version: 3/16/2009

A Design Sequence for Diffuse Daylighting

‘DAYLIGHTING
RULES OF THUMB’

Tiffany ff Otis
O | Christoph
C
Reinhart
Harvard Graduate School of Design

version: 3/16/2009

WHAT IS IT?
- This document presents a sequence of simple equations (rules of thumb) which can be used to identify potentially daylit zones in a building and to assign them dimensions and glazing areas. areas - The sequence is meant to be applied during the preliminary phases of building design (initial massing and programming).
- Its ease of application means that a daylighting concept can be formed for g while it is still being g designed g and that the building’s g an entire building daylighting scheme can be quickly re-evaluated as the design evolves.

INFO
The design sequence is based on a recent paper which compares design sequence predictions to more advanced daylight simulations using Radiance. It can be read under www.gsd.harvard.edu/people/faculty/reinhart/documents/DiffuseDaylightingDesignSequence.pdf version: 3/16/2009

WHEN?
- The method presented in this guide has been developed for buildings that are either located in a climate with mostly overcast (cloudy) sky conditions or in an urban environment whose spaces primarily receive diffuse daylight. - It must be stressed that the repercussions of direct sunlight are effectively ignored by the design sequence. I.e. the resulting spaces should be adequately lit under overcast sky conditions, but may be ‘overlit’, overheated and/or ‘glary’ in the presence of direct sunlight.

INFO

It is recommended that you carry out an initial evaluation of your site to using a sun path diagram and local climate file to quantify the importance of direct versus diffuse radiation for your site.

version: 3/16/2009

DESIGN SEQUENCE
(1) How much daylight is the space receiving?
Calculate effective sky angle.
(2) H
How much h lilight ht d does th the space need? d? Set target mean daylight factor desired for the space.
((3)) Can the target level be achieved?
Calculate the window to wall ratio required to achieve the set daylight factor and choose to continue or abandon daylighting of the space. space (Daylight Feasibility Test)
(4) What dimensional constraints does the target level impose?
Calculate suitable room depths p and surface reflectances.
(5) What quantity of openings are required to achieve the target level in a space with these dimensions?
D t
Determine
i th the required i d glazing l i area.

version: 3/16/2009

SKY ANGLE
(1) How much daylight is the space receiving?
Calculate effective sky angle.

version: 3/16/2009

SKY ANGLE
(1) How much daylight is the space receiving?
Sky angle (θ) = 90º – arctan(y’/x) – arctan(y/d)

INFO
In case you do not know yet where to place your window just pick the center of the face for your sky angle calculation. Please note that more complex obstruction conditions can be analyzed using a 3D model and a raytracer.

version: 3/16/2009

DAYLIGHT FACTOR
(2) How much light does the space need?
Set target mean daylight factor desired for the space.
- The daylight factor (DF) is a metric used to quantify the amount of diffuse daylight in a space. (Diffuse daylight is light that has been scattered in the atmosphere before reaching the Earth’s surface).
- It is usually measured at the height of the workplane (i.e. a desktop), under a standardized CIE overcast sky. It is defined as the ratio of the illuminance of a point in a building and the illuminance at an unshaded outside point facing upwards:
DF = (Ein / Eext ) × 100
Ein : Interior illuminance at a fixed point on the workplane.
Eext : Exterior illuminance under an overcast sky.
- The
Th ‘mean
‘
daylight d li ht ffactor’ t ’ off a room is i th the average d daylight li ht ffactor t value l off a grid of sensors at work plane height that extends across the room.

version: 3/16/2009

DAYLIGHT FACTOR
(2) How much light does the space need?
Daylight factor calculation diagram:

version: 3/16/2009

DAYLIGHT