Window Management Strategies for Energy Conservation
Controlling solar heat gain is of primary concern for energy conservation because of California’s climate. In a typical home, more energy is lost through glass doors or windows than through any other construction element.
This is particularly true in California. The California Solar Energy Center modeled a "base case" house (fairly representative of most single family residences located in central California) to illustrate residential heat and humidity sources. The data gained from this house was consistent with other energy studies. The direction windows face greatly influences the energy gained and lost for heating and cooling.
On a hot summer day, more energy will come through just one square foot of glass than through an entire insulated wall. Solar radiation through window glass is responsible for 19% of the air conditioning load.
The windows in the base house had two-foot overhangs and were shaded by interior draperies. Sixty-two percent of the window area was on the north and south sides, where little direct radiation penetrates. Residences with less shading or different orientations could have much larger loads from solar radiation through windows.
MANAGING HEAT GAIN
It is important to protect windows, walls and sliding glass doors during hot weather; during cold weather, solar heat should be permitted to enter. Cooling and heating energy savings of 10-20% are possible through good shading strategies. Window films, interior shades, curtains and interior insulating shutters help, but exterior devices save more energy because the heat that they absorb remains outside the building.
In general, the most effective way to reduce heat gain is to prevent or block the solar radiation before it enters the room by using exterior shading.
If this is not possible, the closer the shade is to the outside, the more effective it will be. If it cannot be outside the glass, then a shading device between two panes of glass will be more effective than a shade on the room side of the glass. Interior devices that have low transmittance and absorbency and high reflectivity are preferable where interior shading is the option chosen.
There is no one best strategy that will meet all needs or that is appropriate for every person or every location. Trade-offs will be necessary and must be determined for individual site requirements, climate conditions, and personal preferences. What may be appropriate for a south-facing window may be of little or no benefit on an east window. A strategy that provides good diffuse daylight in summer may not provide adequate protection against heat loss in the winter. Techniques helpful to households using air conditioning may not be helpful to those not using air conditioning.
It is crucial that residents analyze their own dwellings and surroundings, checking to see when solar
radiation strikes their windows and how this varies seasonally, determining which shielding strategy would best meet their needs for light, ventilation, comfort, and privacy.
Consideration must be given to any residential restrictions that apply -- such as condominium restrictions on exterior shading.
Resources of time and money must also be considered. Variations in cost exist among the strategies
discussed here. Select the ones that provide the greatest energy savings for the amount of money spent.
Keeping the sun as far out as possible will have the greatest benefit in reducing cooling loads.
Letting the sun enter, then shielding the windows when there is no sun, will have the greatest benefit on heating loads.
Time and human energy available for window management must be considered.
WINDOWS: THEIR FUNCTIONS
Windows have many functions in homes besides contributing to heat loss and gain. Windows provide
contact with the outdoors for people. They also provide ventilation and daylight. Research has found that people tend to have rather negative responses to spaces without windows unless there is some dynamic quality within the space. The qualities that were judged as desirable attributes of windows were the view, sunshine, daylight and a sense of spaciousness.
To achieve these, 20% of the window wall space was determined to be a minimum acceptable size. This information, in addition to data that indicates that windows can make a positive energy contribution under certain circumstances makes it clear that windows can not simply be eliminated but must be used to give positive energy benefits. Research has resulted in data that can be used to achieve these positive benefits. The purpose of this publication is to present these research findings and to suggest applications of this information for California climatic conditions. TOPICS ADDRESSED
The major areas related to windows and energy use that will be addressed in this publication are highlighted below.
Factors involved in heat gain and loss through infiltration. Fenestration is the term used for any
light-transmitting opening in a building wall or roof.
Fenestration includes the glazing material; the framing, mullions, mutins and dividers; external and
internal shading devices, and integral (between glass) shading systems.
Strategies to shield windows for heat gain or loss.
- exterior shading
- glazing and films
- interior shading
Window management.
