Capturing Solar Gain

For four modules the sun was the enemy to be shaded; in Module 5 it becomes the friend to be captured. Where the desert flushed its mass with cool night air, the Himalayan night is the thief to be sealed against. Where the humid coast opened up to ventilate, the cold house closes up to hold its warmth in. Where the warm-humid plan spread out to chase the breeze, the cold house pulls compact to minimise heat loss. And where every hot zone put large openings on its hottest faces, the cold house concentrates its large glazing only on the sunny south.

The first and most powerful cold strategy is direct solar gain: sunlight enters through south glazing, strikes the floor and walls, and is absorbed and converted to heat directly in the living space. It is the simplest solar heating imaginable — no machinery, just glass facing the right way and a surface to soak up the sun. Done well, it carries a Himalayan house through a sunny winter day with little other heating — transformative where fuel is scarce, expensive and polluting. The traditional Ladakhi house already knows this: it opens to the south with timber-framed glazed balconies, the rabsal, and turns a blank insulated back to the cold north. The building leans toward the sun, like a plant.

Picture a Leh room in January. Set the glazing area and its orientation, and watch the low winter sun pour through the glass onto the floor inside. A readout tracks the free solar heat collected over the day — and the cost of facing the wrong way.

Swing the same window to the east and the daily total roughly halves: it catches only the weak morning sun and sits dark through the strongest hours. Swing it to the north and the gain collapses to almost nothing, while the glass keeps leaking heat all night — a pure liability. Drag the glazing area up and the daytime gain rises, but so does the night-time loss the next lesson will reckon with.

This is not a one-lesson toy. The same capture-and-orientation logic returns in 5.2 (holding the heat in) and 5.3 (Trombe walls and sunspaces), where the glass is paired with mass and a controlled buffer.

Direct gain has three working parts. The glazing faces true south and is ideally night-insulated (5.2). The absorber is a dark, exposed, sun-struck surface — a stone or concrete floor, a masonry wall — that converts incoming sunlight to heat. A sunbeam landing on a rug warms only the rug; landing on dark stone, it charges a thermal store.

And the third part is mass, which returns here as your ally. The same heavy material that was a humid-coast villain — radiating its stored heat into the bedroom all night — is here a hero, soaking up the day's solar gain and releasing it slowly through the long freezing night. The cold house wants heavy, sun-charged, internal mass: the exact opposite of the lightweight coastal envelope, and a return to the desert's love of mass — but now charged by the sun, for warmth instead of coolness.

For this to work, the sun must actually reach the mass. Cover the sun patch with a thick carpet or a sofa and you have insulated your battery from its own charger.