Window Buying Guide
A quick guide to strategic product selection

 
Top 5 things to look for:

1. NFRC label: a highly reliable energy performance label that will help you determine how well a window will cool your home in the summer, keep it warm it the winter, and resist wind and condensation year round.
    
2. Efficiency: a window’s efficiency is based on its U-factor, or its ability to resist heat gain and heat loss. The lower the U-factor, the more efficient the product.  
    
3. Glazing: the glazing of a window is its glass pane(s). Look for double or triple-pane windows as these will provide you with a higher insulating value than single-pane. Also look for a Low-e glass coating option and gas-fill in between the panes (see below for details).
    
4. Frame type: the frame of a window, in combination with its glazing, determines the window's efficiency. Some window frames are more efficient (insulated vinyl) than others (aluminum). In addition to how the frame contributes toward efficiency, you should also consider the potential environmental impacts associated with different types of windows (see below).
    
5. Replacement: windows are costly to replace. If you'd like to improve the efficiency of your windows without having to replace them altogether, consider a Window Insulting Panel (see below for details).

ENERGY STAR

The ENERGY STAR label is created and managed by the U.S. EPA and tailored to the United States’ four Climate Zones (Northern: mostly heating; North/Central: heating and cooling; South/Central: cooling and heating; and Southern: mostly cooling).

The ENERGY STAR label displays the product’s energy efficiency for a given climate based on its impact on heat gain and loss in cold weather and heat gain in warm weather. To be eligible for the ENERGY STAR label, products must be rated, certified, and labeled for both U-factor and Solar Heat Gain Coefficient (SHGC) in accordance with the procedures of the NFRC.

• Northern climates: U-factor of less than or equal to 0.35 and any SHGC rating.
• North/Central climates: U-factor of less than or equal to 0.40 and a SHGC rating of 0.55.
• South/Central climates: U-factor of less than or equal to 0.40 and a SHGC rating of 0.40.
• Southern climates: U-factor of less than or equal to 0.65 and a SHGC rating of 0.40.

Solar Heat Gain Coefficient (SHGC)

The Solar Heat Gain Coefficient (SHGC) measures how well a window blocks heat from sunlight, expressed as a number between 0 and 1. The lower a window's SHGC, the less solar heat it transmits.

Low-E

Low-E (low-emissivity) refers to special coatings on windows that reflect infrared light, keeping heat inside in the winter and outside in the summer.

Visible Light Transmittance (VT)

Visible Transmittance (VT) measures how much light comes through a window. VT is expressed as a number between 0 and 1. The higher the VT, the more light is transmitted through the window.

Condensation Resistance (CR)

Condensation Resistance (CR) measures the ability of a product to resist the formation of condensation on the interior surface of the product. The higher the CR rating, the better that product is at resisting condensation formation. While this rating cannot predict condensation, it can provide a credible method of comparing the potential of various products for condensation formation. CR is expressed as a number between 0 and 100 (the higher the number, the more resistant the product is toward condensation). CR is optional however and manufacturers can choose not to include it on their product labeling or spec sheets.

Air Leakage (AL)

Air Leakage measures how much outside air comes into a home or building through a window. AL does not measure insulating value like U-factor. AL ratings typically fall in the range of 0.1 - 0.3. The lower the AL, the better a product is at keeping air out. AL is also an optional rating, and manufacturers can choose not to include it on their product labeling or spec sheets.

Frame type efficiency & toxicity

Aluminum

Aluminum window frames are light, strong, durable, and easily extruded into the complex shapes required for window parts. However, aluminum has very high thermal conductance, making it not ideal as a window frame because it readily conducts heat, greatly lowering the overall efficiency, or U-factor, of the window unit. In cold climates, a simple aluminum frame can easily become cold enough to condense moisture or frost on the inside surfaces of window frames (source: Efficient Windows Collaborative).

The most common solution to the heat conduction problem of aluminum frames is to provide a "thermal break" by splitting the frame components into interior and exterior pieces and use a less conductive material to join them. Current technology with standard thermal breaks has decreased aluminum frame U-factors from roughly 2.0 to about 1.0 Btu/hr-sq. ft-°F. In hot climates, where solar gain is often more important than conductive heat transfer, improving the insulating value of the frame can be much less important than using a higher-performance glazing system (source: Efficient Windows Collaborative).

Wood

From a thermal standpoint, wood-framed windows perform well and typically feature frame U-factors in the range of 0.3 - 0.5. Wood is not intrinsically the most durable window frame material because of its susceptibility to rot, but well-built and well-maintained wood windows can have a very long life.

► Look for FSC-certified wood window frames.

