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Windows: Buyer’s Guide

Buyer’s Guide

Window Shopping: Efficiency Expertise

A few facts will save you time help you see your options clearly when buying new or replacement windows. The chart below defines basic terms about energy-efficient windows. If you’re replacing an existing window, you should at least upgrade to an efficient double-pane window, and consider features like gas filling and low-e coating. Look for a window system with a combined low U-factor (0.35 or less is generally suitable for California’s climate) and a low solar heat-gain coefficient (0.3 or less is generally suitable for California).

Energy-Efficient Window Features


Whole Window
Feature Description Rating Range


Measure of how well the window prevents heat from escaping. The lower the U-factor, the better its insulating characteristics. Consult the  ENERGY STAR® website to find the recommended U-factor for your climate.

0.2-1.2; lower is better

Solar Heat Gain Coefficient

Measure of how well a window blocks heat from the sun. Lower values signify a window that transmits less solar heat. Consult the  ENERGY STAR® website to find the recommended solar heat gain coefficient for your climate.

0-1; lower is better

Visible Transmittance

Amount of light permitted through the window. A higher number means more light comes through

0-1; higher is better

Air Leakage

Amount of air movement, in cubic feet, passing through the cracks in the window assembly (cfm/ft). A lower number signifies a tighter window.

Look for 0.01-0.06

Source: Minnesota Department of Commerce Windows and Doors fact sheet


Window Glass
Feature Description Options

Multiple Glazing (panes)

Window assemblies with more than one pane of glass to help prevent heat loss/gain. Look for double- and triple-paned windows.

Double- or triple-pane

Gas Filling

Often used in multiple glazing window assemblies to fill in the space between panes, boosting insulating properties, and inhibiting heat flow. Argon and Krypton are more efficient than other gasses.

Argon or Krypton

Low-Emittance Coating (low-E)

Microscopically-thin, invisible metallic coating, usually on the inside layers of the glass panes. Low-E coatings help prevent heat flow by reflecting invisible, long-wave infrared radiation, thereby lowering the window’s U-factor.


Insulating Spacer

Insulating material used as a spacer in the assembly to help prevent condensation. Insulating spacers work better than traditional aluminium spacers.


Source: Minnesota Department of Commerce Windows and Doors fact sheet


Sashes and Frames
Feature Description Range

Solid Steel and Aluminium

Least efficient frames because they are poor insulators and expand or contract with weather changes. Choose other material, if possible.

$10 to $15/square foot


A good insulator with minimal expansion and contraction, but requires more maintenance and can get moisture damage. Some wood components are treated with preservatives or are covered in metal or vinyl to decrease maintenance.

$20 to $30/square foot


A good insulator requiring little maintenance, but can be damaged by the sun and elements, leading to peeling, warping and discoloration. Expands and contracts like metal components with weather changes. Sometimes made with a fiberglass core.

$15 to $20/square foot


Good insulator with minimal expansion.

$20 to $30/square foot

Source: Minnesota Department of Commerce Windows and Doors fact sheet

Use & Care

Properly installing and maintaining your home’s windows will maximize efficiency measures. Learn more about Window Use & Care , where to start, and how to care for windows and treatments for the most savings.


If you’re thinking about making efficiency improvements, the Windows Overview is a great place to get acquainted with the basics and your options, before you buy or install. Return to Building and Materials

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