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Movable Insulation

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In recent years, passive solar heating has become popular with people building new homes or retrofitting older homes. In direct gain passive solar homes, solar radiation enters through large south-facing windows and heats the living space. While heat is gained through windows during the day, needed heat escapes through windows at night. The average home loses 10 to 25 percent of its heat through windows. This percentage may be even higher for passive solar homes because they usually have larger window area, although these homes also gain more heat if properly designed.

Movable insulation is one option for windows that can be used in both passive solar and conventional homes to prevent this heat loss. Movable insulation is a device such as an insulative shade, shutter, panel, or curtain that reduces heat loss at night and during cloudy periods, and heat gain during the day in warm weather. When used properly, movable insulation can increase or decrease net heat gain, depending on user needs. These insulation devices may either be removed or adjusted to allow heat gain in the day when it is needed.

Heat Loss Through Windows (410, 563, 937)

To learn how movable insulation works, you must first understand how heat is lost through windows. An entire window consists of the glazing (the glass), the window frames, sash, sill, jamb, and head. The amount of heat lost through all parts of the window varies depending upon the outdoor windspeed, the difference between the indoor and outdoor temperature, and the tightness of the window seal against the frame.

A house loses heat through windows in many ways. Heat travels from warmer to cooler areas. The transfer of heat occurs through conduction, convection, and radiation. Conduction occurs when heat moves directly through materials from molecule to molecule. Most parts of the window, such as the window frame and glazing, conduct heat. Convection occurs when a fluid, such as air, contacts a surface at a different temperature. Warm air particles that touch a cool window become cooler and denser, and sink. The air's variation in temperature causes it to circulate, thereby cooling the interior living space and creating uncomfortable drafts. Radiation occurs when warm objects transfer heat directly to cooler objects. For example, a person standing near a cold window will feel chilled as the warm body radiates heat to the colder window.

You can reduce these forms of heat loss, as well as air infiltration through leaky windows, by caulking and weatherstripping, and by installing exterior storm windows or interior storm windows. If you need to replace windows because they are old, you may want to install windows with low-emmissivity glass, which can significantly reduce heat loss. These methods are usually less expensive than most types of movable insulation. You may want to use movable insulation, however, if conventional methods do not sufficiently lower heat losses.

Most movable insulation improves the energy efficiency of windows by creating an air space between the insulating material and the window. The air space acts as an insulator and reduces heat losses caused by convection and conduction. The insulating material in the movable insulation also lowers heat loss caused by conduction. This is only effective if air does not leak around the insulation, so you should tightly seal the movable insulation around the window frame. You may also add a reflective material to the insulating material to lower heat loss caused by radiation.

Movable Insulation Options

There are four types of movable insulation: shades, shutters, panels, and curtains. Shades, panels, and thermal shutters can be placed on the inside or outside of the home. Curtains, of course, are installed only on the inside of the home. Interior and exterior movable insulation have their own advantages and disadvantages and suit different needs. Most people like interior window insulation because it is reachable, protected from the weather, and usually less expensive than other types of movable insulation. It also doesn't affect the exterior appearance of the house. Since this type of movable insulation is indoors, you should choose a material that is aesthetically attractive.

The exterior units are usually better for reducing summer heat in warm climates. Exterior insulation rejects heat in the summer, improves security, requires little space, and virtually eliminates window condensation. This type of movable insulation, however, must resist seasonal weather extremes, and can affect the outer appearance of the house. It also must be installed and removed outside. Going outside to remove the insulation can be avoided by installing a crank, pulley, or device that can be operated indoors.

Types of Movable Insulation (970, 843, 883)

Thermal curtains are conventional-looking window coverings that save energy. Unlike traditional curtains, they are sealed tightly around the edges of the window frame. The bottom seal is especially important because cold air collects at the bottom of the window. Thermal curtains contain a layer of insulating material, and therefore decrease heat conduction through the window. Thermal curtains may also contain a reflective liner or vapor retarder.