SOLAR HEAT
Glass is a good conductor of heat, and hence a poor insulator. The heat transfer or U-value of glass for
winter is 1.13 with an exterior wind velocity of 15 miles per hour. With still air inside and outside, the U-value in summer becomes approximately 0.90 or below the value of unity (1.00). Materials with high U-values -- close to 1.00 -- are very poor insulators.
A well insulated wall, in comparison, will have a U-value of 0.10 to 0.15.
In addition to the thermal conductivity of the glass itself, energy is lost through and around the fenestration by infiltration and radiation. Infiltration is air leakage around window frames and glass through cracks and air spaces. Infiltration gains and losses are equivalent to about 1/3 of the loss or gain through the glass itself.
Infiltration around sliding glass doors is about three times as much as for a hinged door of the same size.
This is due to the mechanical construction of the door frame plus the loss through the glass. The use of well-fitting frames, caulking and weather-stripping can reduce heat gain and losses from infiltration.
Radiation is the passage of electromagnetic energy over distance. Conduction is the passage of heat through a material. Heat gain or loss through windows via conduction and radiation can be reduced through the use of properly chosen window treatments and careful window management.
Solar heat gain and loss is made more complex for several reasons.
Short-wave solar radiation transmitted through windows will be absorbed by interior surfaces.
Once absorbed, it is given off as long-wave radiation which cannot pass out through the windows and so
is trapped in the room (the greenhouse effect). The heat absorbed can have a damaging effect on wood
and other furnishings.
The room also absorbs heat from lighting, occupants, equipment, and infiltration air as well as from solar
radiation.
At the same time, heat gains or losses through convection -- the transfer of heat created by the motion of air -- are also taking place.
ORIENTATION OF WINDOWS
Orientation refers to the direction windows face.
North, south, east, and west orientations all produce unique characteristics that must be considered when
heating and cooling loads are being evaluated.
South Windows
Windows that face south have many advantages. In summer, south windows are the only ones that can be
effectively controlled by overhangs to block radiation.
This occurs because the summer sun is higher in the sky, and so its rays are coming from a higher angle that can be blocked from vertical surfaces by projecting eaves from the roof or above the window.
In winter, the sun’s rays are coming from a lower angle and can strike the window without being blocked by overhangs and projections.
Winter sun can be used to add heat to the interior if south windows are not blocked by window treatments.
The winter sun never rises as high as the summer sun.
A difference of 47° exists between the summer sun at noon and the winter sun at the same time.
North Windows
In California, a northern window exposure provides excellent and cool illumination. The small amount of direct sunshine reaching these windows from the early morning and late afternoon summer sun comes at such a low angle that very little heat enters the building.
Northern window exposures are particularly suited to southern California’s brief heating season and lengthy cooling season. In winter, windows should be shielded to prevent heat loss and to add to the comfort of persons sitting near the windows. East and West Windows
East and west windows present the greatest extremes. They are more susceptible to heat radiation in
summer because of the many hours of exposure to sunlight in each direction. Carports or porches may be
used to provide shading, as can free standing walls, fences, hedges or trees or trellis vines that tend to lose
their leaves in winter. Louvers, awnings, or shutters can also provide adequate shade for east and west windows.
Even though the solar radiation for east and west directions is about the same, west windows result in the
highest temperatures in the house because of the time of day the sun reaches them in the summer. East and west windows also allow rapid conductivity and loss of heat to extreme cold exteriors in the winter unless carefully shielded and managed.
SOLAR RADIATION
The vertical elevation of the sun directly influences the intensity of solar radiation. This elevation varies
with latitude, day of year, and time of day. The quality of the air will also affect the intensity of solar radiation.
Dust, gas molecules, ozone, and water vapor are the constituents of the air mass that affect the amount of
sunlight that reaches windows throughout the day and seasonally.
Solar radiation on a clear day at sea level is composed of 3% ultraviolet light, 44% visible light, and 53% infrared light. The ultraviolet light is responsible for fading material. Total solar radiation is a
combination of direct radiation and diffuse radiation.
