Note: This page offers a compact presentation of a few principles for selecting from various residential fenestration options. It assumes a reasonably good command of high school physics as well as some familiarity with fenestration terminology. Our Window Glossary may prove helpful in the latter area.
Introduction
The fraction of incident solar light that is transmitted through a window is the visible transmittance (VT). The fraction of incident solar radiant heat that is admitted through the window to the interior of a building as heat gain is the solar heat gain coefficient (SHGC). LSG is the ratio of VT to SHGC, and is normally in the range of 0.6 to 1.4. Glazings at the extreme ends of this range are generally more expensive than those in the middle. U-factor measures the degree of heat conduction through a total window, from the warm or hot air on one side to the cool or cold air on the other. Single pane clear uncoated glass has a U-factor of approximately 1 [in units of Btu/(hr sq. ft.deg F)]. Lower values indicate better window insulation.
Cold climate residential window selection principles
Background:
Admission of solar heat (called passive solar space heating) and the use of insulation to prevent heat loss through a window during cold conditions are generally desired features of a cold climate window.
Recommendations:
1. The SHGC should be as high as possible for maximum admission of solar heat
2. The U-factor should be as low as possible for minimum heat loss on cold nights
3. The visible transmittance can be reduced from a nominal 0.9 (for 1/8" clear single pane glass) in order to mitigate glare from an individual window (if the room brightness remains the same, as from electric lighting or glare-free other windows), but the SHGC (nominally 0.87 for the same glass) should remain high. The LSG ratio in this case should be below 1.0 to maintain high SHGC when VT is reduced for whatever reason. VT should not be too low, however, for this can degrade the purpose of the window, to see outside, and will restrict admission of daylight illumination, possibly causing electric lights to be turned on in the daytime.
4. To achieve low U-factor, a multiple pane sealed insulating glazing system with argon or krypton insulating gas and having a high solar gain low-e coating has been found effective, but moderately expensive. For extremely cold climates, triple pane versions provide still more protection, but at a higher price. Three, or even four, pane windows are commonly used in northern Europe where winters are cold and heating energy is quite expensive.
5. For summer heat gain prevention, the window should be shaded by either fixed or operable shades. When the sun is incident from a relatively high angle, as it usually is at midday in summer, a wide roof overhang will be effective over south-facing windows in the northern hemisphere. This is also true for east- and west-facing windows near mid-day, but when the sun is low—early in the day for east-facing and late in the day for west-facing—additional shading measures are recommended. Generally, exterior shades are best, and those which are operable allow maximum flexibility, depending on the nature of the climate. In areas enjoying mild summers, solar heat gain prevention may be less of a concern.
6. If there is a desire for protection from future energy price rises and from restrictions in the availability of heating energy, the best multiple pane window affordable is recommended, one having the lowest U-factor and highest SHGC available.
7. Select operable windows to permit natural ventilation during months of mild weather and place windows for maximum cross-ventilation.
8. If damage from extreme weather events, or from intruders, is a concern, secure, impact-resistant windows can be purchased. This class includes those where one or more of the panes is made of laminated glass. Exterior shutters may also satisfy this function.
Hot climate residential windows selection principles
Background:
Prevention of solar heat gain and the use of insulation to reduce heat gain through a window during hot conditions are generally desired features of a hot climate window. Thus low SHGC and U-factor values are generally desired. It is important in this case to have the visible transmittance greater than the SHGC value. Though visible transmittance and SHGC have tended to go hand-in-hand in older glazing systems, modern high-tech ones designed for hot climates employ what is called spectral selectivity to separate VT from SHGC. In this case the visible transmittance of the system is made to be different from the solar infrared transmittance (a major component of SHGC). LSG is a measure of this spectral selectivity. It is the ratio of VT to SHGC and is normally found in the range 0.6 to 1.4.
Recommendations:
1. If the window is well-shaded, such as by a heavy tree canopy, a tall building that blocks most of the sky, or an opaque awning with good window coverage, the cost of low solar gain glazing may be avoided. A SHGC value above 0.7 is acceptable in this case, and will be accompanied by a relatively high VT. If energy prices increase substantially, however, and especially if there is concern that the shading may not be permanent, then it might be best to follow recommendation 3 below.
