CALCULATING DAYLIGHT FACTOR

•Daylight – admitted into bldg. through windows / roof-lights.

•Amount of light received inside a bldg. will constantly vary.

•Impracticable to express interior daylighting in terms of illumination actually obtainable inside a bldg. at any time.

•For practical purposes – Daylight Factor (DF) is used.

CALCULATING DAYLIGHT FACTOR (DF)DF – percentage ratio of the instantaneous illumination level at a

reference point inside a room to that occurring simultaneously outsidein an unobstructed position.

Daylight factor is measured as the ratio of illumination at the workingplane inside a room to the total light available outside. This is calleddaylight factor which is expressed in %.

DF – ratio of internal light level to external light level.

DF = (Ei / Eo) X 100%

Where, Ei – illuminance due to daylight at a point on the indoorsworking plane.Eo – simultaneous outdoor illuminance on a horizontal plane from anunobstructed hemisphere of sky.

DAYLIGHT FACTORDF includes the following :

(a) Sky component (SC): direct light from a patch of sky visible atthe point considered.

(b) External Reflected Component (ERC): light reflected from anexterior surface and then reaching the point considered.

(c) Internal Reflected Component (IRC): light entering through thewindow but reaching the point only after reflection from an internalsurface.

Sum of the 3 components gives the illuminance level (LUX) at thepoint considered.

Lux = SC + ERC + IRCD.F. used in bldg. design in order to assess the natural lighting level asreceived on the working plane.

CALCULATING DAYLIGHT FACTORA simple RULE OF THUMB can be used to approximate D.F.

D = 0.1 X Pwhere D = Daylight factor

P = Percentage glazing to floor area

E.g., given a room of 100 sq.m. floor area with 20 sq.m. of glazing.

D = 0.1 X (20/100)X100= 0.1 X 20= 2 %

Calculation of natural illuminance at the reference point inside a bldg.by applying the following formula,

D = (Ei / Eo) X 100Where, D = D.F.Ei = illuminance at reference point in bldg.Eo = illuminance at the reference point if the room was unobstructed.

THE DESIGN SKY CONCEPTWhen the Daylight Factor for a given point has been established, it canbe converted into an illumination value, if the outdoor illumination isknown.

E.g.1:Both factors of E are measured in lux (lumens per sq.m.) with Eo takenas standard 5000 lux for unobstructed sky in the UK, DF = 2%.

Therefore, D = (Ei / Eo) X 100

or Ei = (D X Eo) / 100= (2 X 5000)/100= 100

Therefore, Ei = 100 lux.

THE DESIGN SKY CONCEPTE.g.2: If DF = 8%, & Eo = 6000 lux

DF = (Ei / Eo) X 1008 = (Ei / 6000) X 100Ei = (8 X 6000)/ 100 = 480 lux

•By statistical evaluation of long term illumination records an out-doorillumination level (Eo) can be established for a given location - this istaken as the ‘design sky’ illumination value for the particular location.

•Some typical ‘design sky’ illumination values (in lux) are:

Location Latitude Illumination (lux)

London Lat. 52⁰ 5 000

Sydney Lat. 33⁰ 8 000

Brisbane Lat. 27⁰ 10 000

Darwin Lat. 10⁰ 15 000

Nairobi Lat. 1⁰ 18 000

Delhi Lat. 29⁰ 6 000

THE DESIGN SKY CONCEPT

Previous calculation can be reversed & used as a basis of design:

Establish required illumination level – e.g., 300 lux.

Ascertain local ‘design sky’ illumination, Eo – e.g., 9000 lux.

Calculate necessary daylight factor:DF = (300/9000) X 100 = 3.33%

Manipulate the design variables (window size etc.)to achieve thisdaylight factor.

DAYLIGHTING: Practice of placing windows or other openings & reflective surfaces so

that during the day natural light provides effective internal lighting.

Objective – to maximize visual comfort to reduce energy use. energy saving achieved by reduced use of artificial lighting. or from passive solar heating & cooling.

Use of artificial lighting energy can be reduced byInstalling less number artificial lightsDimming / switching artificial lights automatically in response tothe presence of daylight (Daylight Harvesting).

