Daylighting
Encyclopedia : D : DA : DAY : Daylighting
- This article is about use of natural sunlight for interior illumination. See Daylighting (streams) for the redirection into an above-ground channel of a segment of a creek or stream that was previously diverted into a culvert, pipe, or drainage or sewer system.
Daylighting is the passive solar practice of placing windows, or other transparent media, and reflective surfaces so that, during the day, natural sunlight provides effective internal illumination.
In general passive solar technique, a house will be designed with minimal windows on the north side but more and larger windows on the south side. This is because in the Northern Hemisphere, above the Tropic of Cancer, there is no direct sunlight on the north wall of a house from the autumnal equinox to the spring equinox, north-side windows are ineffective at daylighting. South-side windows receive at least some direct sunlight on any sunny day of the year, so they are effective at daylighting areas of the house adjacent to the windows. One disadvantage of relying on conventional window space for daylighting is that, especially during mid-winter, it tends to be highly directional light that casts deep shadows.
Another important element in creating daylighting is the use of clerestory windows. These are high, vertically-placed windows oriented to the sun to admit sunlight for daylighting: towards the south in the Northern Hemisphere, and towards the north in the Southern Hemisphere. In the case of a passive solar house, these may provide a direct light path to north-side (in the northern hemisphere; south-side in the southern) rooms that otherwise would not be illuminated.
Often, clerestory windows also shine onto interior wall surfaces painted white or another light color. These walls are placed so as to reflect indirect light to interior areas where it is needed. This method has the advantage of reducing the directionality of light to make it softer and more diffuse, reducing shadows.
Skylights are often used for daylighting, but they have half the insulating value of a similarly constructed windows because warm air rises to the ceiling where the skylight is located. In addition, the amount of light skylights deliver peaks in the around midday, when the additional light and heat it provides is least needed. Poorly constructed skylights may have leak problems and single-paned ones may weep with condensation. Still, skylights have a useful place in daylighting, particularly in large rooms or interior rooms with no windows to the outside. Using skylights with at least two panes of glass and a heat reflecting coating will increase their energy efficiency.
Another type of device used are light tubes, also called solar tubes, placed into a roof and admitting light to a focused area of the interior. These somewhat resemble recessed light fixtures in the ceiling. They do not allow as much heat transfer as skylights because they have less exposed surface area. It is also easier to retrofit light tubes into existing buildings, especially those with deep roof constructions.
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