How to reduce heat loss through windows
Energy-Efficient Window Treatments
The awnings on this home shade the windows and generate electricity. | Photo courtesy of ©iStockphoto/jhorrocks
What does this mean for me?
You can choose window treatments or coverings not only for decoration but also for saving energy. Some carefully selected window treatments can reduce heat loss in the winter and heat gain in the summer. Window treatments, however, aren't effective at reducing air leakage or infiltration. You need to caulk and weatherstrip around windows to reduce air leakage.
Window awnings can reduce solar heat gain in the summer by up to 65% on south-facing windows and 77% on west-facing windows. You can use an awning to shade one window or have an awning custom-made to shade the entire side of your house.
In the past, most awnings were made of metal or canvas, which need to be re-covered every five to seven years. Today, awnings are made from synthetic fabrics such as acrylic and polyvinyl laminates that are water-repellent and treated to resist mildew and fading. Whatever the fabric, you should choose one that is opaque and tightly woven. A light-colored awning will reflect more sunlight.
Awnings require ventilation to keep hot air from becoming trapped around the window. Grommets (eyelets) or other openings along the tops and sides of an awning can provide ventilation. The awning may also open to the sides or top to vent hot air.
You can roll up adjustable or retractable awnings in the winter to let the sun warm the house. New hardware, such as lateral arms, makes the rolling up process quite easy.
Window blinds—vertical or horizontal slat-type—are more effective at reducing summer heat gain than winter heat loss.
Because of the numerous openings between the slats, it's difficult to control heat loss through interior window blinds, but the slats offer flexibility in the summer. Unlike shades, you can adjust the slats to control light and ventilation. For example, when completely closed and lowered on a sunny window, highly reflective blinds can reduce heat gain by around 45%. They can also be adjusted to block and reflect direct sunlight onto a light-colored ceiling. A light-colored ceiling will diffuse the light without much heat or glare.
Exterior roller blinds are usually made of wood, steel, aluminum, or vinyl. They're mounted above the window, and side channels guide them as they're lowered and raised. When you lower these blinds completely, their slats meet and provide shade. If partially raised, the blinds allow some air and daylight to enter through windows.
A drapery's ability to reduce heat loss and gain depends on several factors, including fabric type (closed or open weave) and color. With such a wide variety of draperies available, it's difficult to generalize about their energy performance.
During summer days, you should close draperies on windows receiving direct sunlight to prevent heat gain. Studies demonstrate that medium-colored draperies with white-plastic backings can reduce heat gains by 33%. Draperies also stay cooler in the summer than some other window treatments because their pleats and folds lose heat through convection.
When drawn during cold weather, most conventional draperies can reduce heat loss from a warm room up to 10%. Therefore, in winter, you should close all draperies at night, as well as draperies that don't receive sunlight during the day.
To reduce heat exchange or convection, draperies should be hung as close to windows as possible. Also let them fall onto a windowsill or floor. For maximum effectiveness, you should install a cornice at the top of a drapery or place the drapery against the ceiling. Then seal the drapery at both sides and overlap it in the center. You can use Velcro or magnetic tape to attach drapes to the wall at the sides and bottom. If you do these things, you may reduce heat loss up to 25%.
Two draperies hung together will create a tighter air space than just one drapery. One advantage is that the room-side drapery will maintain around the same temperature as the interior space, adding to a room's comfort.
High-reflectivity window films help block summer heat gain. They are best used in climates with long cooling seasons, because they also block the sun's heat in the winter.
The effectiveness of these reflective films depends on:
- Size of window glazing area
- Window orientation
- Building orientation
- Whether the window has interior insulation.
Silver, mirror-like films typically are more effective than the colored, more transparent ones. East- and west-facing windows, because of their greater potential for heat gain, can benefit more from these films. North-facing windows won't benefit from them, and south-facing windows may benefit somewhat, but the benefit could be offset by the reduction of heat from the winter sun.
These films have some overall disadvantages:
- Loss of interior light or visible transmittance
- Impaired outside visibility
- Extra care required for cleaning
These reflective films are available to apply yourself over existing windows. Some window manufacturers also make reflective glazing or glass .
An insulating window panel or pop-in shutter typically consists of a core of rigid foam board insulation. You can push or clip it into the interior of a window. The panels are made so that their edges seal tightly against the window frame. Seals can be made from magnetic tape or Velcro. No hardware, such as hinges or latches, is required.
Insulating window panels have R-values between 3.8 and 7. They are also fairly inexpensive, whether you buy a kit or make your own, but you will need space to
store them when they're not in use.