Daylight harvesting: term used in sustainable architecture for acontrol system that reduces the use of artificial lighting in bldg. interiorswhen natural daylight is available, in order to reduce energyconsumption.

LIGHTING IN THE TROPICSTasks & problems of daylighting in tropical climates:

•To provide adequate daylight even if the windows are protected by louvres for thermal reasons.• To exclude from visual field excessively bright (light coloured, sunlit, etc.) surfaces which would cause glare.

HOT-DRY CLIMATES:Direct sunlight must be excluded from bldgs. – reasons:

•Thermal•Create glare

Windows tend to be small – not much sky will be visible from any point indoors – hence SC (sky component) is insufficient.

Ground & external surfaces of other bldgs. are usually light coloured –in strong sunlight these may create glare – hence ERC can be used with provision to avoid glare.

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External surface

Ground surface

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Internally reflected light – most convenient form of daylighting.

Suitable arrangement –high level window – sill above eyelevel – would admit reflected light to the ceiling.

If the ceiling is white –above method would ensure sufficient & well diffused interior light through a small window.

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sunlit reflective surfaces of shading devices – can cause glare – hence should be non-reflective.

Low level windows – acceptable if they open onto a shaded / planted courtyard.

WARM HUMID CLIMATES: Buildings have•Large openings to ensure cross ventilation & air movement.

•Provided with wide overhanging eaves or shading devices.

Direct sunlight is excluded – thermal reason - bright sky could provide sufficient light - but its high luminance would cause glare -view of sky should be screened by shading device / plants.

Sky luminance - less near the horizon than at higher altitude angles - view of sky up to about 15⁰ may be permissible.

15⁰

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Criteria for the design of shading devices:

•Permit view of sky near the horizon only.

•Exclude view of bright ground & sunlit blade / louvre surfaces.

•Daylight should be reflected from ground & louvres to the ceiling –ceiling should be of light colour.

SUPPLEMENTARY ARTIFICIAL LIGHTING:•In moderate climate (prevalent temperatures are neither too hot nor too cold) – difficult to provide adequate daylight in side-lit rooms - to a depth greater than three times the window head height (6m approx.)•For rooms deeper than this – a system known as PSALI (permanent supplementary artificial lighting of the interiors) has been developed -interior parts of the room are lit permanently by electric lights to provide necessary illumination.•PAL system – Permanent artificial lighting – ignores daylight altogether – windowless environment – window is the weakest point of the building – in both thermal & noise insulation – saving on heating & air-conditioning > cost of artificial lighting.•Counter argument – purpose of window – not only to provide daylight – but also to provide a visual link with outside world•PSALI system – would satisfy the need with reduced windows -insufficient daylight would be supplemented by artificial light.•In hot-dry regions windows are small for thermal reasons – daylight reaching interiors is insufficient.

DAYLIGHTING

•We turn toward the sun, seeking light and warmth - to nourish both spirit and body.

•Position of the sun in the sky / colour of its light - keep us in touch with the time of day / season of the year.

•Intensity of sunlight is so great that no artificial light can match it.

•While building houses - important to locate and organize them in a way that allows all important spaces to receive abundant natural light.

•Placing the house on the site to take best advantage of available light requires a study of the light and shadow patterns created by surrounding structures, by the topography, and by landscape elements.

DAYLIGHT•A primary goal is to bring light into each room from two sides.

A Simple four-square house allows light into two sides of each corner room.

A house with wings creates rooms with the potential for light on two or three sides.

A long thin house allows light to enter from opposite sides of most rooms.

DAYLIGHTINGDaylighting is the complete process of designing buildings to utilize natural light to its fullest. It includes the following activities:

•Siting the building – orienting for optimum solar orientation.

•Massing the building – presenting the optimum building surfaces towards the sun.

•Choosing fenestration to permit the proper amount of light into the building, taking into account seasons, weather, daily solar cycles.

•Shading the façade & fenestration from unwanted solar radiation.

•Adding appropriate operable shading devices, e.g., blinds, curtains, to permit occupant control over daylight admission.

