Reviewed October 1993
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Windows provide your home with solar heat, light and natural ventilation. They also represent one of the greatest sources of heat loss and may account for up to 30 percent of your home's total heat loss in winter.
To reduce losses, you can insulate or improve the management of your windows. The basis for determining the effectiveness of insulation is its R-value or resistance to heat flow. In Missouri, recommended R-values for home insulation are R-30, ceilings; R-19, walls; and R-13, floors. For comparison, consider that a single-glazed window has an R-value of about one and a double-glazed window (or window with storm), about two. In other words, nearly 20 times more heat escapes through each square foot of window than through an insulated wall. Based on current costs window coverings, an R-value of four is considered cost- effective. With this R, the product has about 75 percent efficiency, which means it stops 75 percent of the heat loss through the window. Table 1 below lists the R factors of various window types.
Heat losses occur by infiltration when there are cracks around the frame or glass. Heat is conducted through glass rapidly when it warms the glass and is transmitted to the colder outside environment. Convection losses occur as the air between the window and the heat source cools; this cooled air drops down pulling heated air over the top to be cooled, and the cycle repeats. Radiation is the process of losing heat through space. For example, a person seated near an uncovered window will begin to chill as their body heat radiates to cold glass surfaces.
Table 1
Insulation value of windows and window treatments
| R-value | |
|---|---|
| Single glazing, bare1 | 0.8 to 1.0 |
| Double glazing, bare | 1.8 to 2.0 |
| Single glazing, loose drapery2 | 1.10 |
| Double glazing, pulled shade | 2.05 |
| Double glazing, sealed drapery | 2.35 |
| Double glazing, insulated shutters | 9.50 or more3 |
| Double glazing, insulated quilt, one layer polyester fiberfill | 4.55 |
| Double glazing, insulated quilt, three layers of polyester fiberfill | 6.75 |
The first step in your management plan should be to correct infiltration problems. This can be done with caulking, weather-stripping or a combination of storm windows. Once the weatherizing materials and storm windows are in place, attention should turn to efficient inside window coverings. Your selection should reflect a balance between control of function, improved efficiency and an appearance that meets your satisfaction and budget.
There are two basic types of energy-efficient interior window treatments -- the rigid insulating panel or shutter and the thermal shade.
Rigid insulation board can be styled as a pop-in panel and placed in windows at night for a simple and relatively inexpensive form of movable insulation. Hinged or bi-fold shutters can be attached to the window frame similar to decorative louvered shutters. These shutters can be made using loose-fill insulation, hardboard covering and wood framing, or rigid foam insulation and plywood covering. They can be covered and decorated with paint, stain, fabric or wallpaper.
Thermal shades can be made from a variety of materials using reflective surfaces or insulated with batting fibers, fiberfill and operated like an ordinary roller shade or Roman shade. Common materials and the manufacturer's advertised insulating values are listed below.
Table 2
Insulating materials
| R-value | |
|---|---|
| Foylon | R = 2.02 |
| Fiberfill, 1 layer | R = 2.5 |
| Fiberfill, 2 layers | R = 4.0 |
| Fiberfill, 3 layers | R = 5.5 |
| Window fleece | R = 4.0 |
| Hollofil | R = 2/inch thickness |
| Polar Guard | R = 3.5/inch |
| Thinsulate | R = 4 per 3/4 inch |
| Warm Window | R = 7.5 + (includes single glazing) |
| Window Quilt | R = 4.25 (includes single glazing) |
| Window Comforter | R = 5.0 + (includes single glazing) |
| Texolite® | 1R = 7.0 + (includes single glazing) |
Effective R-values depend on good construction and proper installation and management; loss of value may be due to degradation of materials and air infiltration around and through the product.
An effective insulated treatment must include these features:
A well-designed insulated window treatment's efficiency will depend greatly on management. Daily operation of insulated coverings is necessary and soon will become routine. Otherwise, just a storm window will perform better than a higher-rated treatment that is used occasionally.
