A balance must be struck between limiting heat loss and controlling solar gain when trying to get daylighting right in schools, says Peter Caplehorn of Scott Brownrigg
The provision of daylighting in schools has always been of prime importance. Good daylighting is considered to be educationally significant as well as an obvious physical requirement.
With the need to make all buildings more energy-efficient, schools’ windows have attracted significant scrutiny. The balance between the daylighting level and the energy flux across the sill is a complex issue.
Building Bulletin 95 sets out the basic requirements for Schools of the Future projects, with additional detailed information in bulletins 90, Lighting Design for Schools, and 87, Guidelines for the Environmental Design of Schools.
The prime design criterion is that a space is required to have a daylight factor (the ratio of outside illuminance compared with internal illuminance) of 4-5% with a uniformity (that is, the minimum daylight factor divided by the maximum) of 0.3-0.4. To achieve this standard the specifier will have to consider dual aspect windows and carefully position these. A lighting level of 300-500 lux should also be achieved so consideration must be given to light shelves to help with even light distribution and shading to limit excessive levels to a glare index of less than 19.
The need to produce a cool environment in summer and limit energy losses in winter starts to give the specifier a real challenge when considering all the options for the fenestration.
Also, in the UK there is a great range of solar intensity both on an annual and daily basis which causes the designer further problems. For a quarter of the year, average light levels in excess of 28,000 lux are experienced.
Tempering both the light and the energy into the interior are a major challenge. Low-angle light is of particular concern as control conflicts directly with the need to provide good views out. To achieve best results, a good computer analysis programme will be required. This should be able to model the design accurately.
Meeting the requirements will normally result in a minimum glazed area of 20% and a maximum of 40%. Windows of this proportion on the southern elevation will result in excessive solar gain in summer and if on the north elevation, excessive heat loss in winter. Clearly the design has to be fully integrated with the building and be more than just a “hole in the wall”.
Reducing solar gain
To provide flexibility and performance, many designers are specifying a range of measures to help reconcile the opposing criteria. One solution is the use of triple-glazed windows, which can achieve a U-value in the order of 1.5-1.6 W/m2K and incorporate a blind in one of the cavities. This means the blind is protected behind glass and can limit glare and solar gain.
This will not eliminate all solar gain, however, and the use of a ventilated outer cavity and/or external solar shading will also be required for all but north-facing elevations.
Designers could also consider the use of shutters, particularly ones that allow some light to pass through.
In addition, designers can consider the use of top lighting and light shafts. These make the equation more complex but can achieve a good balance between increasing light without excessive solar gain, but ventilation and shading need to be considered for solutions of this type, too.
The depth of the window reveal is another good design aid, as deep reveals can reduce glare significantly. The colour and type of surface finishes can also play a part in ensuring that the light is well distributed around the space. Details of this can be found in Building Bulletin 90. Clerestory lighting in a Trombe walls may be an answer.
The benefits of combining solar collection and solar shading have yet to be developed in schools.
Daylighting may well also need to be supplemented by artificial lighting and this needs to be carefully balanced. The lights nearest the windows should be controlled separately from the remainder, allowing for changes in external brightness levels.
The specifier needs to consider glass specification that will start to filter the excesses of solar gain; some coatings can cut this by as much as 60%.
Additionally, high-level fritting or ceramic coatings on the glass can also be highly effective. Combining these with blinds that can also reflect light in to the space can help. These are daylight harvesting blinds, but many require automated control systems and therefore raise issues of cost maintenance and reliability.
The specifier should ensure they are clear on the daylighting requirements.
- Make sure these are checked at tender stage, checked pre-site, and checked on site
- Daylighting is difficult to change at a late stage in a project
- Small variations in the window size can significantly affect the daylighting levels
- Careful consideration of the aspect and the level of solar gain is needed on an individual window basis