Daylighting Credit vs. Glare Complaints in a South-Facing Open Office
A 30,000 ft² open office in a hot-dry climate has a fully glazed south facade (U-0.28, SHGC 0.25, VLT 0.40) with no exterior shading. The design team claims a daylight autonomy above 55% of occupied hours for the perimeter zone and is pursuing a daylighting credit. After occupancy, the facilities team logs 60+ glare complaints in the first three months, concentrated 11:00-15:00, and occupants have taped cardboard over the lower glazing. Measured illuminance at the 2.5-ft workplane near the window reaches 4,000-6,000 lux on clear afternoons; interior partitions block daylight beyond ~18 ft. The building has manual roller shades that occupants rarely adjust. The owner wants to keep the daylighting credit and stop the complaints without a facade replacement.
1. Explain why the modeled daylight autonomy and the actual occupant experience diverge, and which metric is missing from the design claim.
Read the worked answer
Why they diverge: Daylight autonomy counts hours above a minimum threshold; it is blind to over-illumination and to luminance contrast. A south facade with VLT 0.40 and no shading can easily clear the autonomy threshold while producing workplane illuminance of 4,000-6,000 lux - 4-6× the 300-500 lux target - and sky/window luminance far above the ~2,000-2,500 cd/m² discomfort-glare range. The design optimized for enough light, not for comfortable light.
Missing metric: Annual Sunlight Exposure (ASE), typically the 1,000 lux / 250 occupied-hours criterion, which flags exactly this over-illumination condition. A credible daylight claim pairs sDA/DA with ASE (and ideally glare probability, e.g., DGP). The design should have failed an ASE screen at the perimeter.
Second-order cause: The glazing choice traded VLT down to 0.40 to control SHGC, which darkens the space overall while the direct sun patch still delivers extreme local illuminance - the worst of both worlds. That mismatch is why occupants reach for cardboard instead of adjusting shades: the shades are manual and the glare episodes are intense but intermittent, so behavior never stabilizes.
2. Recommend a no-facade-replacement mitigation package and state how you would verify it meets both the credit and the comfort goal.
Read the worked answer
Mitigation package (layered, cheapest-first):
- Exterior shading on the south - a light shelf or horizontal louvers/fixed fins sized to cut the high-sun-angle direct beam at 11:00-15:00 while bouncing light to the ceiling for depth. This attacks the root cause (direct sun) rather than filtering it after entry.
- Automated interior roller shades with a closed-loop glare/sun sensor and manual override, zoned by facade orientation and depth. Automation is essential - the current manual shades fail because nobody adjusts them.
- Re-commission the shading control logic so shades deploy on direct-sun incidence, not just on a fixed schedule or exterior illuminance alone.
- Interior light shelf / high-reflectance ceiling to push daylight past the 18-ft partition line and reduce the perimeter-to-core contrast that drives complaints.
Verification:
- Re-run the annual simulation reporting sDA/DA and ASE together - the credit should be re-justified only if ASE drops below the 250-hour threshold while sDA is retained.
- Post-occupancy spot measurement at the 2.5-ft workplane near the window on a clear afternoon: target illuminance in the ~300-500 lux band with no sustained values above ~2,000 lux at the workplane.
- Luminance meter check of window/sky luminance against the ~2,000-2,500 cd/m² discomfort threshold.
- Complaint tracking for 90 days as the acceptance criterion; removal of the cardboard is the practical pass/fail signal.