Daylighting, the illumination of indoor spaces through the means of natural light, has become an increasingly mainstream objective. While the financial impact on the bottom line through decreased reliance on electric lights and associated energy cost savings is noteworthy, the impact on students, teachers and staff is significant. The research speaks for itself.
More than 21,000 students were part of the Heschong Mahone Daylighting Study that showed a strong connection between daylit school environments and student performance. Among the study’s outcomes, the daylit environments yielded 20 percent faster progression in math and 26 percent faster progression in reading. But, the benefits go beyond academics to enhance student education. Retention of the best employees and maximizing productivity are critical factors for school administrators. When potential new teachers are considering their options, features like natural light and a comfortable, well-designed work environment are factors that are certain to be considered.
Guiding Decisions with Modeling
Daylighting strategies, computer-generated building models, and energy modeling are powerful design tools. They enable school districts to grasp how their classrooms, learning centers, collaboration areas, commons, and gymnasiums will provide daylight and views, and how much energy in the form of lighting and cooling can be saved in a proposed school. Modeling should guide design decisions in both new construction and renovations.
Beginning early in the design process, the project team must contemplate the manner in which daylight and views are brought into spaces and how to coordinate that with electric lights, lighting controls, and any potential downsizing of cooling capacity while factoring in the associated first cost. By entering a myriad of product and design options into modeling software, informed decisions can be made.
Look at Lighting Quality
Assessment of the lighting quality and levels in relation to visual tasks being performed is a vital consideration for optimal implementation of daylighting. The control of glare in daylit environments is an important consideration. If the daylight is too bright, users will shut blinds and shades, eliminating views and possibly requiring the use of lights.
Glass selection based on the orientation of each window is a key to optimally taking advantage of daylighting to achieve lighting levels. Insufficient or poor quality lighting may reduce teacher and student productivity while over-lit areas waste energy. Daylight complemented only when needed with an average of forty to fifty-foot-candles of electric light capability measured at night is a good level for most classroom spaces. (Foot-candles are a common unit of measurement in the lighting industry, roughly defined as the amount of light that actually falls on a specific surface. The foot-candle measurement is equal to one lumen per square foot.) For classrooms or offices where computers are the focus, users typically prefer lower light levels -- 20 to 30 foot candles.
Simple Daylighting Strategies
Selecting windows. Windows often comprise a large percentage of the exterior wall area of school or academic building and account for a noteworthy factor in the heating and cooling load of a school. Recently, many technological advances heighten the thermal performance of windows. These technologies include better edge sealing techniques, improved framing materials, low-conductance gas fills, and edge spacers as well as low-emissivity and solar control coatings. These advances, individually or in combination, can optimize performance.
Window performance is typically measured by the following factors: • U-factor which indicates the amount of heat transfer
• Solar heat gain coefficient (SHGC) which indicates the amount of solar energy that a glazing material allows to pass
• Visible light transmittance (TVIS) which indicates the percentage of the visible portion of the solar spectrum that is transmitted through glass.
When selecting windows, try to get the lowest U-factor possible and consider the whole unit U-factor rather than the center of glass U-factor. The whole unit U-factor considers the thermal properties of not only the glass but also the frame and other materials of the window unit.
The proper SHGC differs depending on the climate where the building is located. Schools in colder climates may desire the benefits of passive solar heating gain or higher SHGC. Warmer climate schools will want a lower SHGC to reduce heat from daylighting.
The amount of light transmitted by the windows directly impacts glare or over-lighting. Windows on different sides of a school can be tuned to manage the amount of light based on the amount of sun exposure. A lower TVIS will mean lower daylight levels.
Placing windows. Another consideration is that natural light enhances interior space and creates more interesting environments. Strategic product placement by the design team will manage lighting levels, glare and unwanted solar heat gains and losses. Perhaps more importantly, daylighting and improved views positively impact one’s health and sense of well-being.
Special consideration should be given to windows in spaces with computer screens, marker boards or projections surfaces. Windows immediately adjacent and perpendicular to these surfaces will create unnecessary glare.
Consider replacing or improving inefficient windows. Replacing outdated windows with high-performance windows should be considered for any existing property to improve efficiency, reduce glare, and aid in student comfort. Additionally, daylighting can be improved by controlling glare using window films if existing windows are low-E (low emissivity glass reducing transfer of heat or cold through windows).
Freshen up finishes and improve lighting efficiency. When it comes to updating finishes, it is common to consider how colors will affect the mood and feel of a space. A paint job is one of the most cost-effective ways to freshen up a school and brighten students’ and teachers’ moods. It is also a good way to increase lighting efficiency. Reflectance of natural and artificial lights can use these light sources more efficiently.
Photosensors - Daylight reaping systems use photosensors to detect the prevalent light level. Based on the available daylight, photosensors adjust the electric lighting to provide the proper light level for the space. Skylights – Skylights admit daylight from above, allowing diffused daylight into the space. Many consist of a double layer of material, such as acrylic, for increased insulation.
Tubular daylight devices (TDDs) – Another form of natural toplighting, TDDs bring daylight into the building. Also known as light tubes, they utilize a highly reflective film inside the tube to usher light from a lens at the roof, to a lens at the ceiling.
Siting the building. Positioning the building on an east-west axis, with south and north-facing windows and monitors is a vital daylighting strategy. Since countless other product and design decisions will hinge upon the positioning, few decisions are so key.
Andres, AIA LEED AP, is a Project Architect at Hoffman Planning, Design & Construction, Inc.