Shades Down by Eleven
An 1,800 m² open office has a 6 m deep floorplate and a continuous south curtain wall 2.4 m high with a 3.6 m ceiling. Daylight autonomy is modeled at 62% of occupied hours above 300 lux, yet a shade survey in March finds 78% of shades fully down by 11 a.m. Electric lighting is 4000K linear troffers delivering 480 lux horizontal at the desk and 85 lux vertical at eye level at 10 a.m. Occupants complain of glare on screens and "flat" lighting; several report afternoon sleepiness. The only lighting control is a master switch at the core. Lighting is 28% of plug load in the energy model. The tenant will not fund a facade replacement.
1. Daylight autonomy is 62% yet shades are down by 11 a.m. Diagnose the discrepancy. What would you measure?
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Diagnosis: daylight autonomy measures sufficiency, not comfort. The building has enough light and too much glare.
Likely contributors, in order of plausibility:
- Unshielded window luminance. A 2.4 m south curtain wall with no exterior shading produces very high sky and sun luminance in the occupant field of view; people close shades to stop the glare, not because the room is dark.
- No zone or occupant control. A single master switch means the only local control is the shade, so shades become the de facto lighting control.
- Electric light that fights daylight. 480 lux horizontal from troffers plus daylight pushes illuminance high near the window, increasing contrast and discomfort.
What to measure
- Vertical illuminance at eye level facing the window, at 2 m and 4 m from the glass, hourly.
- Luminance of the window and sky (HDR imaging or a luminance meter) to compute DGP or DGL.
- Shade position logs, not just a one-day survey.
- Occupant survey with a glare question and a satisfaction scale.
What would change the diagnosis: if vertical eye illuminance is low and window luminance is moderate, the problem is contrast and electric-light distribution rather than glare, and the fix shifts to dimming and zoning.
2. Propose a layered strategy covering shading, electric light, and controls. Prioritize it.
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Layered strategy, in priority order:
- Control the source of glare. Exterior shading (louvers, fins, or a screen) on the south facade is the most durable fix, but the tenant won't fund facade work. Interior roller shades with a tight weave and side channels are the fallback; automated shades on a sun sensor with manual override are better than manual only because the survey shows people default to closed.
- Zone the electric light by depth. Separate the perimeter 4.5 m from the core, add dimming, and let the perimeter dim to supplement daylight instead of adding 480 lux on top of it.
- Set a vertical-illuminance target, not just a horizontal one. Aim for meaningful morning exposure at eye level (on the order of 100–150 lux vertical, higher is better) while keeping horizontal task light adequate. This addresses the afternoon sleepiness complaint without over-lighting the whole floor.
- Give occupants local control of at least the perimeter lighting zone; a master switch is not control.
Do not simply add more electric light. The complaint is glare and flatness, and more troffer light would worsen both.
3. What metrics would you track to demonstrate improvement without being misled by a single good day?
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Track four things:
- Shade position: percentage of shades more than half-closed at 10 a.m., 1 p.m., and 3 p.m., sampled weekly for a season. Success is a sustained drop, not a one-day reading.
- Glare: DGP below about 0.35–0.40 in the perimeter zones at the worst hours, or a measured drop in window luminance.
- Circadian-relevant light: vertical illuminance at eye level in the morning, reported as a distribution across the floorplate, not a single point.
- Satisfaction: the same survey before and after, with separate glare and brightness questions so the two do not cancel out.
Report by zone; a floorplate average would hide the perimeter problem that started this.