Window Area and Orientation
South facing windows: South-facing, high-performance windows (U-value less than 0.32 and SHGC higher than 0.40) can collect solar energy and provide a net heat gain over the winter. But even a high performance window, when placed in any other orientation (north, east, or west), will lose significantly more heat over the course of the winter than a standard R-19 wall. Because south-facing windows can collect solar heat and are easy to shade during the summer, their use and area should be maximized.
East/West facing windows: Windows placed in east and west-facing walls are penalized twice. Not only do they lose a significant amount of heat in the winter, but they are also a source of significant heat gain in the summer, increasing air conditioning costs. This is because the sun rarely shines directly through east and west-facing windows in the winter, but shines strongly on these facades in the morning and afternoon during the summer. West-facing windows are particularly problematic in summer because the heat gain is maximized during the later afternoon, typically the hottest part of the day. Moreover, east and west-facing windows are much more difficult to shade than south-facing windows because the sun angle is low in the morning and afternoon. Low SHGC glass (0.3 or lower) can reduce unwanted summer heat gain, but east and west-facing window area should be minimized and placement should be carefully considered.
North facing windows: North-facing windows are the biggest energy losers, but are often valuable because of their ability to provide indirect daylight to interior spaces. Efforts to provide daylighting with east/west windows and even south windows can often be thwarted when people close interior shades to block strong direct light. North windows provide a comfortable indirect light source, which reduces the need for electric lights. In addition, operable north windows pair well with south windows to create cross-ventilation in the summer.
In general, a window-to-floor area ratio close to 15% is recommended for conventional construction. This window-to-floor area ratio balances energy, first cost, and indoor environmental quality. Houses implementing passive solar strategies using thermal mass and south orientation must be evaluated on an individual basis and may require a different overall window-to-floor area ratio to achieve maximum benefit. If significantly more window area is desirable, large expanses of west and east-facing glass should be avoided. To some extent, high performance windows (such as triple-glazed low-E options) can reduce the energy penalty for higher window-to-floor area ratios. In addition, high-performance windows like triple-glazed windows may be desirable where large expanses of glass are used. Otherwise, cold drafts and cold surface temperatures can make adjacent spaces uncomfortably cold.
Depending on orientation, area, and type, windows can account for 10-30% or more of a new, code-compliant home's heating energy load. In addition, windows and window shading (or lack thereof) have the greatest effect on air conditioning loads, not including internal loads such as people and lighting. Finally, if windows are used appropriately they can also play a role in lighting and ventilation. For all these reasons, windows have a large effect on the energy efficiency and environmental impact of a house.
Options and Analysis
alternatives cost of windows percent of budget total house cost 10% of floor area (86sf) $7,020 7.3% $96,230 15% of floor area (130sf) $10,530 10.6% $99,780 20% of floor area (172sf) $14,040 13.6% $102,900 30% of floor area (260sf) $21,060 19.2% $109,580
Costs based on Means CostWorks 2007, including installation.
All windows approximately 30" x 36", with double-glazed, low-e glass
First cost rises sharply as the number of windows increase. A 15% window-to-floor area ratio represents more than 10% of the overall budget for a small single story house, while a 30% window-to-floor area ratio represents almost 20% of the construction budget. From a purely economic viewpoint, homes with lower window area ratios are cheaper to construct. They also reduce energy costs, as seen in the chart below, unless passive solar design is utilized. Window-to-floor area ratios above 18% for standard construction may require increased envelope performance ratings and mechanical equipment efficiencies to meet code. This can increase costs further. Although reducing the window-to-floor area ratio below 15% can save money on construction costs and energy costs, the implications on the quality of interior space, daylighting, and ventilation should also be considered. Homebuilders may find it difficult to sell homes that do not feature adequate window area.
Window orientation generally does not affect construction cost, although it certainly affects energy cost, which is discussed below.
alternatives cooling cost ($/sf floor area) heating cost ($/sf floor area) total energy cost ($/sf floor area) yearly energy cost AREA (equal distribution) 10% of floor area (86sf) $0.05 $0.59 $1.50 $1,292 15% of floor area (130sf) $0.06 $0.61 $1.53 $1,318 20% of floor area (172sf) $0.06 $0.64 $1.55 $1,343 30% of floor area (260sf) $0.08 $0.69 $1.62 $1,400 ORIENTATION (15% floor area) equal distribution (32.5sf all sides 0.06 0.61 1.53 $1,318 north (100sf north, 10sf others) 0.05 0.64 1.55 $1,338 east (100sf east, 10sf others) 0.06 0.62 1.54 $1,325 south (100sf south, 10sf others) 0.05 0.58 1.49 $1,286 west (100sf west, 10sf others) 0.07 0.62 1.55 $1,325
Energy modeling was conducted using REM Rate 12.3 based on an 864sf Minnesota code base house, with wood siding, unshaded windows with U-value = 0.33 and SHGC = 0.3, 80 AFUE furnace, and 10 EER air conditioning.
Energy costs based on average 2007 U.S. residential rates from EIA: $0.106/kWh for electricity and $1.301/therm of natural gas.
The one-story base house demonstrates that modest changes in window area affect energy consumption. Doubling the window area from 15% to 30% increases space conditioning costs (heating and cooling) by 13%, and total energy costs by 6%, or $0.09/sf/year. On the other hand, reducing window area saves energy and reduces energy costs.
The orientation of windows also affects energy consumption. Compared to a house with an equal distribution of windows on all four sides, a house with the majority of windows on the north side slightly reduces cooling costs, but significantly increases heating costs. A house with most of the windows on the east and west facades increases cooling and heating costs slightly. Finally, a house with the majority of windows on the south side can reduce cooling costs and heating costs. Without taking any additional steps to optimize passive solar gain, a house with windows oriented to the south can provide space conditioning savings of 6% at no additional expense. With minor steps, such as optimizing the window shading offered by roof overhangs, using high SHGC glass, and adding additional thermal mass to the interior, savings can be increased dramatically. It is interesting to note that if window area is increased, placing the additional windows to the south side can largely negate the usual energy penalty. For example, a house with a 15% window-to-floor area ratio actually uses less energy than a house with a 10% window-to-floor area ratio if the majority of the windows face south.