Diffuse radiation comes from all parts of the sky and is part of radiation reflected to the window from adjacent surfaces, particularly those to the south of the window.
On completely cloudy days, all light striking the window is the result of diffuse radiation.
Solar radiation striking glass has three effects: part of the energy is transmitted through the glass, part is
reflected off the glass, and part is absorbed by the glass.
The heat transmitted and absorbed can both contribute to cooling loads. Once the transmitted heat is absorbed by furnishings or structure, that heat is trapped inside the room.
Although this can be beneficial in winter, it may add a significant burden to the cooling load in the summer and can have damaging effects on furnishings. If heat is absorbed by a thick mass, such as a 6-inch concrete floor, a continuous time lag, known as thermal lag delays the flow of heat and causes a continuous decrease in the ratio of cooling load to heat gain.
Solar transmittance is the largest portion of heat gain. The radiation absorbed by the glass raises the
temperature of the glass until equilibrium is established and the heat can be diffused either indoors or out
depending on the temperatures and the air movement of both locations. The heat will flow to the cooler
temperature. Heat-absorbing glass absorbs more solar radiation than ordinary plate glass; this reduces
transmittance but causes some of the heat to be transferred to the interior by re-radiation.
Energy that enters the interior through the fenestration is composed of components. Transmitted direct radiation Transmitted diffuse radiation Energy radiated from the inside surface of the glass to surrounding objects within the room Heat transferred from the glass to the room air by convection. This heat, transferred by convection, becomes an immediate part of the cooling load. Air velocity at windows, temperature, and convection air currents also affect heating and cooling loads. In winter the heating load is increased when warm air sweeps over the window glass, creating a greater temperature difference between glass and outside air. This promotes greater heat flow to the outside.
Cooling loads in summer will also be increased when cool air flows over windows and increases the flow of heat inward.
SPECIAL GLASS
The flow of air over a window surface will be further increased by the addition of a shading device set
away from the windows and not blocked at the top.
This increases convective air flow, known as the chimney effect, and causes greater burdens for both
heating and cooling. Heat flows to the cooler surface, thereby carrying heat outdoors in winter where
temperatures are cooler, or indoors in summer if the room temperature is lower due to air conditioning.
WINDOW FRAMES
Solar radiation can be either beneficial or detrimental to heating and cooling loads depending on the season, the orientation, the shading devices, glazing materials, and thermal resistance involved. The size of the window is one of the most critical variables. The interaction of the variables plus the potential losses and gains through the use of daylight must be considered when determining the energy potential of windows.
Daylight can reduce both the amount of energy used for lighting and the cooling load by reducing the heat
generated by the lighting. Optimal fenestrations would balance the thermal benefits and disadvantages of
increased use of daylight, the heat released by room occupants and equipment, the conductive and infiltrative heat gains and losses, and proper window management.
Research results have shown that properly designed and managed windows can result in a reduction in overall operating costs.
EXTERIOR SHADING
A device placed between the sun’s rays and the window prevents solar radiation from being transmitted into a room. This not only reduces transmittance of solar radiation but also gains from absorption and re-radiation as well.
Exterior shading can be provided by a variety of methods. The careful placement of trees, shrubs and
vines, roof overhangs, walls, fences, shutters, awnings, and roll blinds can be effective in blocking radiation.
Decide on the amount of shading that is desired, at what time of the day and the year shading would be provided, and when it is desirable to use the sun for warmth.
Once these decisions have been made, selection of a suitable shading device can begin.
Use of exterior strategies may depend on site and building limitations. Know what restrictions, if any, will
have to be considered. Condominium owners and apartment dwellers, for instance, may be unable to plant
trees or add shutters or blinds. Room overhangs and siting decisions cannot be changed on existing dwellings.
Assuming that restrictions must be considered, suitable choices can be made from the following exterior
strategies to reduce solar heat gain when such a reduction is desirable. Fully shaded windows reduce
solar heat gain by 80%. When using exterior shading devices, be sure to have free movement of air between the shade and the window to carry away heat absorbed by the shading material before it can be transferred through the window.
