Energy Tip of the Week (10/10/11)

Posted: October 7, 2011

Lower Your Energy Bills with Energy-Efficient Windows

Key Points

Drafty windows can reduce the effectiveness of energy-efficiency improvement efforts.

Weatherization and shading techniques can improve the efficiency of existing windows.

Efficiency ratings for new windows measure the amount of heat or light they transmit.

Making energy-efficiency improvements such as upgrading your heating and cooling system, can lower energy costs and increase occupant comfort. Drafty windows, however, can reduce the effectiveness of these efforts and cost you a substantial amount of money through heat loss in the winter and heat gain in the summer. From window maintenance to new window installation, there are a number of steps you can take to reduce energy loss through windows and make your building more comfortable.

Reducing Window Energy Losses

There are three methods to reduce heat loss and heat gain through existing windows; weatherization, internal shading, and external shading:

Weatherization. Weatherizing windows helps prevent air from escaping and also helps control moisture. Inspect windows annually for cracks in caulking or loose weatherstripping, and replace if necessary. Another option is to install tight-fitting clear plastic around the inside of window frames to prevent air leakage during the winter months.

Internal Shading. When properly installed, window shades are one of the most simple and effective ways to save energy. To create an effective sealed air space, mount shades as close to the glass as possible, with the sides of the shades fitting the surrounding wall. Dual shades with light and dark sides offer greater efficiency. The heat-reflecting light side should face inward during winter to keep heat in and outward during summer to keep heat out.

To maximize the effectiveness of shades, open south-, east-, and west-facing window shades during peak hours of sunshine to gain heat in winter. In summer, keep all window shades closed while the cooling system is operating.

External Shading. Overhangs, fins, and window films or coatings (also known as glazing), can help to reduce energy losses from outside. Unlike internal shading, these technologies do not rely on active operation. Overhangs help reflect unwanted solar heat gain and are most effective on south-, east-, and west-facing windows. Fins are similar to outdoor vertical blinds and are often adjustable. They are useful primarily on east- and west-facing windows. Glazing, which is most effective in warmer climates, is used primarily on east- and west-facing windows to reduce heat gain. Glazing technologies can save up to 50% on energy costs, according to the National Institute for Building Sciences.

High-Performance Windows: A Smart Investment

While optimizing the efficiency of your existing windows is important, installing newer, high-performance windows can save substantially on energy costs. The National Fenestration Rating Council (NFRC) operates a voluntary certification program for windows based on a series of ratings that measure methods of heat loss and gain, including unit conductive factor (U-factor), solar heat gain coefficient (SHGC), and visible transmittance (VT).

U-Factor—measures how well a window prevents heat from escaping. The ratings generally fall between 0.20 and 1.20. The lower the U-factor, the greater the window's insulating value.

SHGC—indicates how well a product blocks heat generated by sunlight. The SHGC is a fraction of solar radiation emitted through a window and is expressed in a value between 0 and 1. The lower the coefficient, the less solar radiation is transmitted through the window.
VT—measures how much light comes through a window in values falling between 0 and 1. The higher the VT, the more light is transmitted.

A number of resources and guidelines are available to help in selecting energy-efficient windows that meet your specific performance criteria.
International Energy Conservation Code. At a minimum, windows should be selected in accordance with the 2009 International Energy Conservation Code, which can be obtained from the International Code Council. Windows must be labeled with a NFRC U-factor and the code has specific solar heat gain and air leakage requirements.

ENERGY STAR. This U.S. Department of Energy initiative provides certification for energy-efficient windows. Windows carrying the ENERGY STAR label use up to 15% less energy than standard models and are eligible for federal tax credits.

Efficient Windows Collaborative. This cooperative association of window manufacturers and suppliers promotes the use of energy-efficient products. Their Web site includes information on energy-efficient technologies and a window selection tool.

Be sure to install windows according to the manufacturer's recommendations and in compliance with your local building codes.

Thank you for your continued support.

Thomas Szatkowski
Director of Facilities
Rosemont College
P 610-527-0200 X 2152
F 610-526-2954

Last Updated: October 10, 2011