Green building design models, which maximize the use of natural light sources suitable for visual displays and office work, minimize solar gains during warm months, and maximize solar heat gains during cold winter months, have become a necessity.

Through appropriate design, large glass surfaces are avoided because windows can cost approximately three times more than opaque facades and have about one-third of their thermal resistance.

In this way, the function of natural light is maximized through the use of external horizontal shades, louvers, and light shelves to direct natural light into the building interior.

Increasing the window area beyond a window-to-wall ratio (WWR) of approximately 30% can significantly increase cooling energy after natural light saturation is reached, while heat losses in perimeter areas can be substantial, affecting thermal comfort. This is illustrated in the study "Green Buildings and Potential Electric Light Energy Savings" by Martin Moek, recently published in the Journal of Architectural Engineering.

Current Natural Light Requirements

The LEED rating system offers credits for natural lighting in two areas: optimizing energy performance (energy and atmosphere) and daylight and views (indoor environmental quality). Significant points can be accumulated if energy consumption is reduced below standard requirements.

For example, if the design energy cost of a new construction is reduced to 50% of the standard, 8 points can be earned out of a maximum of 64. A minimum daylight factor of 2% in 75% of all occupied spaces with critical visual tasks earns another point, and direct lines of sight to the exterior in 90% of all regularly occupied spaces earn another category E point.

Issues with Current Natural Light Requirements and LEED Rating System

1. Energy-efficient architectural design relies on software that can simulate advanced natural light systems and various sky conditions.
2. Current requirements are based on daylight factors for an overcast sky condition, which is an insufficient indicator for annual energy performance. Annual electric light lux hours needed based on hourly local weather data are suggested instead, as these are a product of target light levels and office hours.
3. High EA values favor large vertical windows, which can create thermal and glare issues.
4. High daylight factors can be achieved using skylights. While a significant portion of office space is single-story and can accommodate skylights, single-story buildings are not a sustainable future design model. They require extensive highway systems, lead to low urban density, and long commuting times. On the other hand, current multi-story buildings require electric light in at least 75% of each floor's area for most of the year.
5. Eight LEED points can be obtained by reducing electric lighting and heating/cooling loads by 50%. Maximizing daylight factors is not necessarily the best strategy for achieving this goal.
6. Building shape, facade properties, shading, window location, surface reflectance, and transmission and other architectural features can have a more significant impact on electric light consumption and cooling loads than EA, VLT, WWR, daylight factor, and use of specific electric lighting systems. The green building design literature recommends building prototypes, layouts, and forms for different climates. This work focuses on a section of a recommended prototype for a temperate climate.

Objectives of Green Building Design and Architectural Lighting

The proposed office building section should meet the following criteria:

1. Minimum annual electric lighting load, preferably less than 50% of the time, to save electric light and reduce cooling loads throughout the year in the building's core.
2. Visual comfort and maximum display visibility by meeting office lighting criteria, luminance ratios, and luminance limits.
3. Shade level below 0.3.
4. Minimum solar gains from March to October.
5. Minimum direct glare from direct solar penetration.
6. Solar gains in cold months from November to February to warm perimeter areas.
7. Acoustic privacy for workspaces.
8. Exterior views, including from workstations with partitions.
9. Minimal use of blinds, shades, and permanent controllable systems.

Although blinds and shades mentioned in criterion 9 can be useful for reducing glare and solar gain, VLT would decrease while WWR, window size, and EA would increase to maintain the same natural light levels.

This means more heat gain in summer and more heat loss in winter. Additionally, occupants may not adjust blinds and screens to minimize electric light usage. These devices can drastically reduce the amount of natural light admitted into a space, making occupant use of blinds an important factor in estimating daylight energy savings. Occupants rarely adjust blinds.

Another Strategy

Another strategy to control building energy consumption is adding glazing with high transmission and low shading coefficient, using low-emissivity films, reflective coatings, and spectrally selective coatings. These strategies reduce solar heat gains, but external shades are always necessary for direct glare control.

These shades can be designed to completely block direct sunlight for higher sun angles, making advanced costly glazing unnecessary. Additionally, solar heat gains during winter are welcome for elongated buildings like the one discussed.

Since external shades can reduce solar heat gains much more effectively than window glazing exposed to direct sunlight, low shading coefficients are not truly necessary, and affordable glass with very high visible light transmission values, such as 90%, can be used in this study.

This will reduce the necessary window size, window cost, and associated heating and cooling loads and eliminate glare, which advanced glazing does not do. Therefore, this study uses simple glass with high visible transmission and the smallest WWR, combined with an external overhang to admit mostly diffuse light.

View window dimensions can be increased at the architect's discretion, depending on available views. Curtain walls are not discussed here because they are too expensive and require extensive maintenance.

Proposed Daylight Evaluation Protocol

As green building simulations rely on hourly weather data, the following criteria are suggested:

1. Annual lux hours when electric light is needed for each type of space, similar to annual lux hours when light levels are exceeded for sensitive exhibits in museums.
2. Hours per year when ceiling luminance values exceed office lighting limits.
3. Hours per year when luminance ratios do not meet office lighting criteria for visual display units.
4. Hours per year when cooling loads could be too high, as specified by solar gains in warm months.
5. Hours per year when solar gains could contribute to desired perimeter zone heating in cold months from November to February. Lux hours are the product of target lighting and office hours.

Example:

Assuming an office with a target lighting level of 500 lux and 10 office hours from 7:30 a.m. to 5:30 p.m., 5,000 lux hours are needed daily, 25,000 lux hours weekly, and 1,300,000 lux hours annually, assuming 52 weeks per year.

Required electric lighting can be expressed as the difference between target lighting levels and natural light levels multiplied by the hours. For example, natural light levels in lux from 8:00 a.m. to 5:00 p.m. are 120, 200, 300, 400, 600, 750, 550, 450, 200, and 50 lux.

Electric light levels needed to compensate for the difference for corresponding times are therefore 380, 300, 200, 100, 0, 0, 0, 50, 300, and 450 lux. The daily required electric light is the integral of these values, which is 1,780 lux hours.

Therefore, the required fill-in electric light is 35.6% of the total lux hours needed for that particular day. If this procedure is repeated for multiple days throughout a year, the percentage can be used to estimate annual required electric light levels. (Photo: Freepik)