A South Facade That Behaves Differently on Every Floor
A six-story, 150,000 sf office building in Climate Zone 4A occupies a corner site. The south facade is 45 percent vision glass (SHGC 0.32, VLT 0.40); the east facade is 70 percent glass. A 10-story neighbor stands 40 ft to the south and shades the lower three floors until roughly 11:00 a.m. After a year of occupancy, tenants on floors 2 and 3 complain of afternoon glare and heat, while the floor 6 south tenant is uncomfortably warm on sunny winter afternoons. Air handlers and the central plant are at 95 percent of design airflow. The owner will release $900,000 for envelope work only if it resolves the complaints without changing the building's appearance at street level.
1. Why does identical curtain wall produce glare on floors 2–3 and winter overheating on floor 6? What is the first analysis you would run before anyone orders glass or shades?
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Three variables differ by floor: incident radiation, depth of penetration, and air-side response.
- Floors 2–3: the neighbor's shadow line lifts off the facade around 11:00, so these floors take direct sun from mid-morning to sunset at progressively lower altitude. Low-angle sun lands deep on work surfaces and in occupants' field of view. Glare is a luminance problem, not an energy problem - SHGC 0.32 does nothing about it.
- Floor 6: never shaded. At this latitude winter solar altitude drops to roughly 25 degrees, so radiation passes nearly horizontally through 45 percent glass and lands 20–30 ft inside, adding load to a zone whose VAV box is probably at minimum flow with reheat open. That is simultaneous heating and cooling, not a glazing defect.
First analysis: an hourly, full-year shadow and insolation study on the actual massing (Radiance or EnergyPlus), paired with zone-level trend logs of supply airflow, reheat valve position, and space temperature. The simulation predicts when and where; the trends show whether the air side is amplifying it. Do not specify product until the two agree.
2. The owner has $900,000. Compare interior roller shades, exterior shading on floors 4–6, and re-glazing the south facade at SHGC 0.22 / VLT 0.30, then recommend an allocation.
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$900,000 over 150,000 sf is about $6/sf of floor area - roughly $13/sf of south vision glass. That buys one measure well, not three.
- Interior roller shades run $10–15/sf of glass. They cut glare only when deployed, occupants will not deploy them consistently, and they do not address the winter gain problem at all. Low first cost, low effectiveness.
- Re-glazing at SHGC 0.22 / VLT 0.30 runs $45–70/sf of glass. It reduces but does not stop low-angle winter gain through 45 percent glass, and VLT 0.30 pushes daylight autonomy below useful levels, trading a glare complaint for an electric-lighting complaint.
- Exterior shading on floors 4–6 south intercepts radiation before it becomes a load and is typically three to five times more effective per dollar at that job. Set the cut-off angle from the shadow study, not a rule of thumb. Starting at level 4 also preserves the street-level appearance constraint.
Recommendation: exterior shading floors 4–6 south; interior shades only on floors 2–3 for glare control; and a no-cost reduction of VAV minimum flow using the ASHRAE 62.1 multiple-space equation to stop the reheat fight.
3. After the work is installed, how do you demonstrate that the winter condition on floor 6 is resolved, and what do you tell the tenant if it is only partly resolved?
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Establish the acceptance criteria before installation, not after. Baseline two weeks of data now: zone temperature, supply airflow, reheat valve position, and a globe thermometer reading at three occupied workstations between 1:00 and 4:00 p.m., recorded alongside outdoor conditions.
After installation, repeat the same two weeks with matched weather where possible, plus a short-term measurement of direct solar irradiance at the glass to confirm the shading is performing as modeled. Report unmet occupied hours above 76 F as a percentage and compare to the model's prediction. A credible target is a large reduction, not zero - the air handlers are already at 95 percent of design flow, so the envelope cannot be expected to erase a marginal capacity problem.
If the result is partial, say so plainly and separate the two causes: the solar gain is now reduced, but the zone minimum airflow and reheat sequence remain uncommissioned. Present the remaining action as a controls scope with a measured payback rather than a design failure, and put a date on the retest. Never promise comfort that the mechanical system cannot deliver.