The depth rule
Daylight penetrates usefully to about two and a half times the height of the window head. A window topping out at 2.2 m lights the first five metres properly, and no further.
Practical consequence: raising an existing window by thirty centimetres gains more light at the back of a room than widening it by a metre. Head height matters more than glazed area.
The four orientations
- North: constant, cool, shadowless light. The orientation of painters' studios — it falsifies no colour.
- East: grazing light in the morning, nothing in the afternoon. Excellent for a bedroom, difficult for an office.
- South: abundant, but high and hard in summer, low and penetrating in winter. An overhang solves summer without losing winter.
- West: warm, low light at day's end, glaring, with a heavy thermal load. The hardest orientation to live with in summer.
A dual-aspect room beats a heavily glazed single-aspect one. Two sources at different angles remove hard contrast and give volume, at equal glazed area.
Correcting without building work
Matt white ceiling — the largest reflecting surface. Curtains that clear the glazing entirely. A mirror perpendicular to the window, never facing it. Matt rather than gloss finishes, which diffuse instead of returning a point reflection.
And check what is outside: an evergreen tree or a repainted light-coloured wall changes interior light more than any paint choice.
our piece on sensory design sets out the four levers that act on the body.