2. If the window is well-shaded, a high VT is needed to keep the window from appearing dark and to permit entry of adequate daylight. (Values over 0.75 are considered high. To achieve a high VT, a relatively high SHGC is common as well. For example, a glazing system with an LSG ratio of 1.0 and VT of 0.8 has a SHGC value of 0.8. Higher LSG ratio values permit lower SHGC values for a given VT.)
3. If the window receives substantial direct beam sunlight for moderately long periods on hot days it should first be shaded from this sunlight using an effective exterior shade, to stop the sun's heat before it reaches the window. Then the glazing system choice should follow principles 1 and 2. Perhaps the most effective and desirable shade for east- and west-facing windows is thick tree and/or shrub vegetation. The reason is that such vegetation blocks incident solar radiation before it reaches the window, converting a significant fraction of its heat into biochemical energy within the plant. Furthermore, when the sun is on the other side of the house, the vegetation offers a high quality scene to be viewed through the window, one that generally does not reflect a lot of solar heat back to the window. Often a shade, such as an awning or exterior shade screen, will reduce the view to the outside or impair the quality of that view. In such a case, installing an operable shade can solve the problem, if the shade is closed over the window when the bright sun is shining directly on it, but opened for clear views and daylight admission when the sun is not directly incident. This is a particularly effective strategy for windows facing east or west and looking out on a desirable scene or landscape but it has the disadvantage of requiring either automated operation of the shade or a significant effort by the building occupant. A better way of solving this problem would be to reduce the wall (and window) areas facing east and west and to place thermal buffer zones on the east and west ends of the dwelling. For example, an un-air conditioned garage can keep direct beam solar radiation from reaching the west-facing wall of a building. A utility room and/or storage room having a small or no window can be placed on the east end of the building to provide a morning buffer. There are situations, however, where using such shading is precluded by community or physical restrictions, in which cases recommendations 4 or 5 can be followed.
4. If a window receives substantial direct beam sunlight for much of the time on hot days and is only moderately shaded, due to the thinness of vegetative cover, a relatively narrow roof overhang width, or a partially effective awning, sun screen, or other shading device, the SHGC should be reduced somewhat, to an approximate range of 0.5 to 0.65. The VT should be kept high, meaning high LSG glass, unless glare is a problem, in which case the LSG can be lowered slightly, to around 1.0 or so.
5. If the window receives substantial direct beam sunlight for much of the time on hot days and is unshaded, due to condominium or other restrictions preventing the use of exterior shading devices, or for other reasons, then the SHGC should be made as low as possible without overly sacrificing VT. In such a case, a glazing system with as high an LSG as possible should be sought, with a value above 1.2 being essential, and with a SHGC in the approximate range from 0.17 to .3 and a resulting VT in the range from about .2 to .4. VT should not be allowed below about 0.2 or the window will appear dark and insufficient daylight will be admitted, possibly requiring the use of electric lighting in the daytime. To keep the VT from going too low, operable interior shades can be very effective for glare mitigation, in which case, higher VT values, with the accordingly higher SHGC values, can be tolerated. Such shades should have bright, reflective surfaces facing the window, to reflect as much of the admitted solar radiant heat back through the window as possible.
6. If acoustic isolation is desired or needed, then a double or triple pane glazing with the above attributes should be used.
7. If impact resistance is desired or required, then an exterior shade meeting those requirements can be very effective. An operable exterior shade can be closed when needed for heat gain prevention and during extreme weather events and opened for view and daylight entry. Alternatively, the glazing of a single-pane window, or the outer pane of a multiple pane one, can be made of laminated glass. Laminated glass is a single pane made of two sheets of glass sandwiched together with a binder between them to hold them together. Properly designed and manufactured laminated glass is very strong and impact resistant.
8. If extra protection against feelings of radiant warmth in summer or radiant cold in winter near to the window is desired, a double pane solution to the above recommendations can provide this protection.
