Fins, Glass or Panels
An 8,000 m² office block in a hot-dry city has a fixed site with its long axis running east–west, leaving about 3,000 m² of glazing concentrated on the east and west façades. The team must clear 20% energy savings. Option A: high-performance glazing at SHGC 0.22, USD 520,000 extra, modelled at 24% savings on its own. Option B: external shading on the east and west façades, USD 180,000, modelled at 12%. Option C: a 300 kWp rooftop PV array at USD 0.85/W installed, about USD 255,000, offsetting roughly 18% of annual consumption. Option D: roof insulation, USD 60,000, modelled at 4%. The client will fund only one option plus, at most, one small add-on, and wants the lowest 20-year cost.
1. Which option or combination do you recommend, and what does a simple capital-cost comparison miss?
Read the worked answer
Recommendation: B + C. USD 435,000 for roughly 30% modelled savings - cheaper than A alone (USD 520,000 for 24%) and with real margin over the 20% threshold. Neither B (12%) nor C (18%) nor D (4%) clears the bar alone; B + D reaches only about 16%.
What the simple comparison misses:
- Peak versus energy. PV offsets kilowatt-hours but changes nothing about demand. In a hot-dry climate with demand charges and a constrained grid connection, shading reduces the cooling peak, which can shrink chiller capacity and the electrical service - a capital saving that never appears in a cost-per-kWh table.
- PV is not free to own. Inverter replacement around year 10–12, and soiling in a dusty climate can cost 5–15% of yield if the array is not cleaned on schedule. Both need to be in the 20-year cost.
- Tariff and carbon risk. Shading is a physical, permanent reduction; PV's value moves with tariffs, export rules and grid carbon intensity.
- Margin. A 24% modelled result has four points of headroom for value engineering; a 30% result has ten.
2. The shading supplier's drawing shows continuous overhangs on the east and west façades. What do you say, and why does it matter to the modelled 12%?
Read the worked answer
Reject it. Overhangs are the right device for high-altitude sun - principally south façades in the northern hemisphere - but east and west façades receive low-angle sun in the morning and late afternoon, when the solar altitude is shallow enough to pass straight under a horizontal overhang. The shading would look substantial on the elevation and deliver a fraction of the modelled benefit.
What east and west actually need:
- Vertical fins or louvres, which intercept low-angle azimuthal sun; or an egg-crate combination where both altitudes matter.
- Reduced window-to-wall ratio on those façades, if the design can take it - the cheapest square metre of glazing is the one you do not build.
- Higher-performance glass on east and west only, which is often better value than upgrading all 3,000 m² uniformly.
Why it matters: the 12% figure was derived from a shading geometry. If the installed geometry cannot intercept the sun the model assumed it intercepted, the delivered savings collapse and the B + C package may no longer clear 20%. The shading model must be geometric and orientation-specific, and the specification must match the model - this is precisely the kind of gap an auditor finds between drawing and report.
3. The site orientation was fixed before the design team was appointed. How would you express what that constraint cost, and what would you recommend for the next project?
Read the worked answer
Quantify it as the cost of the compensating package: the east–west long axis generates roughly USD 435,000 of shading and PV that a better orientation would have made largely unnecessary, plus the additional cooling plant capacity and its energy draw over twenty years. That figure - not a vague statement about orientation mattering - is what a client can act on.
Put crudely: if the developer had owned two sites, the correctly oriented one would carry a materially lower capital and operating cost, and that difference belongs in the site-selection appraisal, not in the post-rationalisation.
For the next project, the recommendation is a sequence, not a menu:
- Orientation and massing - free, and decided in week one.
- Window-to-wall ratio and shading - cheap, and decided at concept.
- Envelope performance - moderate cost, decided at scheme design.
- Efficient plant - moderate cost, decided at detailed design.
- Renewables - the most expensive way to buy a kilowatt-hour, and the only one that should be reached for last.
The judgement being tested is that a bioclimatic decision made before the sustainability consultant is appointed can outweigh every measure in the model.