Save Energy — Windows

It is always enjoyable to look out a window. Whether it is to check the weather or check on the kids playing outside, windows serve an important purpose in your home. Unfortunately, they also can be the source of huge energy loses. Energy is lost by heat passing through the glass, air infiltrating through cracks, and radiant energy through the glass.

Your Home Energy Living Plan

  • Seal cracks around window.
  • Replace weatherstripping.
  • Remember to close and lock windows when the outside temperature is too high or low.
  • Close shades or drapes at night in the winter and during the day in the summer.
  • If windows have one pane of glass, upgrade to double pane windows. Consider other options like low e glass or triple pane. See discussion below.
  • Consider exterior awnings in areas with high cooling needs.
  • Consider planting trees to shade windows.

Impact of Your Energy Saving Actions

For the average US home, 35% of the annual utility bill is due to heating and 11% is due to cooling. Of that percentage windows are 26% of the heating and 33% of the cooling. Also infiltration is 28% of the heating and 16% of the cooling. Most infiltration will be at the windows.

Assuming 70% of the infiltration at the windows, windows account for about 21% of the annual utility bill. If you have especially leaky windows or single pane windows then your use may be higher. Energy saving actions for your windows can have a major impact on your utility bill.

How Energy is Lost

Glass has a low resistance to heat flow as compared to the wall it is mounted within. Windows with just one pane of glass (single glazed) and a wood frame have an U-Value of 0.84 (R = 1.19). The insulation in a typical wall has a R-Value of 11.0 (3.5 inches of fiberglass batt insulation). Every square foot of window (single glaze) has about 10 times more energy passing through it than a square foot of wall.

The frame of a moveable window may not close tightly or trim may have pulled or eroded away from the house. This creates small cracks that allow air will to pass through. Air movement through cracks or unintended openings is called air infiltration. When the wind is blowing on the window, air will be forced inside while air is sucked outside on the opposite of the house. During a cold winter night a freezing gust of wind can enter the house, causing a cold draft that can be felt. Not all air infiltration is easy to notice. The air you just paid to condition can flow out of the house through window cracks, but this air will not be felt.

The energy of the sun can pass through the transparent window. If sunlight is passing through the glass, then energy is entering your home. This is an advantage in the winter, the heat from the sun will warm your house, reducing the run time of your furnace. However, in the summer, this sunlight can add tremendously to the running time of your air conditioner. This sunlight entering through the window is called solar gain. It is radiant energy transfer directly from the sun. At night, radiant energy will transfer through the window to the colder sky. In the winter this will need to be compensated by running your furnace.

Due to the high heat transfer of a window, the inside glass surface temperature may be well below the indoor air temperature in the winter. The indoor air next to the glass will cool and fall to the floor because it is denser than the room air. As this air falls, warm interior air replaces it, which in turn is cooled. A circulation of cool air is created and can be felt by you. This is not infiltration, but can still feel like a cold draft. The lower glass temperature can condense the moisture out of the air. Seeing condensation, especially if it is frozen, is an indication much heat is being lost through the window.

Window Characteristics

Any item used to fill an opening in the outer structure of your home is referred to as a fenestration. Common fenestrations are windows and doors. Energy codes define a fenestration as anything which allows light to pass. However, the definition of a fenestration differs between various organizations. By all definitions, windows are fenestrations.

Windows have two major components; glazing and frame(including sash). The transparent portion is called glazing. Glazing can be any transparent material, but generally it is made of glass. Older windows may have only one pane of glass in them. They are called single glazed windows. Due to the poor heat transfer, modern windows may have two parallel panes of glass, called double glazed. The glass will be separated by a small air gap. This gap increases the insulation of the window. A second glazing can decrease the U-value from 0.84 down to about 0.45. This is close to doubling the insulating performance of the window. Air is a good barrier to thermal conductance. Some manufacturers offer windows with a third pane of glass called triple glazing. The third glazing can reduce the U-value down to about 0.14. This is a significant reduction but the cost of the window is much higher than double glazed. Typically triple glazing is only used in northern climates to insulate from the extreme cold winter temperatures.

Due to the large amount heat transferred from the sun shining through the window another term is used. This term is SHGC, which stands for solar heat gain coefficient. The SHGC is a fraction from 0 to 1, which indicates the amount of heat gain from the sun. A SHGC of 1.0 means 100% of the heat gain is transferred through the window. Single glazed windows have a SHGC of about 0.64. While transparent, glass still reflects some light. When you see your reflection in a window, this is reflected light. Double glazed windows will reduce the SHGC to about 0.56. The second pane adds more reflection.

Special coatings can be added to the glazing to further reduce the SHGC. Glazing with these coatings are called low-e glass. The e stands for the emissivity or ability to radiate through the glass. So a low-e glass blocks more of the radiant transfer resulting in a lower SHGC. Low-e double glazing can reduce the SHGC from 0.56 down to 0.30.

Do not forget about the frame when trying to save energy. The sash is the part that holds the glazing in place. For an operable window the sash and glazing move together. The sash slides or pulls away from the frame. If the edge where glazing meets sash leaks air, then replace the caulking at this location. The points where sash meet frame require weatherstripping. This weatherstripping must remain pliable to provide a good seal when window is closed. Over time the weatherstripping can become brittle, not pliable, or simply fall off. Movement of the house can cause gaps between the frame and the house resulting in air leaks. Or the frame may move causing the sash to be misaligned. This can also create air leaks.

Energy Solutions Explained

Sealing your windows will aid in reducing energy. As your home ages, some movement occurs and seals become hard and inflexible. The older the window the more likely weather-stripping will be a large benefit. Estimating the potential savings is difficult because evaluating the infiltration is difficult. If you have single pane windows, replacing with double pane would be beneficial.

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