate be for the entire floor area in an open-plan office to be considered sufficiently daylit?

The daylighting illuminance performance of two classes of blinds, passive and active, are investigated to generalize this design guidance. A representative model of each class of blind is used. Through the use of a high-performance multi-storey open-plan double-perimeter zone office building in Golden, USA (40°N, 105°W) as a case study, a simplified simulation model using the radiosity method is validated.

The simulation model is used to examine the effect of different parameters such as blind type, location, glazing properties, building depth, façade orientation, window to wall ratio, and window head height on daylighting illuminance in the office space. Simple correlations between building geometry and interior daylight illuminance sufficiency are sought that can be used as design guidance in early schematic design in lieu of simulations.

Based on the results, the conclusion is that for most combinations tested active blinds will perform as well as or better than passive blinds. While a passive blind may be acceptable for mild, temperate climates, it may cause excessive overheating in climates with high cooling loads. In this respect, the greatest flexibility is offered by the class of active blinds which can control when daylight or solar heat is desired in the interior. Using the sDA300/50 metric from the IES LM-83-12 standard, the study found that the maximum building depth for South-oriented open-plan space that provides ‘nominally acceptable’ daylight illuminance is 14.5 m for Golden (actual building depth is 18 m). This calculated maximum building depth is between 11.5 m (Vancouver) and 15 m (Montreal) for different locations. This variation is due to the different total annual sunshine hours and visible transmittance of the glazing and blind at different solar incidence angles at each location. A correlation is made between window head height and maximum building depth for an open-plan office space.