Designing for Delhi's Climate

Delhi asks a building to do two opposite things. In May it must keep 45°C out. In January it must keep 5°C out, in the mornings, without much sun to help. In July it must handle humidity that neither of those strategies addresses. And through much of the year it must deal with air that is genuinely bad.

This is a composite climate, and the reason so many buildings here are uncomfortable is that they were designed for one season and air-conditioned through the others.

Orientation, which is free

The longer axis running east-west, presenting the long faces to north and south.

North light is diffuse, consistent and carries very little heat. It is the best daylight available and it is essentially free. South can be shaded effectively with horizontal projections, because the summer sun is high.

East and west are the problem. Low-angle sun, morning and afternoon, striking vertical surfaces almost perpendicular. West is the worst: it arrives in the afternoon when the building has already absorbed a day's heat, and horizontal shading does not work against it because the sun is low.

The instruction that follows: minimise the west face, and put the things that do not need comfort against it, stairs, stores, services, circulation. A west-facing bedroom in Delhi is a design failure that the occupant pays for every summer afternoon for the life of the building.

Shading, which is the highest-return element in the envelope

Glass gains heat far faster than wall. An unshaded west window is the single largest thermal liability in most Delhi buildings.

• Horizontal projections work on the south, where the summer sun is high.
• Vertical fins work on east and west, where it is low. Horizontal shades do not.
• Deep reveals, recessing the window into the wall, shade with the building's own thickness and cost almost nothing.
• The jaali, which is the regional answer and remains the correct one. It shades, admits diffuse light, permits air movement and gives privacy simultaneously. Four functions from one element, and it is the reason it appears in this region's architecture for six hundred years.

Shade the glass on the outside. Internal blinds stop light, not heat, by the time the radiation is inside, it is already inside.

Mass and the daily swing

Delhi's dry season has a large diurnal range. A May day can move fifteen degrees between afternoon and dawn.

Thermal mass exploits this: a heavy envelope absorbs heat slowly through the day and releases it at night, flattening the internal swing. Combined with night ventilation, opening up after dark to flush the accumulated heat, it is the traditional strategy and it still works.

The critical qualification: mass only works in the dry months. In the monsoon, when the night is barely cooler than the day and the air is saturated, night flushing does nothing. Delhi needs a building that can switch modes, mass and night ventilation in May, sealed and dehumidified in July, opened up in October.

Designing for one mode and running the others on machinery is what most buildings here do, and it is why energy bills look the way they do.

Ventilation, and its limits

Cross-ventilation needs openings on opposite or adjacent faces with a clear path between. A single-aspect flat cannot cross-ventilate however many windows it has.

Stack ventilation, warm air rising and drawing cooler air in low, works without wind and is what the courtyard and the light well have always done.

But there is now a constraint that classical practice did not face: for a significant part of the Delhi year, the air outside is worse than the air inside. A design that depends entirely on opening windows is a design that cannot be operated in November. The honest contemporary answer is a building that ventilates naturally when the air permits and filters mechanically when it does not, with the envelope tight enough that the second mode actually works.

Six mistakes we see repeatedly

1. Unshaded west glazing, usually because of a view.
2. Single-aspect flats marketed as cross-ventilated.
3. Full-height glass on a residential envelope in a composite climate. It is a specification for a different latitude.
4. Dark external surfaces on the west and roof.
5. An unshaded, uninsulated roof, the single largest gain surface on a low-rise building, and the cheapest to fix.
6. Air conditioning as the design strategy rather than as the fallback for the weeks when passive measures genuinely cannot cope.

The order of operations

Orientation first, because it is free. Then shading, because it has the highest return per rupee. Then envelope and mass. Then ventilation strategy. Then, and only then, size the mechanical systems, against a building that has already done as much as it can on its own.

Done in that order, the plant gets smaller, the running cost falls for the life of the building, and the rooms are pleasant on the days when the power is out.

Done in the reverse order, which is the common order, you get a glass building with a large chiller and an occupant who never opens a window.