Exterior Roller Blinds
The exterior roll blind is a series of slats that can be made of wood, steel, aluminum or vinyl. The roll blind is mounted above the window and has side channels to guide the blind as it is raised and lowered. During summer months, the blind is lowered during the day to provide shade and raised at night (if desired). The operation can be reversed in winter, allowing maximum window management and increasing insulation value.
When the blind is fully lowered, the slats meet and provide complete shade. When the blind is partially
raised, the slats part slightly, allowing some daylight and air to enter between the slats. Experimentation with exterior roll blinds has determined that these blinds can improve the U-value of the window area from the standard 1.13 for uncovered glass to 0.57 with a lowered blind. This shading method can require a substantial outlay of money, and changes the exterior design of a building.
Awnings
Awnings have been used as exterior sun shades for many years in south California. In recent years, energy considerations and more variety and availability have generated more interest in them in other parts of the
state. When selecting awnings, consider shape, color, venting, and the specific materials of which the awning is constructed. Awnings can add a great deal of aesthetic interest to a simple home.
Canvas awnings are available in four basic styles.
The simplest style is the venetian awning, good for east/west exposures. A similarly shaped awning, with
sides added to block out additional sun, is referred to as a hood awning. A hip roof awning projects out and then down (incorporating two angles) to accommodate outward opening casement windows.
Roller awnings are of the same shape as venetian awnings, but can be rolled up for self-storage when they are not needed, a feature particularly helpful in the winter.
Two other types of awnings have shapes similar to the canvas styles just described, but are made of
aluminum. One style is of a solid design; the other is slatted. The slatted design allows greater air circulation to dissipate heat that can build up as the awning absorbs sunlight.
Aluminum awnings may have open or closed sides.
Awning shapes should be selected to best suit the windows being protected and the orientation of those
windows.
Color choice and material for awnings are also important considerations. Awnings with low solar-absorbing surfaces (white) will maintain temperatures closer to the outdoor air temperature because of the
reflection from the light color. As a result, air temperatures under the awning are not raised appreciably. Awnings that absorb solar radiation may need to be vented to reduce this heat build-up.
For south-facing windows, during the period of the day when the sun falls directly on the window, use of a canvas awning reduced heat gain by 55-65%. For west exposures, the reduction in heat gain was 72-77%.
During the period of the day when the sun would directly strike the window, metal awnings reduced heat
gain by 70-75% on a western exposure, 75-80% on an eastern exposure, and 50-60% on a southern exposure.
Exterior Shutters
Window shutters, mounted to be functional rather than just decorative, can be effective in blocking solar
radiation. Two types of top-hinged shutters are Sarasota and Bahama shutters. Sarasota shutters are most effective on south-facing windows where less of a projection is needed to shade the entire window.
Bahama shutters extend the full length of the window and would be appropriate for east- and west-facing
windows because of the protection provided when the sun is low in the sky.
Side-hinged and rolling shutters are usually designed to have two vertical sections that meet at the center of the window. Shutters may be either slatted or solid. Slatted shutters allow diffuse daylight to enter. Shutters used in winter can provide extra insulation when they are closed. They can be opened during the times when sunlight strikes the windows.
Mesh Screens
Mesh screening materials reduce the intensity of the sun striking the window and provide some diffusion of solar radiation. This reduces heat gain and helps control cooling loads. The mesh screens are mounted in exterior frames and should cover the entire window.
The reduction in sun intensity that they provide is a function of the relative amount of opaque area in the
fabric to open areas between the fibers. Screens absorb heat outside the window, preventing it from entering.
The outward view is not seriously impaired, especially with the darker meshes.
Screens mounted on the exterior of the windows are more effective at reducing heat gain than those mounted inside. Screens are particularly effective when installed on the outside of windows facing east or west. This option can reduce the radiant heat gain by up to 70% in the summer.
Landscaping
Because of our abundant sunshine, landscaping is one of the most attractive energy conservation measures.
