Good Daylighting Reduces Lighting Energy Use and Improves Occupant Wellbeing - Bad Daylighting Just Creates Glare
Daylighting is among the highest-value passive design strategies available to architects: it reduces artificial lighting energy consumption (which accounts for 20-30% of commercial building energy use), reduces cooling loads associated with artificial lighting heat gain, and improves occupant wellbeing, productivity, and circadian health. But daylighting done poorly - oversized south or west windows without shading, skylights without diffusion, clear glass facades with direct solar beam penetration - creates disabling glare that occupants remedy by closing blinds, eliminating any daylighting benefit entirely. Effective daylighting design requires understanding how light enters the building, how it is distributed, and how to separate the desired diffuse daylight component from the undesirable direct-beam solar radiation. The ARE PPD tests daylighting design as part of the sustainable design and building performance content area.
Sidelighting
Sidelighting is daylighting through vertical windows in exterior walls. Key principles: (1) Daylit zone depth: Effective sidelighting typically penetrates to a depth of approximately 1.5-2.5 times the head height of the window. A window with its head at 9 feet provides useful daylight to approximately 15-22 feet from the window. Beyond the daylit zone, artificial lighting is required. (2) Window head height: Higher windows penetrate daylight deeper into the space; raising the window head from 8 to 12 feet significantly increases daylit zone depth. (3) Orientation: North-facing windows provide consistent, non-direct diffuse daylight (excellent for studios, galleries); south-facing windows provide direct sun in winter (valuable for passive solar heating) but require shading for summer glare control; east and west windows deliver low-angle morning and afternoon sun that is nearly impossible to shade without blocking the view - these require the most careful shading design.
Light Shelves
A light shelf is a horizontal reflective surface mounted on the exterior or interior of a window (or both) at approximately the height of the window's midpoint. The light shelf shades the lower portion of the window from direct sun (reducing glare at workplane level) while reflecting sunlight from its top surface upward onto the ceiling. The ceiling then becomes a secondary light source that distributes light deeper into the space. Light shelves work best on south facades with relatively high sun angles and require the ceiling surface to be highly reflective (white or light-colored). They are ineffective on north facades where there is no direct sun to redirect.
Toplighting
Toplighting - skylights, roof monitors, and sawtooth roofs - provides daylight from above and can achieve more uniform distribution across a deep floor plan than sidelighting alone. A flat skylight admits more total daylight but also admits direct solar beam radiation (heat and glare); a diffusing glazing or diffusing light well below a clear skylight converts direct sun to diffuse light. Roof monitors (raised roof sections with vertical glazing) and sawtooth north-facing glazing provide diffuse north light without direct sun penetration - highly effective for uniform daylighting of large floor areas (factories, art studios, retail).
Key Exam Points
- Sidelighting daylit zone depth: approximately 1.5-2.5x window head height from window.
- Higher window head height = deeper daylit zone penetration.
- North glazing: consistent diffuse light; no glare; preferred for art galleries and studios.
- South glazing: direct sun in winter; requires horizontal shading (overhangs) for summer control.
- East/west glazing: low-angle sun nearly impossible to shade effectively; most challenging orientation.
- Light shelf: shades lower window; redirects sun to ceiling for deeper penetration; effective on south facades only.
- Toplighting: skylights and roof monitors; more uniform distribution for deep plans; diffusing glazing reduces direct beam glare.
AREprep's PPD daylighting content covers aperture design, orientation strategy, shading devices, light shelf design, and daylighting metrics (daylight factor, illuminance) - giving ARE PPD candidates the daylighting design knowledge the exam tests in building performance and passive design strategy questions across multiple building types and climate contexts.
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