In the winter, there are about 16 hours a day when windows aren't performing their intended function. Instead, they are an enormous heat drain, which provides a good reason for finding ways to insulate them.
South windows
Open the window treatment for the six best hours of sunshine. These windows gain more heat during the day than they lose at night. Night insulation is very effective.
East and west windows
Open the window treatment for the three best hours of sunshine. Solar heat is gained through east windows during morning hours and through west windows in late afternoon.
North windows
Close unless there is sun in morning or evening. North windows receive no direct sunlight and are always losing heat.
All window coverings closed all hours the air conditioner is operating. If air conditioner is not in use, close those windows and window treatments receiving direct sunlight. Use shaded windows for ventilation.
Safety
Many insulating treatments incorporate substantial quantities of plastic foam and films and synthetic fibers. If exposed to extreme heat or flame, these may constitute a smoke and fire hazard. During a fire, some foam products may give off toxic fumes; these products should be covered with fire-retarding material such as hardboard or plywood. (The International Fabricare Institute has this information on wet-side applications to improve the flame-retardant qualities: mix a solution of 7 ounces of borax with 3 ounces of boric acid per gallon of water. Immerse items in solution for 5 minutes. Extract excess solution and air dry. The family should be aware that retardants reduce flame spread; they do not prevent burning.)
Windows are often used as an emergency exit. Coverings which obscure the window location and whose operation is not apparent may present a safety hazard to family members and visitors. All families should install smoke detectors throughout the house.
Appearance and operation
Will the window coverings coordinate with the existing or planned interior design? Are the seals and controls convenient and easy to operate?
Installation and maintenance
Multiple layers, composite layers and dissimilar materials require special handling. Fiber batting may fuse if ironed. Dry cleaning solvents can destroy tape adhesive. Water washing may cause uneven shrinkage. Check the manufacturer's information carefully. Fabric-covered window treatments may be sprayed with a soil retardant and cleaned regularly with the appropriate vacuum cleaner attachment. Installation must be precise for effective edge sealing. If you have professional installation, it should be included in payback calculations. Also include your hidden costs of gathering supplies and materials for do-it-yourself projects. Pop-in panels will need to be stored when not in position.
Payback
Insulating window treatments are considered temporary devices and may not be eligible for federal tax credits as are storm windows. But you may check to see if regulations have changed.
Simple payback can be calculated by dividing the total cost of the project by the yearly savings. Depending on where you live in Missouri, adding an R-value of 4 to a double-glazed window can save from 40 cents to 60 cents (1983 fuel costs) a square foot of window area each year, depending on cost of fuel used. Not reflected in these figures is the escalating cost of fuel and the improved comfort level when cold window surfaces are covered.
A useful formula for estimating yearly savings is:
R added = the insulating value of the window treatment you are considering.
R existing = the insulating value of your bare window (Table 1).
DD = the number of heating degree days where you live. Your electric utility or a heating contractor should be able to provide this.
24 = 24 hours each day.
Square feet= area of the window to be covered.
For example: (the 2 and 1/2 in the example come from the existing window)
At $12.50 per million Btu (or $1.25 per 100,000 Btu), your yearly savings at that window would be about $7.50. Divide this figure into the cost of the new window treatment to find the payback period in years.
Use the checklist below to evaluate different window treatment options, either purchased commercially or home constructed.
Table 3
Evaluation checklist for window treatment options
| Rating | Yes | No |
|---|---|---|
| Does it provide a minimum R added of 4? | ||
| Is there a provision for a continuous side seal? | ||
| Is there a vapor barrier present directly behind the decorative or face fabric? | ||
| Is it possible or convenient to open and close window treatment as needed? | ||
| Have you protected potentially harmful materials from contact with heat and flame? | ||
| Are the products easy to remove and clean? | ||
| Can you afford special installation and cleaning? | ||
| Does the family like the product's appearance? | ||
| Does it match your decor and furnishings? | ||
| Is the cost of the product justified when compared to the value of energy saved? |
GH2815, reviewed October 1993