Complete shading for windows may be provided by careful landscape planning. Trees on the east and west
sides of a home provide shade and prevent the sun from striking the glass. In addition, shrubbery can be placed to capture the natural breezes.
Sun Control Films
Window film, if placed on the outside of the glass, is another shading option. (If the film is placed on the
inside, the glass tends to heat up.) Sun control films on the market today fall into three basic categories: high reflectivity film, heat saving film, and fade protection film.
The high reflectivity films are most effective at blocking summer heat gain. Because they are designed
to reflect solar radiation, some of the benefits of the use of winter sun for warmth will also be lost. For climates with long cooling seasons this is not usually a problem.
The higher the reflectivity of the film, the more effective it will be at blocking heat gain. The silver, mirror-like films are more effective than the colored, more transparent films. Reflective film lowers the U-value of the window to about 0.96.
The heat-saving or winter films are designed to reduce the problem of winter heat losses through glazing.
These films can lower the U-value of glass to 0.72, improving the insulation value of the glazing.
The fade-protection films are designed to filter out ultra-violet rays. Ultra-violet rays are responsible for 60-65% of the fading of most home furnishing fabrics.
The effectiveness of sun control film depends on a number of factors. These include the type of film, the
glazing to which it is applied, the size of the glass area to which the film is applied, the orientation of the
windows, weather conditions, building surroundings, and whether the window also has interior shading.
Rooms with large window areas will benefit more from the use of film than those with smaller areas of
window. East and west windows, because of the higher heat gain experienced, can benefit more from film.
North windows can be left without film. South windows not protected in any other way may benefit somewhat, but that benefit may be offset by the reduction of beneficial winter sun.
Optimum savings such as are given in performance claims will only be realized on clear, cloudless days for windows with no obstructions such as trees. Use of sun-control film on heat-absorbing glass can cause glass temperatures to get too high.
Results of a survey of film users in 10 cities, including Los Angeles, indicated that the chief advantages of using sun-control film are heat and glare control, elimination of sun damage, and to a lesser extent,
reduced visibility into buildings during the daytime.
Disadvantages cited were reduction in interior light available, some loss of visibility to the outside, cost,
extra care required in cleaning, and the reflections.
INTERIOR SHADING
Most window treatments have been designed for decorative purposes and privacy. Until recently little
emphasis was placed on energy conservation. Progress has been made with commercial shades and blinds due to the large amount of glass that is used in new construction and the popularity of these treatments in
commercial installations. Designing fabric treatments for home settings can be done with limited skills in drapery making and without a large investment of time and money.
Exterior shading is most effective in reducing solar heat gain. However, people who rent or live in
condominium or high rise facilities may not have any control over the exterior of their windows. Therefore, the only means available to them to modify solar radiation may be interior treatments.
Interior devices do offer the advantage of being more easily accessible and managed then exterior
shading devices. When appropriately mounted and managed, interior shading can reduce heat gain in
summer and heat loss in winter.
The principal disadvantage of interior shading is that radiation does penetrate into the interior of the room at least as far as the shade, and the heat absorbed by the shade is then reradiated into the interior and also
delivered by convection. If the shading device is translucent, additional radiation will be transmitted and
absorbed by room surfaces and furnishings, and as such, will be trapped within the room by the greenhouse effect." If the shading device is not mounted to effectively trap air between the device itself and the window, its insulation value will be minimal and it will not be effective in preventing heat loss in colder months.
A shading device mounted to establish a tight closure and then managed to achieve maximum solar benefits can be effective in improving the performance of a window.
Several types of interior shading devices will be considered. These include roller shades, venetian blinds,
draperies and interior shutters.
Roller Shades
Roller shades are one of the simplest and most effective types of interior window treatments for energy
conservation. In selecting roller shades, the color and material of the shade and its positioning in relation to the glass are the factors most important in influencing heat flow.
Shades made of opaque materials prohibit the passage of solar radiation. If the shade side closest to
the glass has a low solar absorbency, solar heat gain is further reduced because much of the sun’s rays striking the shade will be reflected off the glass before they can become part of the cooling load. Very light colors, and especially white, have the lowest solar absorption rate.