Shades are convenient to use, and unlike shutters, allow you to place plants and furniture near the window. Interior shades are either fiberfilled or quilted. Interior rolls shades are bulky and can be difficult to use. To avoid this problem, roman insulated shades, which fold up, are now available. Thin membrane materials with high R-values are also available, but some varieties are highly flammable. Exterior insulative shades are designed to resist the harsh effects of the wind and sun. Shades with foam-filled aluminum slats are an example of this type of movable insulation.

Shutters are popular because they can be installed in a variety of ways. Homeowners can purchase shutters with hinges at the sides, top, or bottom. Homeowners also can install tracks that allow the shutters to slide into place. Shutters are effective and generally easy to use. Exterior shutters reflect extra sunlight into a room if they are open and hinged at the bottom.

Panels cost less than other types of interior window insulation. Pop-in panels are made of rigid, lightweight insulation such as polystyrene or polyurethane and fit snugly inside the window frame. They may be "popped out" and stored away when they are not needed. Panels also may include face seals that directly press the insulation against the glass. This allows little space for cold air convection or infiltration because the insulation touches or almost touches the glass. Pop-in panels made from polystyrene and polyurethane burn easily and should be covered with a fire retardant.

Insulation for skylights, clerestory windows, sunspaces, and greenhouse prevents heat losses at night in winter. Skylights and clerestory windows are especially vulnerable to heat loss because warm air collects at the ceilings where these windows are placed. A clerestory window is a window on a wall that rises above an adjoining roofed section of a building or room. The type of movable insulation chosen for these applications depends on how convenient the system is to operate and maintain, and how well it meets the criteria for selection listed below. Skylights also need shading in summer, although reflective films may also be used. Clerestory windows can be designed with an overhang to reduce heat gain in the summer.

Sunspace and greenhouse insulation can significantly reduce heat loss at night, eliminating the need for supplemental heating during the cold months. In summer, insulation, along with cross ventilation, helps prevent the greenhouse or sunspace from overheating. Overheating is usually less of a problem if these structures use only vertical glazing and overhangs.

Purchasing Movable Insulation

Movable insulation is practical if you are buying new curtains and shades because window insulation costs only slightly more than conventional curtains or draperies. If you have already installed curtains or draperies, you should invest in movable insulation only after assessing the local climate; the amount of window area; the location, use, and condition of the windows; local fuel costs; aesthetics; and the amount of money you are willing to spend.

Movable insulation in some homes may quickly produce impressive energy savings, while in others, it may only produce marginal savings. If you do not properly install, use, or maintain the device, it will probably produce only low or moderate savings on heating and cooling costs. You must be willing to spend time opening and closing the devices during the appropriate times of the day, depending on whether you want to increase or reduce heat flow. Movable insulation should also fit the window tightly to function properly.

You may want to calculate savings before making a purchase to determine the cost-effectiveness of movable insulation in your home.

Selection Criteria

Each type of movable insulation has its strengths and weaknesses. To make a wise purchase, you need to carefully select the type of movable insulation that best suits your needs. You should examine the material's ability to resist heat flow. You should make sure that the insulation fits tightly without causing moisture problems, operates easily, and looks attractive. The following section describes the criteria for selection that purchasers of movable insulation often examine.

Window insulation must provide a barrier against the cold. Its ability to resist heat flow is indicated by the material's R-value. The higher the R-value of the insulation, the better it performs. Window materials with R-values from R-2 to R-5 per inch significantly reduce heat loss through conduction. Determine how the value was established when comparing the R-values of movable insulation. R-value ratings for movable insulation are not currently based on a standardized test. Ask the dealer to give the R-value test results for each product. Make sure that the advertised R-value applies to the actual product being examined, and not for a standard 1-inch thickness of the insulation. For example, the vendor may claim the insulating material has an R-value of 4. The R-value advertised may apply to 1-inch of the material. The product, on the other hand, may have only 1/2-inch of insulation. The movable insulation, therefore, would only have an R-value of 2.

