Design Strategies for Effective Daylighting

Modernist Beach House, Malibu, CaliforniaDaylight should be used as much as possible to light a home, both for energy efficiency and for the health and comfort of occupants. Design requirements for daylighting must be balanced with the homeowner’s requirements for views and privacy. Daylighting must also be considered alongside building location, orientation and layout, in order to control solar access for passive heating and cooling.

Principles for utilizing natural light include:
• use diffused rather than direct sunlight.
• avoid over-glazing which may cause glare and heat gain in summer and heat loss in winter.

Sources of daylight
Daylight inside a building comes from three sources:
• Direct sunlight provides both light and solar gain. If the sky is overcast, solar gain is reduced but the sky dome continues to be the source of daylight.
• External reflection comes from light reflecting from ground surfaces, adjacent buildings, wide window sills and light shelves. Excessive reflectance may cause glare and should be avoided.
• Internal reflection comes from light reflecting from the internal walls, ceiling and floor. High reflectance surfaces such as smooth or gloss surfaces, light-colored finishes and mirrors reflect light around the room, increasing penetration and also reducing extremes in brightness contrast.

Increasing daylight penetration
The higher the window head, the deeper the daylight will penetrate into the interior. A window will produce useful illumination to a depth of approximately 1.5 times the window head height. Therefore, increasing the height of the window head can increase the depth of light penetration up to two times or more.

Other ways light can be allowed to penetrate into the building interior include:
• installing full-height windows with the head as high as possible (but ensure that the minimum distance between the head height and the underside of the overhang is maintained).
• using roof-lights for top lighting – insulated glazing units (IGUs) must be used for roof glazing to minimize heat loss.
• designing open plan spaces.
• installing light shelves.
• including high level, clerestory windows.
• using glazed internal walls or borrowed lights.
• installing lightpipes (proprietary tubular rooflights).
• designing so all spaces requiring daylighting have access to an external wall.
• using glass that admits light but reduces glare and UV.
• arranging spaces so natural lighting comes from two or more directions.

Natural light from two directions

Natural light from two directions

Improving interior lighting levels
Use reflective finishes such as light colors, gloss finishes and mirrors to increase the penetration of natural light into the interior. Reflectance values of light colors increase daylight penetration up 80% from ceilings, 50–70% from walls and 20–40% from floors. The reflectance value for white paint is approximately 75% when new but drops off as it discolors with age.
Window frames, jambs and sills should also be painted in light colors, and where windows are deep set, jambs should be splayed or angled towards the interior. Although floors contribute the least reflectance, a dark-colored floor finish will reduce the light levels in a room.

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