Why does daylight fall off so quickly from a window?

Because the window is a limited aperture and light spreads. A position twice as far from the glass sees a much smaller apparent area of sky, so the daylight reaching it drops far faster than distance alone suggests. This is why a deep floor plate has a bright perimeter band and a middle that depends on artificial light regardless of how good the glazing is.

What affects it most?

Window area and head height, first. A window's head height matters disproportionately — light entering high up travels further into the room than light entering low down, which is why a tall window daylights a deeper zone than a wide one of the same area. Then glass transmission, external obstructions, internal surface reflectance, and anything blocking the path such as cabins along the facade.

Does higher daylight factor always mean a better space?

No. Daylight factor says nothing about glare, direct sun or heat. A perimeter with a high figure and unshaded east or west sun can be unusable at certain hours, and the blinds get closed permanently — at which point the daylight is gone anyway. It has to be read alongside orientation, shading and solar control.

How does it relate to energy?

Daylight only saves energy if the lighting responds to it. A well-daylit perimeter with lights on a single switch for the whole floor saves nothing. Daylight-linked control — dimming or switching the perimeter row as daylight rises — is what converts daylight into reduced consumption, and ECBC 2017 addresses lighting and control provisions for commercial buildings.

What does the layout have to do with it?

A great deal, and it is the part most within a fit-out's control. Enclosed rooms along the window line stop daylight at that line and leave the open floor behind them on artificial light all day. Placing enclosed rooms toward the core, or glazing them so light passes through, is usually the single most effective daylight decision available at fit-out stage.

Is the overcast sky assumption a limitation?

Yes, and it is worth knowing. Daylight factor is defined against an overcast sky, which makes it a stable, comparable metric but means it does not describe sunny conditions, orientation or seasonal variation. It is a good tool for comparing how deep daylight reaches; it is not a full description of what a space will feel like through the year.

Daylight Factor Explained | Hagerstone International

Why it matters on a project

Daylight is the thing occupants respond to most strongly and the thing fit-out decisions most often destroy without anyone noticing.

Daylight factor is useful because it makes the loss measurable. It shows how far into the plate daylight usefully reaches, which in turn tells you where enclosed rooms can sit without cost, and where they will darken the whole floor behind them.

What raises and lowers it

Raises daylight factor	Lowers it
Larger glazed area	Deep floor plate
Higher window head height	Low window heads
Higher glass light transmission	Heavily tinted or reflective glass
Light internal surfaces	Dark ceilings and walls
Clear sky view	External obstructions, adjacent buildings
Open layout near the facade	Cabins along the window line

The head-height point is underused

Two windows of the same area do not daylight a room equally. Light entering high travels deeper. A tall window daylights a deeper band than a wide, low one of the same glazed area.

That has a direct fit-out consequence: dropping the ceiling near the facade, or running a deep bulkhead along the window line, cuts the top of the window and removes the part doing the most work. It is a common casualty of services coordination, and it is worth defending.

What it does not tell you

Daylight factor is defined against an overcast sky. That makes it comparable and stable, but it says nothing about:

Direct sun and glare at particular hours and orientations
Solar heat gain, which is SHGC
Seasonal and daily variation
Whether blinds will end up permanently closed

So it answers "how deep does daylight reach", not "is this a comfortable place to sit". Both questions matter.

Interior illumination practice is set out in IS 3646, with lighting and daylighting control provisions in ECBC 2017 and building services in NBC 2016, Part 8.

Common mistakes

Lining the facade with enclosed rooms. The most damaging daylight decision available at fit-out stage.
Cutting the top of the window with a bulkhead or dropped ceiling. Removes the most effective part of the aperture.
Optimising glass for solar control alone. A dark facade moves energy from cooling to lighting.
Daylight with no lighting control. No daylight strategy saves energy if the lights do not respond.
Treating daylight factor as a comfort measure. It does not describe glare or direct sun.

Standards referenced

Interior illumination practice in IS 3646; lighting, control and daylighting provisions in ECBC 2017; building services provisions in NBC 2016, Part 8. Daylight analysis and compliance for a specific building must be established by the project's lighting or energy consultant.

Daylight Factor: How Much Daylight Reaches Into a Floor

Why it matters on a project

What raises and lowers it

The head-height point is underused

What it does not tell you

Common mistakes

Standards referenced

Standards referenced

Frequently asked

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