Dark colors will be more absorbent. Translucent shades will allow some penetration of radiation into the room.
Experimentation with roller shades has determined the properties for shade materials listed in Table 1.
From these results, it can be noted that white opaque shades have the highest reflectance and one of the lowest absorbencies of the materials tested.
The mounting of roller shades is also extremely important. Shades should be positioned as close to the
glass as possible. Mounting the shades so that the sides of the shades are held close to the wall by means of channels will result in the establishment of a sealed air space, which will increase the insulation value of the shade.
Another method is to mount the shade with outside brackets and a conventional roll so that the shade fits
closer to the window. A cornice can be added to the top of the shade and decorative curtains on the sides.
Fully drawn roller shades, during periods when solar radiation is striking the window, can effectively reduce heat gain. For south-facing windows, during the period from 8:00 a.m. to 4:00 p.m., white opaque shades can reduce heat gain by 64%; white translucent shades can reduce heat gain by 56%; and dark green shades can reduce heat gain by 30%. On a west-facing window, during the period from 12:00 noon to 6:00 p.m., white opaque shades can reduce heat gain by 68%; white translucent shades can reduce heat gain by 60%; and dark green shades can reduce heat gain 33%. These figures were calculated for regular plate glass at optimum conditions.
Reverse Shades with Seasons
The heat-absorbing properties of dark colors can be used to an advantage during cold months. In this
situation, placing a shade with a dark color next to the glass, and establishing a venting system to allow the heat collected by the shade to be convected into the room can reduce heat losses. To be effective, a shade of this type would need to be mounted with side channels to create a dead air space; the shade should be able to be attached at the bottom at night to prevent cold air from being carried into the room by convection.
A dual shade, dark and absorbing on one side and white and highly reflective on the other would provide
maximum solar control. The shade would need to be reversed with the seasons, and would have to be drawn all day to be effective. Shades with low transmittance (5-10%) would permit some daylight to enter the room.
These shades have the advantages of regular blinds without the disadvantages of two separate blinds --
needing to be changed and stored between seasons. The dual blind would simply be reversed with the seasons.
Fabric Shades
Various types of fabric shades can be designed.
Even simple shades, carefully constructed and installed can yield an R-value of 2.0. A value of 4.0 can be
achieved with quilted fabric construction, and some types of Roman shades -- sealed at the edges and incorporating several layers of fiber batting -- have been rated 2.5 to 5.5.
By comparison, a single pane of glass has only a 0.89 R-value. A variety of plain shades and Roman
shades can be designed for interior use.
Blinds
Slat-type interior shading devices, either vertical or horizontal, are more effective in regulating summer heat gain than at reducing winter heat loss. Heat loss is more difficult to reduce with slat-type blinds because of the numerous openings between the slats, even in a closed position. For summer, however, slat-blinds offer flexibility in regulating the light and ventilation not available with roller shades while still deflecting direct radiation.
With venetian blinds, when slats are set to reflect sky radiation, ground radiation is not blocked. Ground
radiation may or may not be a problem, depending on the surroundings. Ground radiation from a concrete
surface, which is a good reflector, would be greater than that from a surface with grass or shrubbery. In
experiments, venetian blinds produced slightly better reflectance measurements than draperies under the same conditions.
The angle of the slats, the shadow-line angle, and the spacing ratio of the slats affect the proportion of straight-through radiation and reflected-through radiation.
Reflected radiation is also dependent upon the absorbency of the slat for solar radiation. Shade
performance can be improved by the use of highly reflective paints and colors on the slats.
Increasing the reflectance of the slats will result in an increase in the amount of solar radiation admitted to
the room because of the increase in the reflected-through radiation. The increase in reflectance to the outside, however, will counteract this reflected-through component so that the total overall heat gain is therefore reduced.
