Light
Daylight
Natural light admitted into a building, and the design decisions governing how much arrives, from which direction, and at what times of day. Distinct from a view, and distinct from the solar heat that travels with it.
In more detail
Daylight is natural light admitted into a building, and the design that determines how much arrives, from where, and when. It varies by orientation: north light is diffuse and stable, south light is strong and seasonally variable and can be shaded predictably, east and west light is low-angle, intense and difficult to control.
It is measured several ways because there are several questions. Illuminance in lux answers how much light is on a surface at a moment. Daylight factor expresses interior illuminance as a percentage of available exterior illuminance under overcast conditions. Annual metrics such as spatial daylight autonomy, defined in IES LM-83, describe what share of a floor area meets a light level for a given fraction of occupied hours across a year — the closest available answer to whether a room is genuinely daylit.
Daylight arrives with two companions. One is solar heat, governed by glazing SHGC and shading, which is why glazing chosen to maximize light can also raise cooling load. The other is glare, a function of contrast and luminance in the field of view rather than of quantity — which is why a very bright room can be uncomfortable and a well-daylit one is usually shaded.
Why it matters
Daylight is the largest and cheapest source of light a house has, and effectively the only one available at the intensities relevant to morning light exposure. It is also settled almost entirely at design: window placement, sizing, orientation, glazing selection, overhangs and interior openness. No fixture specification recovers a floor plan that put the primary living space on the north side with small openings.
The common misunderstanding
That more glass means better daylight. Large unshaded openings produce glare, overheating and rooms whose blinds stay closed — which is how a heavily glazed house ends up dimmer in practice than a modest one with well-placed, well-shaded windows. Daylight quality is a matter of distribution and control, not glazing area.
Related terms
On Compound
Where this term does real work — and where the decisions get made.
Sources
Last reviewed July 2026 by Compound Editorial. Definitions are educational and not project-specific engineering or medical advice.