9. If the energy company powering your air conditioning system provides financial incentives for reducing your peak load on its system, this can be accomplished with low SHGC double pane glazing and insulated frame windows. Perhaps the financial incentive will pay the extra cost of double pane glass and insulated frames. In the future, the utility may collect what are known as demand charges, extra payments to compensate for the consequences of higher peak loads than desired. To protect from future demand charges, insulated glass and frames should be installed now.
10. If the air conditioning system will be installed or replaced at the same time as the windows (or is anticipated at a later date), reducing the peak loading on that system from the windows can reduce the size and hence cost of the air conditioner required. This is accomplished by using an insulated window: double pane glass and an insulated frame. If the window receives direct beam sunlight in the summer, a low SHGC should also be used. The cost savings from the smaller air conditioner can partially or wholly offset the extra cost of the better windows.
Mixed (Hot & Cold) climate residential window selection principles
Background:
There are energy and other benefits associated with multiple pane glazings for both warm and cold conditions. In mixed climates, the coldest conditions are usually experienced at night, when passive solar space heating is not possible. Thus high LSG glazing systems are recommended. They can keep solar heat gain within acceptable limits on hot days while also providing low U-factors for both hot and cold conditions.
Recommendations:
1. If the window is well-shaded, such as by a heavy tree canopy, a tall building that blocks most of the sky, or an opaque awning with good window coverage, the cost of low solar gain glazing may be avoided. This is also true for north-facing windows which do not receive strong direct beam solar heat gain. A SHGC value above 0.7 is acceptable in this case, and will be accompanied by a relatively high VT. For an unshaded north-facing window, an SHGC below 0.7 will help protect from the solar radiant heating produced by beam radiation reflected from white clouds to the north of the building. If energy prices increase substantially, however, and especially if there is concern that the shading may not be permanent, then it might be best to follow recommendation 3 in the previous section.
2. If the window is well-shaded, a high VT is needed to keep the window from appearing dark and to permit entry of adequate daylight. (Values over 0.75 are considered high. To achieve a high VT, a relatively high SHGC is common as well. For example, a glazing system with an LSG ratio of 1.0 and VT of 0.8 has a SHGC value of 0.8. Higher LSG ratio values permit lower SHGC values for a given VT.)
3. Principles 3 through 7 above for hot climates should be followed in mixed climates.
4. For protection against feelings of radiant warmth in summer and radiant cold in winter near to the window, a low U-factor (below 0.5) double pane, frame-insulated window is recommended.
5. If the energy company powering your heating and cooling systems provides financial incentives for reducing your peak load on its system, this can be accomplished with low SHGC double pane glazing and insulated frame windows. Perhaps the financial incentive will pay the extra cost of double pane glass and insulated frames. In the future, the utility may collect what are known as demand charges, extra payments to compensate for the consequences of higher peak loads than desired. To protect from future demand charges, insulated glass and frames should be installed now.
6. If the air conditioning system will be installed or replaced at the same time as the windows, reducing the peak loading on that system from the windows can reduce the size of the air conditioner required. This is accomplished by using an insulated window: double pane glass and an insulated frame. If the window receives direct beam sunlight in the summer, a low SHGC should also be used. The cost savings from the smaller air conditioner can partially or wholly offset the extra cost of the better windows.
7. A flip window for mixed climate zones contains a glazing system with a spectrally selective absorbing outer pane and a clear glass with low-e coating inner pane. The outer pane admits visible radiation while absorbing solar infrared radiation, which heats it up. This is the configuration used for hot summer conditions. The trapped heat from the IR part of the spectrum is released mostly to the exterior, so that the SHGC of the system is low. If the window is flipped over for cold winter conditions, the IR-absorbing hotter pane is now in contact with the interior air, heating it from most of the solar radiation absorbed within the glass. When flip windows are available in the U.S., they can provide an excellent solution to the different needs of cold and hot climate windows.
© 2007-2014 University of Central Florida. The Florida Solar Energy Center (FSEC)
is a research institute of the
University of Central Florida.
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