Mesh Window Screens
Mesh window screens can diffuse solar radiation, reducing heat gain in the summer. Screens should be mounted in an exterior frame and should cover entire windows. They are particularly effective on east- and west-facing windows.
Properly sized and installed roof overhangs can most effectively shade south-facing windows from the summer heat. If oriented properly, overhangs will allow the sunlight in through the windows during the winter, providing more warmth to a house.
Construction and orientation of an overhang can be tricky, because it involves many passive solar design considerations, including:
- Solar radiation transmittance
- Illuminance levels
- Window size and type.
It's easy to incorporate overhangs into a home design before or while it's under construction. Adding an overhang to an existing home, however, can be quite difficult and sometimes impossible. Window awnings, louvered patio covers, or lattice-type panels can be considered as alternatives for existing homes.
When properly installed, window shades can be one of the simplest and most effective window treatments for saving energy.
Shades should be mounted as close to the glass as possible with the sides of the shade held close to the wall to establish a sealed air space. You should lower shades on sunlit windows in the summer. Shades on the south side of a house should be raised in the winter during the day, then lowered during the night.
For greater efficiency, use dual shades—highly reflective (white) on one side and heat absorbing (dark) on the other side—that can be reversed with the seasons. The reflective surface should always face the warmest side—outward during the cooling season and inward during the heating season, and they need to be drawn all day to be effective.
Quilted roller shades and some types of Roman shades feature several layers of fiber batting and sealed edges. These shades act as both insulation and air barrier, and control air infiltration more effectively than other soft window treatments.
Pleated or Cellular Shades
Several manufacturers have designed two- or three-cell pleated or cellular shades with dead air spaces, which increase their insulating value. These shades, however, provide only slight control of air infiltration.
Window shutters—both interior and exterior—can help reduce heat gain and loss in your home.
Interior shutters need a clear space to the side of the window when they're opened. They also require hardware that is fastened to the window jams or trim. Properly designed exterior shutters may provide the best possible window insulation system. They offer several advantages:
- Weather protection
- Added security
- No use of interior space
- No thermal shock to windows if left closed.
Exterior shutters must be integrated into your home's architecture. Their mounting, drainage, and hinging will require special consideration, and it's easier to address these design issues in new construction.
Most exterior shutter systems include a mechanical crank, rod, or motor to allow operation from indoors. This can help encourage daily use of the shutters, and may be required by local fire codes.
Roll-down metal exterior shutters are often used as protection against storms and/or vandalism. While metal shutters provide protection against these hazards, they don't provide much of a barrier against air infiltration and heat.
Like window blinds, louvered shutters work best for summer shading. Movable or fixed louvers allow ventilation and natural daylight to enter a room while blocking some direct radiation. However, they won't provide much insulation against heat loss in the winter.
Solid shutters will decrease both heat loss and summer heat gain. These insulating shutters consist of wood panels, a vapor barrier. and sometimes a decorative covering. If you fit them tightly against a window frame, they'll provide an insulating air space between the shutter and the window.
You can combine shutters with other window treatments such as draperies for greater insulating ability.
A storm panel added to a single-pane window can reduce winter heat loss by as much as 50%. They are also less expensive than double-glazed windows. You can add them to the exterior or interior side of windows.
There are two types of exterior storm window panels: single and combination. Single storm panels are made of glass, rigid plastic, or plastic sheeting. You typically put them up in the fall and take them down in the spring. A combination panel consists of two windowpanes and a permanent screen over the window. In the summer, you can slide one of the panes up and the screen down for ventilation. Exterior storm window panels need to be custom-made.
Interior storm window panels consist of flexible (like polyethylene) or rigid plastic. Rigid plastic panels are typically mounted using Velcro, magnetic, or snap-in seals. You can easily install the flexible type in window frames using snap-in retainer seals or double-faced tape. Despite their ease of installation, interior panels are usually not as clear as their rigid counterparts. Flexible panels may also wrinkle or sag after installation.
Heat-shrink film, however, doesn't wrinkle. This type of flexible film adheres tightly against the seal as it's heated using a hair dryer.
Interior storm window panels should go up before the heating season and come down before the cooling season. They are more useful for windows with awnings or for those that crank-out, where it's difficult to use an exterior storm window panel.
Unlike exterior storm window panels, interior panels don't have to be custom-made to fit windows, though custom-made ones are available from some window suppliers. Therefore, interior panels usually cost less. You can purchase interior panel kits from building suppliers or hardware stores.Source: energy.gov