If you associate high-performance daylighting techniques with new construction and expensive lighting controls, think again. While a facility’s footprint, glazing and daylight apertures may be fixed, FMs have low-cost strategies available to them for retrofits that save money and energy on lighting and HVAC loads.
A successful plan balances natural light with heat gain and integrates architectural features with artificial lighting and controls. When adequate daylight is available, lighting levels are reduced by either dimming lamps or switching them off (entire luminaires or individual lamps within a luminaire). Natural light may be introduced through toplighting (skylights, clerestories) or sidelighting (windows). Toplighting tends to provide better performance than sidelighting but will likely have fewer applications in multistory buildings.
1) The Reflectance Factor
A starting point is improving interior reflectance values. Office Daylighting Potential, a study for the California Energy Commission, found that improving interior reflectance was one of the most consistent variables providing energy savings.
The study is based on the California Commercial End-Use Survey (CEUS) dataset of 536 existing office buildings in the state. The investigators found that improving reflectance values for ceiling tiles, walls and carpet had a double-digit savings effect. Specifically, replacing off-white acoustic tiles (70% reflectance) with brighter white tiles (85%), changing wall color from 50% to 60% reflectance, and carpet from 20% to 30% can reduce square-foot energy use by 15% for lighting and HVAC. Ceiling and floor reflectance each have a greater impact than floor reflectance. These results assume two-level switching control for the artificial lighting.
The savings is an average across all daylit zones, which included primary zones (those within 8 feet of an exterior wall with windows), secondary (8-16 feet from windows) and tertiary (16-24 feet). These zones correspond to a typical 8-foot by 10-foot luminaire grid. Most daylighting tactics have their greatest impact on primary zones.
2) Managing Furniture Heights and Layouts
Although lower partition heights may have a negative effect on acoustics, they help direct daylight to the interior. The California study found that reducing partition heights from 60 inches to 45 inches decreases square-foot energy an average 20% on a baseline wall/window ratio. Reducing heights from 60 inches to 30 inches yields an average 24% savings.
The layout of open-office workstation partitions can be optimized to maximize daylight without creating occupant discomfort. The Daylighting Guide for the Commercial Office recommends that workstation partitions be kept at 42 inches or lower and parallel to the window wall. Higher partitions for privacy (48 inches and greater) should have an orientation perpendicular to the wall (see floorplan below). Such perpendicular panels can be as high as 65 inches without creating shadows.
Replacing opaque panels with transparent or translucent panels also directs daylight to the interior. Light shelves can help bounce sunshine inside, but the California study found that the energy savings are miniscule. Although secondary zones received marginally more daylight, the primary zones received less.
3) Evaluating and Commissioning Lighting Controls
The study also evaluated the performance of dimming vs. switching controls. Generally, a dimming system will save more energy than a switching system when illuminance is adjusted many times during the day due to climate conditions and variable illuminance needs. When the supply of daylight is stable (consistently sunny or overcast and unaffected by shadowing or reflections from other structures), switching controls often saved more energy.
If your daylighting plan includes new controls, they should be commissioned and re-evaluated continuously. When configured incorrectly, controls will either waste energy by not reducing artificial light or cause occupant complaints if illuminance is insufficient. Start with conservative setpoints so occupants gain familiarity with the new lighting, which will avoid possible tampering and requests to disable or override the system.
The best way to assess the potential of a daylighting project is to have a consultant do an analysis with lighting simulation software. The Whole Building Design Guide notes that an analysis can simulate a given climate, predict illuminance levels, determine dimming and switching response, and estimate annual savings.
The Productivity Factor
If you are weighing the benefits of a daylighting retrofit, keep in mind that energy savings may not be the sole outcome. A World Green Building Council report says that workers with access to daylight are 18% more productive. A study in the Journal of Clinical Sleep Medicine found that access to natural light during the work week accommodates circadian rhythm, leading to better sleep with fewer disturbances. Researchers have also found that natural light increases occupants’ alertness and memory.
RESOURCES ON DAYLIGHTING
These resources cited in the article provide more information on daylighting tactics.
Daylighting Plans for Existing Buildings, a free, on demand webinar presented by Mudit Saxena: www.buildings.com
Daylighting, an article in the Whole Building Design Guide (WBDG), a program of the National Institute of Building Sciences: www.wbdg.org/resources/daylighting.php
Daylighting Guide for the Commercial Office, a guide from the Efficiency Vermont nonprofit: www.efficiencyvermont.com
Office Daylighting Potential, a report prepared for the California Energy Commission by the Heschong Mahone Group: www.energy.ca.gov