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Energy Efficiency in Remodeling: Windows

November 1996     

Opportunities

In older homes, windows are often one of the largest sources of heat loss in winter due to their low insulating ability and high air leakage rates. Windows are also generally the major source of unwanted heat gain in the summer. As a result, windows are typically net energy losers, and can be responsible for 25 to 50 percent of the energy used to heat and cool homes. However, improved windows, combined with proper consideration of their placement and other details, can result in windows that provide a net energy gain.

Benefits

In addition to dollar savings, better windows can:

  • Improve comfort
  • Inside temperature of the glass is closer to room temperature
  • Drafts are reduced or eliminated
  • Eliminate condensation
  • Allow installation of smaller capacity heating and cooling equipment
  • Reduce fading of carpets and furniture fabrics

Factors to Consider

Whether you are replacing windows or adding new windows, some of the basic factors to consider include glazing type, orientation, shading, total area of windows, total insulating value, and installation methods. The National Fenestration Rating Council (NFRC), a non-profit organization, publishes a directory of over 22,000 tested/certified windows, skylights, and doors. he rating they provide indicates the whole-window U-values for two standard window sizes. In addition to the insulating value of the glazing and frame, these U-values include the effects of low-e glazing, gas fills, tinting, and films. For each manufacturer/window, information is provided which includes the product type, number of glazing layers, spacer type, presence of low-e and its value, and the gap fill and gap spacing.

Installation Issues

Proper detailing when installing new or retrofit windows can virtually eliminate air leakage around windows (between the window frame and the wall). Employ one of the following methods when installing windows:

  • spray low-expansion foam between the window frame and wall framing (not stuffing insulation, as is commonly done)

    OR

  • where air barriers or foam sheathing is used, seal around the exterior of windows with air barrier tape or caulk.

It is also important that windows are properly squared when installed to avoid resulting gaps that increase air infiltration around sash.

Related Issues

Design considerations - windows can provide a net energy gain by allowing the sun's rays to fall on the glass during the winter (when the sun is lower) and blocking them with shading in the summer (when the sun is higher). As much as possible:

  • Reduce the area of windows on east and west sides to avoid summer heat gain.
  • Properly shade windows with overhangs, awnings, or vegetation.
  • Limit the use of skylights, which almost always add more heat in the summer and less in the winter.

Air leakage - air leakage can occur through and around windows, and can have a significant impact on energy use and comfort.Air leakage at existing windows may be improved through weatherstripping and caulking. New windows are often tested for their air leakage through the window (between frame and sash). Better windows have tested air leakage rates between 0.01 and 0.06 cfm/ft of perimeter. Casement, awning, and hopper type windows tend to have lower air leakage rates than double hung or slider types due to the compression of the seal when latched closed. Information on air leakage should be available from the window manufacturer.

Material/Equipment Options

Manufacturers may report R-values for center-of-glass only or for whole the window (frame and glazing). When selecting windows on this basis, make sure you are comparing the equivalent values.

The graph below indicates typical whole-window U-values for the following types of windows:

POOR = single glazing, aluminum frame
GOOD = double glazing, wood frame
BETTER = double glazing, wood frame, argon gas fill
BEST = two glass layers plus two film layers with low-e, argon gas-filled


Window U-values graph


Typical whole-window U-values range from about 1.0 to 0.33.

Frames

Several materials, sometimes in combination, are used to make window frames:

  • Fiberglass frames, generally the best energy performers.
  • Wood frames, including vinyl-clad wood.
  • Vinyl frames, generally similar to wood in energy performance.
  • Metal frames, available with and without a "thermal break". Metal frames without a thermal break are the worst energy performers.

Glazing

Single and double glazing is most common. Single glazing allows the most unwanted heat loss and gain. Triple and quadruple glazing is available. Some windows have two glass layers with one or two plastic films suspended between them.

Coating/tinting

Glazing (either glass or plastic films) may have a low-emissivity (low-e) coating or may be tinted, or both. Low-e glazing has a special metallic coating that reduces both heat loss and solar energy transmission (typically an advantage except on south walls in passive solar designs). Low-e is available in different types and values and can reduce window energy loss by as much as 35 percent. Low-e coatings are rated for the amount of heat they radiate -- the lower the number, the less heat is radiated. These emissivity values range from around 0.04 to 0.35.

Low-e coatings can be placed on different glass surfaces depending on the climate and window orientation. For example:

Use low-e on the outside of the inner pane on south facing sides if the windows are designed to provide heat energy in the winter and have overhangs or other shading to reduce summer heat gains. If windows on the south side will not have proper shading, select low-e on the inside of the outer pane. West and east facing glazing should preferably have low-e on the inside of the outer pane. Tinting also helps reduce heat gain and is available in a range of colors/values.

Gas Fill

Multiple-pane windows can be filled with various gases that insulate better than air. Argon and krypton gas are typically used. R-values can be increased 50% by using these gasses.

Spacers

  • Spacers are used to separate multiple panes of glass.
  • Traditionally made of metal only.
  • Improved "warm edge" spacers are less conductive than traditional metal spacers.
  • These newer spacers can result in a net 2-10% improvement in window heat loss/gain.
  • Most glazing or window manufacturers offer a ten year warranty against seal failure of these spacers (seal failure results in condensation/fogging between panes of glass and some loss of insulating value).

Remodeling Scenarios

All Scenarios

Energy efficiency can be improved by:

  • Exterior shading by overhangs, awnings, sunscreens, and vegetation, especially on east and west sides
  • Interior shading with drapes or blinds
  • Air sealing around windows

Remove Old Window Entirely

All options for glazing layers and type, spacer type, gas fill, and frame and sash material are available with the use of new windows. Attention should be paid to proper air sealing around the windows.

Leave Existing Frame

Options include:

  • New windows set into the frame of the old window
  • Sash-only replacement

The glazing used in replacement sashes is available with low-e coating. When selecting replacement sashes, as with new windows, give consideration to the number of glazing layers and type, spacer type, gas fill, and frame and sash material.

Leave Existing Window

Options include:

  • Storm windows (low-e coating available)
  • Plastic films
  • Weatherstripping

New Construction

All options for glazing layers and type, spacer type, gas fill, and frame and sash material are available with the use of new windows. Attention should be paid to proper air sealing around the windows.