Lesson 4.3Lesson 4.3 · Composite and Temperate Strategies
Selective Mass
Don't ask how much mass — ask where it sits, whether the sun can charge it, and whether you can flush it.
How much mass should a Delhi wall have?
The contradiction this course has circled: in 2.1 thermal mass was the hot-dry hero, storing the day and releasing it into a cool night flush; in 3.3 the same mass was the humid-coast villain, a night radiator with no cool night to discharge into. So how much mass should a Delhi wall carry, when Delhi is hot-dry in May and warm-humid in August? "How much" is the wrong question — a fixed quantity cannot be right for both seasons. The right questions are where the mass sits, whether the sun can charge it, and whether you can flush it. Mass that is shaded, internal and flushable helps in the dry summer while doing almost no harm in the humid monsoon. Selective mass is not a quantity. It is a strategy of placement and control.
Selective mass is an old instinct — the haveli kept its mass in deep-shaded internal walls, never baking on an exposed west face.
Break the charge–discharge link and the problem disappears
Mass hurts in the monsoon for exactly two reasons. It gets *charged* — the sun strikes it, or warm humid air soaks into it. And it *can't discharge* — a warm night gives it nowhere to dump that heat, so it radiates inward all night. Break either link and the problem mostly vanishes.
Shade the mass so the sun never charges it, and place it internally so it couples to the controlled indoors rather than the hot outdoor air, and it stays near a comfortable temperature year-round — a stabiliser, not a store. Make it flushable when a cool night does arrive, common in the composite spring and autumn, and it earns its keep in the seasons that reward it.
Walk the three placements through both seasons. External, sun-struck mass stores and lags in the dry summer but is risky, and in the humid monsoon it fails badly — it charges in the sun and re-radiates at night. External but shaded mass still helps in summer (it stores coolness and can be flushed) and is only mild in the monsoon — less charging, but still coupled to warm outdoor air. Internal, shaded mass helps in summer by stabilising and flushing, and is neutral in the monsoon — it stays near indoor temperature and does little harm. The winning move is the third: shaded, internal, flushable mass — a flywheel the sun cannot wind up, sitting at indoor temperature, actively dischargeable. The summer benefit of 2.1 without the monsoon penalty of 3.3. The old haveli knew this: its mass lived in deep-shaded internal north walls and around the courtyard, never baking on an exposed west face.
Stop asking how much mass. Ask whether the sun can charge it and whether you can flush it. Those two questions decide everything.
Controlling mass, not just placing it
Placement is the first lever; control is the second. Three controls turn a fixed lump into a managed device.
Shading control — deep eaves, the sun geometry of 1.1, operable shutters — decides whether the sun can charge the mass at all. Set it to admit the low winter sun onto internal mass while excluding the high summer sun. Ventilation control — the night-flush openings of 2.4 — decides whether you can discharge the mass on a cool night. Coupling control — insulation placed *outside* the mass, so the mass faces the room and not the weather — decides whether it tracks indoor or outdoor temperature.
Together they let you charge the mass only when you want winter warmth or summer-night coolness stored, and isolate it when you want it to do nothing at all. Mass becomes one more thing the switching building switches.
Shading, ventilation, coupling — three switches that turn a dumb wall into a managed battery.
Three altitudes on the same idea
Read the band that fits you — or all three.
Most Indian homes are already heavy — RCC frame, brick walls. So your job is not to add or remove mass; it is to keep the sun off the mass you have. Deep shades, light external colours, shaded internal walls — so the walls don't bake all day and then radiate heat at you on a warm night. Shaded, cool mass is a quiet helper; sun-roasted mass is a night heater you never asked for. Shade the walls, and open the house up on cool nights to let the mass cool down.
Treat mass as a controllable subsystem, not a fixed quantity. Place the primary mass internally and shaded — party walls, internal cores, courtyard walls, north faces. Externally insulate any sun-exposed mass so it couples to the interior rather than the weather. Size shading to admit the winter and exclude the summer sun onto any passive-solar mass (1.1). Provide night-flush paths (2.4) for spring, autumn and dry-summer nights. Above all, avoid the worst combination: heavy, sun-struck, un-flushable external walls. Charge the mass only when a cool night or winter sun makes it useful; isolate it otherwise.
The monsoon penalty (the decrement factor f and time lag phi from 3.3) only matters if the mass actually charges, and charging is dominated by absorbed solar radiation: q_solar = alpha * I, where alpha is surface absorptance and I the incident irradiance. Shade the surface and I collapses toward the much smaller diffuse value. An exposed west wall sees roughly I = 500-600 W/m^2 direct in the late afternoon; shaded, it sees only I = 50-100 W/m^2 diffuse — so charging heat drops about 5 to 10 times. A *large shaded internal* mass therefore charges far less than a *small sun-struck external* one. Placement beats quantity: f and phi describe what mass does with the heat it receives, but shading and placement decide whether it receives any heat at all.
“In a mixed climate, just use a medium amount of mass — not too heavy, not too light.”
Run the method yourself
Run the placement test on a real plan before the next lesson.
- 1Test all three placements — external sun-struck, external shaded, internal shaded — in both seasons. Which single placement is good in summer and harmless in the monsoon?
- 2Explain why external sun-struck mass fails in the monsoon, in one sentence about charging versus discharging.
- 3For a real heavy RCC home, identify one afternoon-baked wall. How would you shade it or externally insulate it to neutralise its night-radiator effect?
- 4List the three controls — shading, ventilation, coupling — and say what each one switches on or off for the mass.
↳ Use the worksheet below to record your answers.
Take it with you
Mass is a behaviour to control, not a quantity to tune
We've now made air, courtyard and mass switch with the seasons. The last and trickiest element is glass — the one place the envelope is deliberately weak. In a composite climate a window must do contradictory jobs at once: admit the low winter sun for warmth and daylight, reject the high summer sun, and still serve monsoon ventilation. The final Module 4 lesson tackles the composite façade — shaded glazing tuned, like everything else here, to be selective by season.