Case Study 4 - A West-Facing Glass Wall and Four Ways to Spend the Budget
A school in Ahmedabad (hot-dry climate; May afternoons above 42 °C) is adding a library along an 18 m long west facade that faces the playground. The principal wants full-height glazing - a band 18 m long and 2.4 m high, from 0.9 m sill to 3.3 m head - so that children can see the trees and the room does not feel like a box. The architect proposes 6 mm single clear glazing with a 50% reflective film, arguing that the film halves the heat, that the trees will shade the glass within a few years, and that the extra daylight will cut the lighting bill. The committee can fund only one option: (i) the film as proposed; (ii) external vertical fins, 0.6 m deep, at 1.5 m centres; (iii) reduce the glazed band to 1.0 m high with a light shelf and deep reveal; (iv) a 3 m deep open verandah along the facade. They ask for a written recommendation with reasons.
1. Of the architect's three claims, identify the most defensible, the least defensible, and state the assumption each depends on.
Read the worked answer
Most defensible: 'the film halves the heat'. Directionally reasonable, but unsupported as stated. A 50% reflective film typically brings a single clear glazing from an SHGC near 0.85 down to roughly 0.45–0.55 - close to a halving of solar gain through the glass. The claim is defensible only if (a) the figure is the certified SHGC of the actual glass-plus-film assembly, not a visible-light transmittance figure quoted as heat, and (b) it is understood as a percentage reduction of a very large number, not as a small number.
Do the arithmetic that the claim hides. The band is 18 × 2.4 = 43.2 sq m. At a May afternoon west irradiance of about 700 W/sq m, incident power is 43.2 × 700 ≈ 30.2 kW; at SHGC 0.42 the gain is still about 12.7 kW into one room. Halving a large number leaves a large number.
Least defensible: 'the trees will shade the glass in a few years'. It assumes a species with the right growth rate and mature canopy height, planting on school land that will not be built on, a canopy positioned to intercept 3.3 m head height sun in the late afternoon, and no pruning or felling. It is a landscape promise doing the work of a shading device, and it fails exactly in the years the library is new.
Also weak: 'daylight will cut the lighting bill'. West daylight arrives with glare. Occupants will close blinds, and the saving disappears at precisely the hours the claim depends on. The assumption is that daylight quantity and daylight quality are the same thing.
2. Evaluate options (i) to (iv) on heat gain and glare, and recommend one in a way that answers the principal's wish rather than dismissing it.
Read the worked answer
Option (i) - film only. Heat: roughly 12.7 kW peak as computed. Glare: unchanged or worse, because the film darkens the view while the sky behind it stays bright, increasing the luminance ratio the eye must cope with. Rejected.
Option (ii) - vertical fins, 0.6 m deep at 1.5 m centres. Depth-to-spacing ratio = 0.4, so the fins shade only when the sun's azimuth is more than arctan(0.4) ≈ 22° away from the facade normal. On a due-west facade the afternoon sun is close to normal for much of the critical period, so the fins do little when it matters and do help only in late afternoon when the summer sun swings north of west. Partial and expensive. Keep as a supplement, not a solution.
Option (iii) - 1.0 m band with light shelf and deep reveal. Glazed area falls from 43.2 to 18 sq m, a 58% reduction, so peak gain falls to about 5.3 kW before any shading benefit; the shelf bounces light onto the ceiling and shades the glass above eye level. It keeps a view at seated eye height, but only a strip of it, and the deep reveal needs care to avoid a dark slot. Strong, though it narrows the principal's view.
Option (iv) - 3 m verandah. The glazing stays full height, so the principal keeps the view and the sense of openness; the verandah intercepts the low west sun before it reaches the glass and removes most direct gain - a realistic 80–90% reduction of the 12.7 kW, plus a shaded outdoor room the school can use. Cost: 54 sq m of ground and a deeper, more expensive structure.
Recommendation: (iv), with (iii) as the fallback if the site cannot spare 3 m. The verandah answers the principal's actual wish - a generous view and a room that does not feel like a box - without pretending that a film solves a west facade. The argument to make is not 'no glass'; it is 'glass with something in front of it'.
3. What single piece of evidence would most change your recommendation, and how would you obtain it before the committee meets?
Read the worked answer
The decisive evidence is the certified SHGC and visible-light transmittance of the exact glass-plus-film assembly, paired with the actual sun-and-shade survey of the site. Either alone is insufficient.
Why these two.
- A certified SHGC below about 0.25, at visible transmittance above about 0.40, would make option (i) genuinely interesting on heat. But even then, glare would remain: film cannot reduce the luminance of the sky and trees the children are looking at, and that ratio is what causes discomfort, not the total watts.
- A survey showing an existing mature canopy already shading the facade at 15:00–17:00 in May would weaken the case for the verandah, since the shade is already there.
How to obtain it quickly.
- Ask the film manufacturer for the test report for the specified film on 6 mm clear glass; refuse a brochure percentage.
- Visit the site at 15:00 and 17:00 on a clear day with a camera and a tape, and photograph and measure the shaded area on the facade plane. One visit answers the tree question definitively.
- If time permits, a simple hourly simulation of the room gives peak gain and daylight autonomy, but the site visit plus the test report is enough to make a defensible decision.
Framing for the committee. State what would change the recommendation and what would not. Nothing about glare would change it; only measured shade plus a very low certified SHGC would. That is a stronger position than asserting an opinion, and it shows the committee exactly which risk they are carrying.