Attaching reflective material to the movable insulation may reduce radiant heat transfer. As long as it faces an air space, reflective material may be attached to either side of the movable insulation.

Movable insulation's air-tightness is just as important as R-value. Edges of the insulation must be tightly sealed to eliminate air flow between the window and the room. The edge seals are a component of movable insulation that prevent convection and infiltration by immobilizing air between the window and insulation. Edge seals also help prevent condensation on the windows. Edge seals must be precisely installed to fit each window's measurements. This is especially important for older windows that are not perfectly rectangular or whose corners are not perfectly square.

Edge seals vary according to the type of movable insulation. The main types of edge seals are roller tracks, wooden clamp strips, hook-and-loop tape, magnetic seals, or sail tracks. While all edges should be held tightly against the window frame, the bottom edge seal is the most crucial for insulating windows in the winter.

Window condensation is another concern with movable interior insulation. Moisture in the air passes through the insulation and condenses on windows when the relative humidity in the home reaches 40 to 50 percent, or 25 to 30 percent in extremely cold weather. Freezing condensation forms frost that can rot the window frame. Tight seals and vapor retarders prevent moisture from condensing on the window or the insulation. The insulation should be lined with a vapor retarder on the side facing the room. A vapor retarder is especially necessary for single-glazed windows or in areas of the house with high levels of humidity such as the kitchen and bathroom.

As its name implies, movable insulation requires a great deal of handling. It must, therefore, be durable and easy to operate. The more convenient the system, the more likely it is to be used. The movable insulation should be simple and safe to handle, and should easily open and close. The insulation and edge seals must withstand rapid temperature swings, moisture, everyday wear and tear, and prolonged exposure to ultraviolet light, which fades colors in fabrics. Durability is especially important for sunspace and greenhouse insulation systems, which usually have large window areas and sometimes cover slanted surfaces that place extra stress on the edge seals. While quality movable insulation should last many years, expect repairs and replacements. You should purchase movable insulation, therefore, that can be easily and quickly repaired.

Window insulation that replaces or supplements curtains, drapes, or shutters often affects the interior appearance of your home. Rooms with the greatest need for movable insulation are often the most frequently used areas of the house. You may prefer movable insulation that you can coordinate with your home and its furnishings. You should consider the effect that movable insulation will have on the house's appearance when choosing a system.


Adding movable insulation to several windows can be expensive, so many people make and install their own systems. Do-it-yourself projects can be a less expensive solution if the consumer has sewing or carpentry skills, and realistically assesses the project and the time required. Some systems, such as pop-in panels, are relatively simple to make. Kits also are available for those who need some instruction.

R-Values for different types of window treatments

  1. Single-pane glass, wood sash, 80% glass with no treatment - Total R-Value is R-1.01.
  2. Single-pane with either storm or double-glazing (i.e., glass, plastic or polyethylene sheeting) - Total R-Value is R-2.2.
  3. Single-pane with insulative roman shade (with reflective plastic thermal back) - Total R-Value is R-4.2.
  4. Single-pane with insulative thermal shutter (if insulation used has R-Value of 8) - Total R-Value is R-10.0.
  5. Single pane with rigid foam expanded polystyrene pop-in panel (approximate R-4 per inch), 1" thick - Total R-Value is R-5.9, 2" thick - Total R-Value is R-9.9, 3" thick - Total R-Value is R-13.9, 4" thick - Total R-Value is R-17.9.
  6. Single-pane with plastic foam (flexible) pop-in panel (R-3.69/inch), 1" thick - Total R-Value is R-5.6, 2" thick - Total R-Value is R-9.3, 3" thick - Total R-Value is R-13.0, 4" thick - Total R-Value is R-16.7.