The size of windows should amount to at least 10% of the room's floor surface, according to many building codes. That's a recognition that we need natural lighting in our homes; that’s the very function of windows or skylights.
But we should not forget that windows are a common cause of significant energy loss and unwanted solar heat gains and glare.
Windows shouldn't be too large, to not compromise the energy efficiency of the building. There is a compromise to be found between our need for daylighting and the need of reducing their size to protect our living spaces from the elements.
Natural lighting without compromising energy efficiency
Here are a few basic rules:
Do not oversize your windows. That's important to prevent heat loss, in the heating season, or overheating and high energy bills; and also to prevent glare, fading of furnishings or loss of privacy. See Window size and placement
Skylights. Do not use skylights unless you do need them; skylights can be thermally disruptive, even with the right glass. Sun-tunnel skylights are a good alternative to traditional units; they can’t provide views or ventilation, but can be very effective at delivering daylight without causing unwanted heat transfer. See: Skylights Guide for Energy Efficiency
Visible Transmittance: In the non-sunny side of the house, in cold and moderate climates, windows should have high-visible transmittance glazing; in the other sides, namely in east- and west-side, the VT (Visible Transmittance) coefficient should be properly chosen. That’s important to control glare. See: Low-E Coating Glasses
Sun-facing side of the house (the South, in the Northern Hemisphere; the North, in the Southern hemisphere): in cold and moderate climates consider relatively large windows in the sun-facing side of the house, but also protect the windows from the summer's sun with properly sized and positioned overhangs, vegetation, or awnings and other shading devices; that's very important to control summer heat gains. Do not use skylights in sun-facing roofs. Select carefully the Solar Heat Gains and the Visible Transmittance coefficients of the glazing. In hot climates, reduce the size of the windows, and protect them from the sun.
Non sun-facing side of the house: in cold and moderate climates consider small windows (to minimize energy loss); Choose windows with high Visible Transmittance glazing. In very hot climates windows can be larger, for natural ventilation purposes.
There isn't a single "best" window size.
It depends on the part of the house where the windows are going to be installed (North, South, East, West), the sun’s angles, the climate zone, the layout of the house, the type of building (stand-alone single family houses…), the prevailing winter winds and breezes and the intended goals (daylighting, passive solar heating, natural ventilation, views…).
There are anyway a few broad guidelines; we list them further below.
General sizing rules
Some building regulations require a glazing area of at least 10% of the room’s floor area.
But 10% is just the minimum, and it doesn't take into consideration the climate, or the construction methods and the side of the house where the windows are installed. Most guidelines (including those for energy-efficiency) recommend values between 20% and 30% of the home’s living space in moderate and mixed climates.
In colder climates (and in some hot climates) these values should be lower (less than 20%).
North, South, East and West Windows
More important than the overall size of the windows, is their size in each side of the house.
In regions with extreme temperatures, the overall size of the windows is typically small, to protect the living space. But that’s not a rule: in very hot climates, the use of natural ventilation in cooling strategies can make large-sized windows advantageous, while the use of air conditioning can make a small overall size of the windows a better option.
In cold and moderate climates you should locate a significant part of the glazing surface in the sunny side of the building (the south in the Northern hemisphere; the North if you live in the Southern hemisphere).
The size of these windows may reach 30% of the room’s floor area, which is important for winter heat gains (the windows should also benefit from properly sized overhangs to protect them from the summer’s sun). In new homes, locate the main living areas of the house at this side as much as possible.
The North side of house (the south, in the Southern hemisphere) should have a reduced glazing area, say, 5%-10% or the room’s area. New homes should be designed to have their secondary service areas (closets, garage, laundry room, circulation spaces) in this side, whenever possible.
Windows in the West and East side of the house, should also be relatively small (10%-15% or the room’s floor area); their glass should have a low VT (visible transmittance).
The values mentioned above are broad guidelines, that should be tailored to the particulars of the climate, site and home design and features (landscape, porches, room buffers…). Just do not underestimate them. They are critical, and one of the most important elements for comfort and energy efficiency.
The RESFEN window-energy software may help you to optimize your choices; though designed for the US climate regions, it can also be used by non-American homeowners.
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