Air Leakage (AL)

The amount of air passing through a window.

Cool Climate Rating (Energy Performance)

The Cool Climate number – or rating – indicates the amount of energy that you can save by using the window attachment if you live in a cooler climate (where you turn on your heat most of the year), compared to not using a window attachment at all. In other words, if the window attachment you are interested in buying has a Cool Climate rating of 64, installing the attachment over an existing window in your home will save 64% of the energy that would otherwise be lost without the attachment.

Double Pane Window

Window unit with two panes of glass sealed into a single glazing unit; gas between the panes insulates the window.

Double Pane Window with Clear Glass

Double Pane with Clear Glass is a type of window that provides strong thermal performance: good resistance to heat loss, high solar heat gain, and high visual transmittance.

Double Pane Window with Low-E Glass

Double Pane windows with Low-E glass provide high resistance to heat loss and can be utilized for solar gain but with some loss of visual transmittance.

Emissivity

How readily the surface of a material gives off heat by radiation. Many common building materials have “high” emissivity (around 0.9 out of 1.0 being a perfect emitter), while materials or objects with metallic coatings or surfaces have “low” emissivity (around 0.15 or less).

Low-Emissivity (Low-E)

Nearly invisible metallic coating on inside surface of double-pane glass windows, storm windows, and interior window panels that improves the energy efficiency of the window and the thermal comfort of occupants.

Single Pane Window with Clear Glass

Single Pane Clear Glass windows, commonly found on older homes, provide the weakest thermal performance but highest visual transmittance amongst window types. The window unit has just one pane of glass.

Solar Heat Gain Coefficient (SHGC)

The SHGC is the fraction of incident solar radiation admitted through a window, both directly transmitted and absorbed and subsequently released inward. SHGC is expressed as a number between 0 and 1. The lower a window’s solar heat gain coefficient, the less solar heat it transmits. A product with a low SHGC would be more effective reducing cooling needs in the summer, while a higher SHGC would be more effective reducing heating needs in the winter.

U-Factor (U-value)

A measure of how readily heat transfers through a material or component, or the heat loss.

Visible Transmittance (VT)

A performance property of glass/windows that is measured as the amount of light transmitted through a window.

Warm Climate Rating (Energy Performance)

The Warm Climate number – or rating – indicates the amount of energy that you can save by using the window attachment if you live in a warmer climate (where you use your air conditioning most of the year), compared to not using a window attachment at all. In other words, if the window attachment you are interested in buying has a Warm Climate rating of 64, installing the attachment over an existing window in your home will save 64% of the energy that would otherwise be lost without the attachment.

The AERC Energy Rating Label