Light Control
With slat shades, light control is also an important consideration. Slats can be adjusted so that direct
sunlight is blocked and reflected sunlight is directed to the ceiling. This will cause the ceiling (if it is a light color) to act as a diffuser and provide a more uniform illumination without glare. Use of day-lighting for illumination can further contribute to energy conservation, both in reducing the use of electricity
needed to provide light and in reducing the heat generated by the lights.
Use of blinds on a sunny window in the summer can reduce heat gain by about 45% if the blinds are highly reflective, fully lowered, and completely closed. Venetian blinds between glass or to the outside of glass will be more effective than those used on the interior.
Draperies
Draperies are popular residential window treatments.
Their interaction with solar radiation is a complex one because of the variables of weave, color, reflectance, transmittance, absorbency, and convolution of pleats.
One drapery classification system recognizes comfort factors as well as shading coefficients. Open weave is defined as fabric that allows good outward vision, partial protection against solar radiation, and uncomfortable reradiation from warm glass. Dark, open weaves provide the best outward vision, and light colors have higher reflectance. Semi-open weaves do not permit details to be seen, but larger objects are definitely outlined.
Closed fabrics are those through which no objects are visible, but light or dark areas may show.
Draperies contribute to the comfort factor in a room during the summer because they generally stay cooler
than other shading devices. The many folds of the drapery lose heat by convection to keep draperies cooler, and this in turn protects those sitting close to the window. In addition, draperies can contribute to the comfort level of the room by absorbing room noises.
(Draperies do little to reduce noises entering from outside.) The noise-absorbing quality of draperies is
related to the openness of the weave: the tighter the weave, the greater the absorption.
Ceilings and carpets are the major sound absorbers in a room. The third way in which draperies contribute
to the comfort factor is by modification of view and glare. Semi-open weaves can allow light to enter while still providing privacy by day. Brightness control is required for eye comfort. For this purpose, off-white is better than white, as off-white does not appear as bright when exposed to the sun.
The insulation value of draperies affects how much outdoor heat is added to the summer cooling load.
Tightly woven draperies will be more effective than looser weaves. Also of importance in establishing the
insulation value of draperies is the method of mounting.
Draperies should be hung so that they are as close to the window as possible.
The sides of the draperies should be sealed to the wall or window frame, and the draperies should contact
the floor or the window sill. The top of the drapery should either touch the ceiling or be enclosed by means
of a closed-top cornice. Mounting draperies in this fashion prevents the chimney effect between the drapery and the window, improves the U-value of the window, and decreases heat gain or loss by convection.
Air from heating or cooling vents should also be deflected from the window so it cannot flow freely
between the drapery and the window. Sealing the top and sides of the drapery not only reduces heat loss in
winter, but also reduces total heat gain in summer.
Double draperies (two pairs of draperies used in tandem) can further reduce the U-value of the
fenestration to about 0.5 by incorporating an additional air space into the configuration. The use of double
draperies usually provides a tighter air space than the use of single draperies or a closed venetian blind. With the double drapery arrangement, the solar heat delivered to a room is a combination of the transmitted heat, the heat absorbed in the room-side drapery, and the heat absorbed in the window-side drapery less the heat radiated to the glass from the window-side drapery. One major advantage to this is that the room-side drapery will be more nearly the temperature of the interior space, which adds to the comfort of the room occupants.
Double drapery arrangements frequently consist of a sheer or semi-open drapery and a heavier, closed-weave drapery. It is customary to place the sheer drapery closer to the window, with the heavier drapery
roomside. The heavier drapery is then kept open by day and the sheer gives privacy and reduces glare while allowing daylight to enter. The heavier drapery is closed at night for more privacy.
A more effective way of hanging the draperies to deal with heat gain and loss would be to reverse them,
with the heavier and light colored one window-side and the sheer room-side. By doing this, the tighter weave drapery would be in a position to block solar radiation by reflecting radiation back through the window before it could be absorbed. The sheer drapery would add the extra insulation layer when it was closed.
Reflectance is the dominant factor in determining the solar heat gain potential of drapery fabrics.
Transmittance is less important because a decrease in transmittance usually results in an increase in heat
absorption, so there is still a contribution to heat gain.