Wood clad

A variation of the wood-framed window is to clad the exterior face of the frame with either vinyl or aluminum, creating a permanent weather-resistant surface. Clad frames create lower maintenance requirements while retaining the wood finish on the interior.

Vinyl

Vinyl, also known as polyvinyl chloride (PVC), is a versatile plastic with good insulating value resistance to moisture. However, vinyl/PVC is not an environmentally-friendly product. From its manufacture to its disposal, PVC emits toxic compounds. When it burns, two extremely hazardous substances, hydrogen chloride gas and dioxin, are formed, which present both acute and chronic health hazards to humans (some 100 different toxic compounds are released during the burning of vinyl PVC). Vinyl/PVC is also the largest source of chlorine (needed for dioxin production) in municipal waste incinerators (source: Greenpeace).

Also, less than 1% of post-consumer PVC is recycled in the US. Recently, the Association of Post-consumer Plastic Recyclers (APR) stated its position on PVC, "We're going to view this material as a contaminant, and you sure won't find PVC packaging listed as a recycled plastic in APR's revised design guidelines."

Insulated Vinyl

Insulated vinyl frames are identical in most of their characteristics to standard vinyl frames. The major difference is improved thermal performance. In insulated vinyl frames, the hollow cavities of the frame are filled with insulation, creating superior thermal performance compared to standard vinyl and wood frames. However, these types of windows do have the negative environmental impacts associated with PVC vinyl.

Composite wood

Composite wood products, such as particleboard and laminated strand lumber, consist of wood particles and resins that are compressed to form a strong composite material. These composites are very stable, and have the same or better structural and thermal properties as conventional wood, with better moisture resistance and more decay resistance.

Although this approach has the added environmental advantage of reusing a volume of sawdust and wood scrap that would otherwise be discarded, they may contain formaldehyde in their bonding agents. Most fiberglass products use phenol formaldehyde as a binder, which should be avoided because the formaldehyde out-gasses, contributing to indoor air quality problems. There are options using formaldehyde-free binders or no binders at all. Fiberglass is also a skin and respiratory irritant as airborne fibers are small enough to be inhaled and lodged in the lungs. Also, airborne fibers are a suspected carcinogenic hazard.

ABS Plastic

Acrylonitrile-Butadiene-Styrene (ABS) is used as a hard plastic in many applications, including windows and uses a number of hazardous chemicals in its creation. These include butadiene and styrene and acrylonitrile. Acrylonitrile is highly toxic and readily absorbed by humans by inhalation and directly through the skin. Both the liquid and its vapor are highly toxic. Acrylonitrile is classified as a probable human carcinogen as are styrene and butadiene.

Fiberglass

Fiberglass frames are made of glass-fiber-reinforced polyester, which is extruded into lineal forms and then assembled into windows. These frames are dimensionally stable and have air cavities similar to vinyl. When the cavities are filled with insulation, fiberglass frames have thermal performance superior to wood or vinyl (similar to insulated vinyl frames). Because the material is stronger than vinyl, it can have smaller cross-sectional shapes and thus less area.

However, methods of fiberglass production are not only energy-intensive, but typically entail the use of styrene, which causes hazardous air pollution that is harmful to breathe at excessive levels. Also, some fiberglass products use phenol formaldehyde as a binder which should be avoided because the formaldehyde out-gasses, contributing to unhealthy indoor air. Fiberglass is also a skin and respiratory irritant as airborne fibers are small enough to be inhaled and lodged in the lungs.

Multiple panes (e.g. double or triple pane)

Multiple pane windows feature two or three panes of glass. They have an air or gas filled space between them, creating an insulating effect that a single pane window can’t match.

Gas fills

Some energy efficient windows feature argon, krypton or other gases between the panes. These odorless, colorless, non-toxic gases insulate better than air alone.

Maximize window performance

Proper installation

To get the most out of your windows, have them installed by trained professionals according to manufacturer instructions. Ensure that the windows are tightly sealed using low-VOC caulking and weather stripping. If installation requires scraping lead-based paint, be sure to take proper precautions.

Orientation

Orienting windows to the south and using roof overhangs can help reduce energy use by providing shade from the sun in the summer and solar heat gain in the winter. Overhangs are much less effective against the lower angles of the east and west sun, therefore reducing the size and number or east and west facing windows can help reduce energy use.  

In lieu of window replacement: window insulating panels

How to dispose of your old windows

Recycle, reuse, sell or donate your old window glazing and framing when they are no longer functional or efficient. Visit Earth 911 to search for a location near you.

Questions? Feedback?

Visit our Forum to ask questions, share your thoughts and offer feedback about energy-efficient, less toxic window products.