Description

Windows play vital roles in our homes: bringing in natural light, connecting us visually with the outdoors, providing fresh air during warm months, helping to slow down heat loss, controlling unwanted temperature increases from sunlight (solar heat gain), and contributing to overall appearance. Balancing these attributes can be complex because some conflict with others; for example, a feature that optimally blocks unwanted summer heat gain may negatively affect views, daylighting, and beneficial solar heat gain in the winter.

Windows’ impact on energy consumption is huge. In older homes, windows typically account for 25% of annual heating and cooling costs but can be responsible for as much as 40%. In new construction, a homeowner can easily minimize windows’ energy impacts by choosing highly energy-efficient products.

But what should be done about existing windows?

This fact sheet lays out a process for evaluating existing windows and deciding among a wide range of options for either replacing windows or retrofitting them with interior or exterior coverings, or attachments, depending on the particular situation and needs.

Names and Parts of a Typical Window

Do Windows Need to Be Replaced?

The first step in deciding how to address existing windows in a home is to evaluate their condition. If the windows are in poor shape, the best course of action may be to replace them with new high-performance windows. The difference in price between standard and “high-performance” replacement windows with low-emissivity (low-e) coatings and gas fill will, in most cases, be quickly paid back through energy savings. In colder climates, triple-glazed replacement windows with two low-e coatings may be justified.

Windows are Good Candidates for Replacement if:

• Wooden frames are rotted or decayed.
• Windows are extremely leaky and weatherstripping is not practical.
• Aluminum or vinyl sashes are broken, cracked, or out of square.
• Sliding or hinged sashes can no longer be operated because of failed tracks or mechanisms.
• Seals on insulated glass panels have failed, as evidenced by condensation or fogging between the panes of glass.
• Wooden frames and sashes are covered with lead paint that is subject to abrasion.

The Efficient Windows Collaborative is the best source for information on window replacement.

If windows are in good shape, it’s often hard to justify the cost of replacement, and one or more of the window coverings described in this series of U.S. Department of Energy (DOE) fact sheets may be a better investment. The other fact sheets in the Window Coverings series recommend options based on the assumption that the homeowner has decided to keep existing windows.

Terminology

Spectrally selective means transparent to some wavelengths of the solar spectrum and reflective to others. Typical spectrally selective coatings are transparent to visible light and reflect short-wave and long-wave infrared as well as UV radiation. Spectral selectivity can be achieved with low-e coatings and/or high-performance tints.

Mean radiant temperature (MRT) is roughly the average temperature of all the objects or surfaces in a space. The temperature contribution of each individual surface is weighted by its area, spatial relationship, and temperature difference from the skin temperature of a person in the space. An MRT that is warmer than the skin temperature of a person will cause the person to feel warmer, and an MRT that is colder than the skin temperature of a person in the room will cause that person to feel colder. MRT and air temperature are the two most important factors in assessing thermal comfort.
 

What are Window Coverings?

Window coverings are elements added to existing windows to improve energy performance, reduce glare, provide privacy, or enhance the appearance or comfort of a home. Window coverings include: interior shades and drapes, plastic films applied directly to the glass, exterior shades, shutters, awnings, and storm windows.

Deciding Among Various Window Covering Options

In considering the various options for window coverings, it is important to understand how these treatments can affect window properties and performance. The attributes described below are keyed to a summary table at the end of this fact sheet.

Thermal Performance and Ventilation

For the existing Window Covering selection tool, a team of industry members and window/window covering researchers used a consensus process to qualitatively and comparatively eveluate window covering performance. This qualitative tool will soon be augmented by a quantitative selection tool, using industry-leading simulation software to predict or comparatively evaluate diffferent window coverings based on a standard home and for a variety of climates.

Photo credit (infrared image): LBNL/BuildingGreen

Insulation

The degree of insulation a window provides is a major determinant of the window’s overall thermal performance. Window retrofits can boost window insulation by adding layers of glazing, coatings that reduce radiant heat loss, and even moveable insulation panels.

Air tightness

Air leakage through windows is another major determinant of overall window thermal performance, particularly in cold climates. Some window attachments improve airtightness by reducing air leakage.

Solar heat gain

Many window coverings add significant solar heat gain control; they can be selected and adjusted depending on whether heat gain is desired or not.

Comfort – Winter

The thermal comfort provided by windows is a function of air leakage, surface temperature, and solar heat gain. During cold weather, some window coverings improve comfort by reducing air leakage and keeping the window surface facing the room warmer.

Comfort – Summer

During hot weather, some window coverings improve comfort by controlling solar heat gain and keeping the innermost window surface cooler. Spectrally-selective glazing or coatings can improve the insulation, winter thermal comfort, and summer thermal comfort performance of many window covering options.

Condensation

When warm indoor air comes in contact with cold window surfaces, condensation can result. Window coverings can prevent or reduce condensation by keeping the window’s interior surface warmer. However, some can increase condensation problems by either decreasing the temperature of surfaces that are in contact with warm, moist interior air, or by making the home more airtight and reducing the amount of dry, outdoor winter air leaking into the home.

Ventilation

Some window coverings, such as low-e storm windows with integral screens, facilitate the use of windows for ventilation; others can impede airflow through otherwise operable windows.