The CIBSE Applications Manual Window design, published way back in 1987, has been crying out for an overhaul. Right on cue, Daylighting and window design is about to see the light of day. Brian Sims previews the Institution's revised guidelines for architects and designers.
Although the CIBSEs' new Lighting Guide LG10: Daylighting and window design is primarily a daylighting design guide, architects and designers borrowing from its pages of wisdom must remember a salient fact. When daylighting decisions are made, they will impact upon other, interrelated aspects of window performance including solar heat gain, winter heat loss, acoustic performance, security and fire protection. Designers must always take these variables into account when drafting their project outlines.

Rest assured that the new guide will not leave you high and dry when attempting to do this, as it provides detailed advice on all the main daylighting concerns associated with both new build and refurbishment projects.

Building on the strengths of the well-known Applications Manual Window design (which it now supersedes), LG10 is split into three sections, concentrating on the role of windows in building design, the design issues themselves (eg building form and layout, window and rooflight sizing and the integration of daylight and electric light) and daylight calculations.

Window design: the key issues

Windows are far more important to building design than might appear at first glance. Yes, they are essential for views out, but they are equally important when serving as ventilators (to improve indoor air quality), for providing mass cooling ventilation and when acting as a climate moderator, insulator and/or glare protector.

The level of daylight ingress through windows is crucial, of course. A host of studies have shown this to be true, including many applications of the Building Research Establishment's Environmental Assessment Method. These have documented conclusively that good or bad daylighting provision in offices has major implications for staff absenteeism and productivity.

To this end, LG10 stresses that window design must be considered as part of a building's lighting design and ventilation strategy at the earliest possible stage.

The guide provides a useful checklist on using windows in the design process, in terms of both sidelit and rooflit spaces, and also outlines the special requirements for refurbishment projects. In the latter case, if the building interior is too dark designers should check the room depths and no-sky lines and, if the results are not satisfactory, rooflights or the addition of an atrium should come into the equation.

What about all those attempts to substantially increase daylight ingress by using light shelves and special glazing types? Well, for the most part these have not been a resounding success, although the guide states that they can help to improve light uniformity in spaces that are only of medium depth.

No-sky lines, atriums and rooflight sizing

LG10 contains a useful table detailing the maximum depth for different room widths and window head heights. In specific terms, higher reflectances and higher window heads will allow deeper-plan rooms. For wider rooms the allowable depth is markedly greater, of course.

There is also a useful worked example of how to plot the no-sky line. Where there is more than one window, states the guide, the final no-sky line will surround those areas which cannot receive direct skylight from any of the windows. It can then be calculated by considering each window on its own, and then combining them.

The no-sky line and the limiting room depths on each side will set a limit to the depth of the building that can be satisfactorily daylit. If the building needs to be deeper than this, options include the use of rooflights, an atrium or courtyard space or a non-daylit core with daylit perimeter.

If an atrium or internal courtyard is planned, its dimensions must be chosen with care. In a tall, narrow atrium, the base will receive less light and little light will penetrate the adjoining lower floors. If the atrium is wide compared to its height, adjoining spaces will have a better chance of receiving light through it.

Crucially, LG10 includes a concise, fully-updated section on window and rooflight sizing, shape and positioning, and looks at the contribution that rooflights can make. Rooflights can be a useful supplement to side windows in an interior. The light can brighten the back of an excessively deep room or "bring a welcome balance to strong sidelighting".

The guide also provides an excellent table listing the advantages and disadvantages of various types of rooflight, including the horizontal and sawtooth varieties. Methods of predicting daylight and the criteria against which to judge the values are then outlined, while guidance is given on the visual considerations of view and glare.

The facts about window components

The choice of glazing material is crucial to a building's successful daylighting design, and the guide outlines all the main types. LG10 also runs through the various solar shading devices which are available, listing their applications, advantages and disadvantages, their cost implications and maintenance considerations.

One area which has generated much interest of late is the subject of daylight redirecting systems, including light shelves, prismatic glazing and mirrored louvres. They can provide a degree of shading with better use of sunlight, and can also improve the uniformity of daylight in the space.

Integrating daylight and electric light

The integration of daylighting and electric lighting needs thorough planning, the correct choice of light source and the installation of a suitable controls strategy. If successful, states LG10, the advantages of a successful strategy are a variable illuminance distribution, visual interest in the space and, most importantly perhaps, reduced lighting energy consumption.

The guide takes the reader through the various types of lighting control and where the different systems are best applied.

The final section of Daylighting and window design deals with an important topic, that of daylight calculation.

Seasoned engineers will know all about daylight factors – the ratio of the indoor illuminance at a given point in the building to the outdoor, unobstructed horizontal illuminance. Very often, the lighting designer will be asked to provide a detailed distribution of daylight factors in a building's interior, or energy use calculations for lighting may call for the minimum daylit factor at the worst lit workstation.

The daylight factors can be calculated by manual methods, computer programs and scale modelling, all of which are comprehensively explained in LG10.

Thanks are due to Dr Paul Littlefair of the BRE for his help in the preparation of this article.

Daylighting design for the Queens Building, De Montfort University

Following occupation of the School of Engineering at Leicester’s De Montfort University – better known as the Queens Building – the quality of the daylighting provision was assessed by researchers at the BRE. The building’s electrical laboratories are of most interest, forming two projecting wings at the eastern end. Each contains shallow-plan laboratories on four floors, with the two wings separated by this small courtyard. Each laboratory is lit mainly from two sides, the courtyard side having been fitted with a low internal light shelf. The laboratories contain many pcs, so good lighting levels were important. The researchers found that the shallow-plan space (6·4 m from wall to wall) results in uniform daylighting, although daylight factors are low. The lack of adjustable shading devices has meant that reflections on pc screens have been a problem. This scenario could have been avoided by running the laboratory workbenches perpendicular to the windows and not parallel to them.

Western Morning News in the shade

The Nicholas Grimshaw & Partners-designed Western Morning News building in Plymouth. Low angle sun was a potential problem on the second floor, so engineer Cundall Johnston & Partners (CJP) used the Tas dynamic simulation program to simulate the sun’s movement. The program was also used to calculate the sun’s effect on computer-based working at the building’s south and south eastern elevations (ie the ‘bows’ of the building). The issue of glare control was handled by the architect. CJP’s designers proposed Colt’s helio shades, but the architect wanted something similar to an adjustable ‘sail’. The resulting indecision forced the occupants to use giant sheets of canvas as ad hoc anti-glare devices. Not an ideal scenario. Manually retractable dark blue blinds were installed at a later date by way of a remedial measure. Clearly, the desire for a translucent structure can be incompatible with screen-based working if glare is not tackled early on.

Lightshelves at APU’s Learning Resource Centre

The 6000 m2 Learning Resource Centre at Anglia Polytechnic University makes use of fabric light shelves. These were specified to enhance light levels deep into the space, while cutting out direct sunlight and helping to minimise glare. Although the twin shelves do provide glare protection, the flat ceiling slab was replaced by a waffle slab in the main study areas and light is reflected into the coffers. Single opaque versions of the light shelves might have provided sufficient protection against glare. Measurements1 have also suggested that incoming daylight levels drop rapidly from the building’s perimeter. Reference 1‘PROBE 8: APU Learning Resource Centre’, Building Services Journal, December 1996.