All transmitted energy, and 75% or more of the absorbed energy, becomes part of the cooling load.
Draperies are less efficient at reducing winter conducted heat loss. Medium-colored draperies with
white plastic backing were found to reduce conducted heat loss in winter by 6-7%. This type of drapery
reduced conductive and radiant heat gains in summer by 33%.
Drapery management is important in producing the desired effect. Windows receiving direct sunlight in
winter should have the draperies completely open so that the window area is not blocked. Draperies and rods should be wide enough for complete stack-back to allow for this complete opening. Draperies should be tightly closed during winter nights and hours when the sun is not on the window. For summer, draperies should be closed during sunlight hours and can be opened at night to provide ventilation.
Interior Shutters
Shutters can be used as an interior shading device as well as an exterior one. Several types of shutters have
been in use for many years, and wooden shutters are increasing in popularity. Louvered shutters are
appropriate when the primary concern is with summer shading. Movable or fixed louvers prevent direct
sunlight from entering the rooms throughout most of the day. At the same time, daylight can still enter to lessen the need for artificial lighting.
Louvered shutters allow for natural ventilation while blocking direct radiation. Louvered shutters will not
provide insulation in colder months. Shutters can be combined with other window treatments such as
draperies.
Solid Shutters
Solid shutters decrease winter heat loss and summer heat gain, but limit the amount of daylight, ventilation
and view available. If the shutters are installed or placed so that they fit tightly to the window frame, they can create an effective air space to act as insulation.
The insulation value of the shutter will depend on the insulation value of the shutter material and the air
space. Plywood shutters can improve the U-value of a single glazed window from 1.13 to 0.36. Other materials can provide even better insulation.
Materials with good insulating qualities, such as urethane and polystyrene foam panels, can be fitted to
form close fitting shutters. Materials such as these should be used with caution because of their high
flammability and the toxicity of the smoke and fumes they create when they burn. Such materials should be clad with some protective covering to reduce this hazard. They will help to reduce winter heat loss and can improve the comfort of those sitting next to the windows.
The elderly are particularly vulnerable to hyperthermia when exposed to cold air and drafts.
Jalousie windows used in older California homes do not seal tightly and are a source of cold air. The solid shutters are relatively easy to make and fairly inexpensive, particularly for small windows. They
can be very attractive and appropriate for homes or certain rooms that are not occupied during the day --
thus with no need for lighting.
Mounting insulating shutters so that they are easy to operate and store can be difficult. Shutters may be
hinged to fold back out of the way when not in use.
Shutters made of foam insulating boards are easily crushed and must be handled with care.
REFERENCES
Bernabei, Carole A. Window Treatments For Energy
Conservation, L.E.T. Project Technical Bulletin,
November 1980.
Cook, Gary. Building and Remodeling to Save Energy,
California Energy Extension Service, FED-35, July 1986.
Fairey, Philip, and Ross McCluney. Techniques for
Shading Residential Walls and Windows, California Solar Energy Center, FSEC-CN-8-86, May 1986. McCluney, W.R. Window Management For Energy
Conservation, California Solar Energy Center, FSECEN- 4-80, January 1985.
Ozisik, N., and L. Schutrum. Heat Flow Through Glass
with Roller Shades. ASHRAE Transactions, 1959,
65, 697-710.
Pennington, C., W. Smith, E. Farber, and C. Reed.
Experimental Analysis of Solar Heat Gain Through
Insulating Glass with Indoor Shading. ASHRAE
Journal, 1964, 6, 27-40.
Parmelee, G., and D. Vild. Design Data for Slat-Type
Sun Shades for Use in Load Estimating. ASHVE
Transactions, 1953, 59, 403-434.
Rosenfeld, A.H. Some Comments on Dual Solar-Control
Venetian Blinds, Energy and Building, 1977, 1, 97-
98.
Vieira, Robin. Dealing with Heat and Humidity in
California Homes, California Solar Energy Center, FSECEN- 14-86.
Compiled by YJay Draiman
