Daylight & Shading

Daylight earns its place on two independent grounds - energy and wellbeing - and both are substantial. On energy, artificial lighting is a large slice of a building's electricity use, and in commercial buildings it also adds internal heat that the cooling system must then remove, so good daylighting saves twice: less lighting energy and less cooling to reject the lighting's heat. A well-daylit space with daylight-linked controls (dimming or switching as the sun rises and falls) can cut lighting energy by a large margin across the year - the strategy captured in metrics like daylight autonomy, the percentage of occupied hours a space is lit by daylight alone.

On wellbeing, the evidence is strong and growing. Access to daylight and views is linked to better mood, alertness and sleep (through the body's circadian response to bright morning light), faster recovery in hospitals, better learning outcomes in schools, and higher satisfaction and productivity in offices. This is why standards increasingly treat daylight as a health requirement, not a luxury: the European daylight standard EN 17037 sets recommended levels and view access, and green rating systems from LEED and BREEAM to GRIHA, IGBC and the WELL Building Standard all award credits for daylight and views.

The measure that ties it together is the daylight factor - the indoor daylight level as a percentage of the outdoor level under an overcast sky - with roughly 2% marking a usably daylit space and 5% a generously bright one, though modern practice increasingly uses climate-based metrics that account for the real sun and sky over a year. The point for the designer is simple: daylight is not decoration. It is a measurable energy saving and a measurable health benefit, and it deserves to be designed with the same rigour as any other system.

Good daylighting is about getting light deep and evenly into a space, not just making big windows, and a few robust rules of thumb govern it. The most important is that daylight penetration depends on window head height, not window area: daylight reaches roughly 1.5 to 2.5 times the height of the top of the window into a room. A tall window high in the wall daylights a deep room far better than a wide window low down of the same area - which is why ceiling height and window head height are such powerful daylighting levers. Beyond about twice the head height from the window, a single-sided room falls into gloom and needs a second source.

That is why deep plans are the enemy of daylight: a floor plate too deep to daylight from its perimeter commits the core to permanent electric light, one of the quiet reasons big boxy buildings use so much energy. The strategies to get light deeper are worth knowing. A light shelf - a horizontal element partway up the window - bounces sunlight off its top surface onto the ceiling and deep into the room, while shading the area just inside the glass and evening out the distribution. Clerestory windows and rooflights / monitors bring daylight down into deep or single-storey spaces from above. Atria and light-wells daylight the centre of deep buildings. Splayed reveals and light-coloured interior finishes spread and multiply what comes in.

Orientation shapes the quality too. North light (in the northern hemisphere) is soft, even and nearly free of glare and heat - the reason artists and studios prize it. South light is abundant and, crucially, controllable with a horizontal overhang. East and west light is low, direct, glaring and heat-laden - good for neither comfortable daylight nor easy shading. So the daylighting strategy dovetails exactly with the orientation logic of lesson 1.2: gather daylight from the north and the shadeable south, and be sparing and careful on east and west.

The number that captures the daylight-versus-heat trade is the window-to-wall ratio (WWR) - the proportion of a facade that is glazed. Push it up and you gain daylight and view; push it too far and you gain unwanted solar heat in summer, heat loss in winter, and glare, because glass is thermally the weakest part of any wall. The fully glazed curtain-wall tower is the anti-pattern this whole module argues against: it maximises the worst of both worlds in most climates and can only be kept comfortable by brute mechanical force.

There is no single right WWR - it depends on climate, orientation and the shading and glass you pair with it - but useful ranges exist. For hot Indian climates, a moderate WWR of roughly 15-40%, concentrated on the well-shaded north and south and minimised on east and west, typically balances good daylight against manageable heat. India's residential energy code, Eco Niwas Samhita, encodes this directly through the Residential Envelope Transmittance Value (RETV) and openable-area limits, and the commercial code ECBC caps effective glazing performance by climate zone - both are really formalising the daylight-versus-heat balance.

Crucially, WWR interacts with two glass properties you specify together. Solar Heat Gain Coefficient (SHGC / g-value) is the fraction of solar heat the glazing lets through - low SHGC glass (with a spectrally selective low-e coating) rejects heat while staying clear. Visible Light Transmittance (VLT) is the fraction of visible light it passes. The prize is glazing with a high VLT and a low SHGC - lots of light, little heat - and the ratio of the two (the light-to-solar-gain ratio) is a good single figure of merit. But glass coatings only reduce heat gain; they never beat keeping the sun off the glass in the first place, which brings us to shading.

Shading is where daylight design is won or lost, and the first principle is decisive: external shading beats internal shading, every time. A shade outside the glass stops solar radiation before it enters the building; a blind or curtain inside stops it only after it has already passed through the glass and become heat in the room. External shading can cut solar heat gain through a window by a large fraction - often more than half - while internal blinds do far less for heat (though they remain useful for glare and privacy). So the primary move is always to shade outside.

The correct external device depends on the sun's geometry at each orientation, which is why there is no universal shade. On the south facade the summer sun is high, so a horizontal overhang (or a brise-soleil of horizontal louvres) sized to the sun angles blocks the high summer sun while still admitting the low, welcome winter sun - a single element that shades in summer and heats in winter. On east and west the sun is low and comes in side-on under any horizontal element, so horizontal overhangs are nearly useless; the answer is vertical fins, deep reveals, screens (the traditional jaali), operable shutters, or planting - or, best of all, simply having few openings there. Fully adjustable devices (movable louvres, external roller shades) handle changing conditions but cost more and need maintenance. The traditional chhajja - the deep projecting eave of Indian architecture - and the perforated jaali screen are climate-tuned shading devices refined over centuries.

And shading must be reconciled with the daylight and glare it governs. Over-shade and you plunge the room into gloom and switch the lights back on, defeating the purpose; under-shade and you overheat and dazzle. Glare - excessive brightness contrast, especially from low sun or a bright sky in the field of view - is a real failure that drives occupants to close blinds permanently, killing both daylight and view. It is controlled by shading, by placing and shaping openings well, by daylight-diffusing devices like light shelves and jaalis, and by bringing daylight from more than one direction so no single bright patch dominates. The whole module comes to rest here: read the climate, set the form, run the passive systems, and then light the building with free daylight while refusing the heat and glare - a bright, comfortable, low-energy space that the people inside feel the benefit of every day.