•Designing electric lighting controls – energy saving benefits of daylighting.

DAYLIGHTING – TOPLIGHTING CONCEPT

SKYLIGHT TOPLIGHTING•Use of skylights to introduce light from above.•Best done with diffusing skylight to prevent direct sun rays from causing overly bright spots.•Skylights should be no more than 5% - 6% of the roof area.

CLERESTORY TOPLIGHT•Use of high windows, above ceiling line.•Best done when the windows faces north to prevent direct solar radiation.•With north facing fenestration, ceiling aperture can be very large.

DAYLIGHTING – TOPLIGHTING CONCEPT

SAWTOOTH CLERESTORY TOPLIGHTING•Angled ceiling produces more indirect light –increasing the efficiency of skylight.•Best if north facing.

DOUBLE CLERESTORY TOPLIGHT•Best if long axis is oriented east & west•Use passive shading on south side to prevent direct solar radiation into the space.

DAYLIGHTING – SIDELIGHTING

•Side lighting employs vertical fenestration (usually windows) to introduce natural light.•Unlike top lighting, side lighting tends to introduce light that can be too bright –causing glare.•Desirable view provided by windows makes glare an acceptable side effect.•Sidelighting on east, south & west can permit direct solar glare & heat gain -necessary to shade windows to prevent excessive glare & gain.

•Overhang soffits provide a limited amount of shading.•Best employed on the south façade (northern hemisphere) of the building.

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SOFFIT OVERHANG

DAYLIGHTING – SIDELIGHTING

•Awnings / other extended shades offer additional protection.•Generally needed on the east & west façade of the building.

•A light shelf provides both shading & indirect lighting for the space –increasing the amount of daylight depth penetration.•Most effective on the south façade –can be employed on east /west façade.

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AWNING

LIGHT SHELF

BASIC PRINCIPLES OF DAYLIGHTING DESIGN

1. Plan the building such that every regularly occupied living space has access to a window/skylight - high priority to windows that provide a view – effective day lighted area extends about 2 times the width of a window & about 2 - 2.5 times its height.

2. Minimize the size of east/west sides of the building, maximize the south/north sides. North facing windows present no solar heating problems, south facing windows can be protected with overhangs/awnings/light shelves.

3. If a large area of the building is not near a window – provide toplightskylights in single story buildings / top floor of multistory buildings – skylights should occupy 3% - 5% of roof area.

4. Protect the interior from too much natural light – employing appropriate window glass, external shading devices, internal shading devices etc.

5. Provide an electric lighting system and or automatic lighting controls to permit daylight harvesting. The best way is to dim the electric lights rather than switch them on & off.

SUN’S PATH

•This house plan is one that takes full advantage of the light and the spectacular views.

•Basic form of the house - a two-story rectangle with chamfered corners facing south.

SITE PLAN

FIRST FLOORGROUND FLOOR

•By setting thehouse with itsleading cornerfacing south, twofull sides of thebuilding receivesouth sun, fillingthe interior withlight frommorning untilnight.

•A wide verandah with a balcony above wraps the south corner.•When the sun is lower in the sky - early morning and evening & winter –the low angle sun allows light to penetrate the verandah and enter the rooms beyond.

•Three bedrooms have windows to the east; as the sun rises it gradually fills the rooms with light. Each bedroom also has windows on a second wall, balancing the light in the room and increasing the period of direct sunlight. By mid-afternoon, the bedrooms are protected from heat gain by the placement of the bathroom and fireplace to the west.

FIRST FLOORGROUND FLOOR

•At breakfast, the eastern sunlight enters the kitchen & adjoining dining room.

•Verandah shades the south side on the lower floor, giving midday protection to kitchen & living room.

•By late afternoon, the sun moves to the west and enters through the corner windows of the living room - it is important for afternoon / evening activities to have a sense of the light but also to be protected from heat gain - protection is created by dense landscaping outside the corner windows, which blocks the sun as it gets low in western sky.

•Placing the house diagonally to the south takes maximum advantage of available sunlight. Every room gets direct sun at some point during the day.

FIRST FLOORGROUND FLOOR