Windows are an Important Feature of Your Home

Windows are an important aspect of the thermal shell of your home, and they provide our homes with light, warmth and ventilation. Windows can have a big impact on a home's energy use. Heat gain and loss through windows are a significant contributor to a home's heating and cooling needs.

Installing energy-efficient windows can have a big impact on reducing energy costs. The technology and energy efficiency of windows has changed significantly. When properly selected and installed, energy-efficient windows can help minimize your heating, cooling and lighting costs and increase comfort in rooms with high levels of direct sun.

Windows protect you from the elements just like a winter coat. But like a winter coat, you should pick the window that makes sense for our climate and what direction it faces. Some windows are better at keeping you warm, others excel at keeping you cool.

We will explore the components of an energy-efficient window and how to make the right selection for your home. The important factors to consider are the durability features like cladding or fiberglass, the window's design (double hung, casement, etc.), and energy performance rating of the frame and the glass.

For windows to be eligible for the ENERGY STAR® label, they are rated on their U-factor and Solar heat gain coefficient for the climate zone they will be used in (Northern, Southern, etc.). The rating is done by a third party National Fenestration Rating Council (NFRC) to ensure that they meet strict energy efficient guidelines set by the EPA.

Heat Gain and Loss

Windows gain or lose heat by direct conduction through the glass, glazing and frame. Sunlight changes from UV to IR (heat) when it passes through the glass into the house. Heat passes out of the house through the glass from room-temperature objects such as people, furniture and interior walls. Air leakage through poorly fitting window sashes and gaps around the windows also adds to heating and cooling loads.

These properties are measured and rated according to the following energy performance characteristics.

U-factor is the rate at which a window conducts non-solar heat flow and expresses the insulation value specifically of windows. The lower the U-factor, the more energy-efficient the window is. The temperature difference between the interior and exterior of a building creates the non-solar heat flow that results in windows loosing heat or gaining heat.

Solar heat gain coefficient (SHGC) is the fraction of solar radiation admitted through a window either transmitted directly or absorbed, and subsequently released as heat inside a home. Determining the SHGC will depend on your climate, the home's orientation, and external shading. These are important factors in determining the optimal SHGC for a particular window.

  • The lower the SHGC, the less solar heat it transmits and the greater its shading ability.
  • A window with a higher SHGC rating is more effective at collecting solar heat during the winter.
  • A window with a lower SHGC rating will be more effective at reducing cooling loads during the summer by blocking heat gain.

Knowing the orientation of your home, the placement of your windows, and the exterior shading will help you design windows that will increase your solar gain in the winter and be shaded in the summer. Blinds help decrease solar gain, and landscaping with trees can also help provide shading. If you have large west-facing windows, you will want to reduce your solar gain through low SHGC glass, retractable shades if maintaining a view is important, and internal blinds. This will help reduce cooling loads during the summer.

Air leakage is the rate of air movement through the seals of a window. A window with a low air leakage rating will provide a tighter window.

Improved Window Technologies

Improved Frame Materials

Improving the thermal performance of the frame can contribute to a window's overall energy efficiency. Wood, vinyl, fiberglass, and some composite frame materials provide greater thermal resistance than aluminum or metal.

Wood window frames insulate relatively well. They do expand and contract in response to weather conditions, and as such, do require regular maintenance. Aluminum or vinyl cladding on the outside helps reduce maintenance requirements. Some climates are better suited to wood windows than others.

Vinyl window frames are usually made of polyvinyl chloride (PVC) with ultraviolet light stabilization to keep sunlight from breaking down the material. Vinyl frame cavities can be filled with foam insulation, increasing the thermal performance rating of the window.

Fiberglass window frames are dimensionally stable, very durable, and have air cavities that can be filled with foam insulation giving them superior thermal performance.

Composite window frames consist of recycled wood by-products like saw dust with a glue binder. These composites are very stable. They have better structural and thermal properties than conventional wood frames, and they have better moisture and decay resistance.

Window Technologies

Low-E Glass

Special coatings reflect infrared light (heat) back into the inside during the winter and help keep the summer heat outside. A super-thin metallic coating is utilized in low-E glass and applied in a special process on selected sides of the glass to reduce the transfer of heat through the window.

There is the added benefit of reflecting ultraviolet light which helps protect your home's furnishings.

Low-E glass coating is microscopically thin and is a virtually invisible metal or metallic oxide layer that is applied to the selected glazing surfaces to control heat transfer. This reduces energy loss by 30% or more. The low-E coating is typically used on multi-pane windows which can also have argon gas-filled spaces to further reduce heat transfer.

  • In hotter climates, the low-E coating is applied to the outer layer of the glazing to help keep the heat out.
  • In colder climates, it is applied to the outer surface of the inner pane of glass.

Gas-Fills

Some energy efficient windows have gases between the panes. Argon and Krypton are the gas fills used most often to displace the air between the panels in the windows. These gases are odorless, colorless, and non-toxic.

  • Argon comprises slightly less than 1% of the Earth's atmosphere. Its thermal conductivity is roughly 67% that of air and is inexpensive.
  • Krypton has the same properties and is an even better insulator albeit more expensive to produce.

These gases are pumped inside insulated glass units to improve their thermal efficiency because these slow-moving gases insulate better than regular air. They are used in double and triple pane windows.

Warm edge spacers keep a window's glass panes the correct distance apart, thereby keeping the edge of the glass warmer to improve thermal performance and to prevent condensation.

Multiple Panes

The standard for most windows is two panes of glass with air in the middle. The step up is two panes of glass with gas-filled space in the middle and suspended films that act like another high-performance pane of glass.

Three or more panes of glass with gas-filled spacers have even greater energy efficiency with increased impact resistance and sound insulation.

The final decision about what type of energy efficient window to use will come down to many factors, and this is where your builder, working with their third party rater, can evaluate all the factors and costs to help make the decision. The cost of all the performance and style options for the windows is a big part in any comparison for energy savings.

If you are building a new home or replacing existing windows, the impact of choosing the right window for your needs will have a big impact on your home for years to come.

Sources:
Environmental Protection Agency (EPA) Energy Star Program
U.S. Department of Energy
International Association of Certified Home Inspectors

20 Years of ENERGY STAR
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