natural lighting with windows
Energy-efficient windows, skylights and clerestories, conveniently dimensioned and located, can reduce the need for artificial lighting without colliding with energy-efficiency purposes. But you have to implement the right strategy.
Natural lighting (or daylighting) strategies involves a careful and well planned use of windows, skylights, clerestories and other transparent media in order to bring sunlight into home, providing a better indoor illumination with lower electricity bills.
Lighting strategies vs. energy-Efficiency
Natural lighting strategies - based on windows and traditional rectangular skylights - aren't easy to define and can collide with energy-efficiency strategies - by having a negative impact in the cooling or heating of the house during some parts of the year.
That's why they should be carefully planned, taking into account overall energy efficiency considerations and also shading strategies (involving overhangs, landscaping, awnings, etc.).
A convenient shading strategy can avoid too much glare or too much sunlight and heat during the summer.
Natural Lighting & Energy efficiency & Climate
When considering natural lighting we can't forget the climate and the energy-efficiency issue.
Natural lighting considerations should be part of a larger approach. The size and location of the windows is something that is closely dependent on climate and of an overall energy-efficiency assessment - and not mainly on purposes related with natural lighting...
In northern hemisphere countries, in cold or even temperate climates where cold is a dominant feature, the major window should face south*, to collect the sun's heat during the winter, when the sun is low. This same side of the house should also benefit from overhangs or awnings or other shading devices to prevent unwanted heat gains during the summer, when the sun is higher in the sky. That's basic energy-efficiency goals, and natural lighting purposes can't violate them.
In hot climates that's basically the opposite. Cooling considerations are particularly important in these climates, while natural lighting is a typically secondary goal... In these climates glazing areas should be mostly north-facing** (to protect the house from the heat gains trough south-facing windows), while the window area in the opposite side of the house should be limited and benefit from as much shading as possible...
* Or north, in southern hemisphere countries.
** South-facing, in the southern hemisphere.
North-facing windows* & Lighting
In cold northern-hemisphere countries, north-facing windows are typically used for natural lighting purposes*. They collect natural light and little glare and heat gains (either in summer or winter).
These windows should not have large dimensions, to avoid unwanted loss of energy in winter.
* South-facing windows, in southern hemisphere countries.
East and west windows and The VT (Visible Transmittance) coefficient
The VT (Visible Transmittance) coefficient is crucial in a natural lighting approach. VT is a measure of how much visible light is admitted by the glass of a window or skylight. You should require a high Visible Transmittance on the north side of a home (in northern hemisphere countries) though not on the east and west sides of the house, where sun is a cause of glare.
East- and west-facing windows can be good elements of a natural lighting strategy in the morning and evening, but their number and size should be limited since it's difficult to effectively control sunlight and unwanted heat gains and glare when the sun is low in the sky...
These windows should be shaded or involve glass with very low VT (Visible Transmittance), to avoid glare. A low SHGC - to avoid unwanted heat gains - is also desirable.
The Type of glass
In cold climates the glazing elements of the windows in the sunny side of the house must have a high SHGC, that is, a high Solar Heat Gain Coefficient (0.6 or more). That's important to improve solar heat gains during the winter.
In these climates also prefer a glass with a U-factor of 0.3 or less (to reduce conditioning heat transfer).
A high visible transmittance (VT) glass is also important on north-side* windows (in northern hemisphere countries), though not in east and west facing windows, where sunlight is a cause of glare, difficult to avoid...
*South-side windows, in southern countries.
Clerestory windows
Clerestory windows can be a good element of natural lighting strategies in moderate and hot climates. See: Clerestory windows for lighting and ventilation
Skylights
Skylights are a crucial element in any natural lighting strategy. That's their main goal, after all. If interested in skylights, take a look at non-rectangular skylights: pyramidal, dome, circular, oval, diamond, multi-sided and mainly tubular skylights…
This last type of skylights (also called sun-tubes and light-tubes) are extremely efficient, easy to install and do not pose problems like unwanted heat losses, infiltration, overheating and moisture.
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