What is “Low-E” Glass?

Low-E glass products give you year-round energy savings and comfort by helping manage the sun’s energy and the heating system energy in your home. Low-E glass products are coated with microscopically-thin, optically transparent layers of silver sandwiched between layers of antireflective metal oxide coatings. In the summer, Low-E glass products let in visible sunlight while blocking infrared and ultraviolet solar energy that drives up cooling costs and damages curtains, window treatments, carpeting, and furnishings. In the winter, Low-E glass products offer greater comfort and reduced heating costs by reflecting room-side heat back into the room. Emissivity is a measure of how much heat is emitted from an object by radiation. Heat is transferred to and from objects through three processes: conduction, convection, and radiation. For instance, on a hot night, heat will be conducted through a window from the outside, causing the inside pane to become warm.

Convection, or natural circulation, of the air in the room past the window will transfer some of that heat into the room. But the window will also radiate heat as infrared waves, which will warm objects throughout the room. This radiative heating is why you can feel the heat of a red-hot piece of metal (for instance, a heating element on an electric stove) from several feet away. Low-emissivity, or low-e, coatings are put on window panes to reduce the amount of heat they give off through radiation. In hot climates, where the outside of the window will typically be hotter than the inside, low-e coatings work best on the interior of the outside window pane. In cold climates, where the inside of the window is typically hotter than the outside, the low-e coatings work best on the inside window pane, on the side that faces toward the outside.

What is U-Value?

The U-value, also called the U-factor, is a measure of how well heat flows through an object (thermal conductivity). It is also referred to as the heat transfer coefficient or the coefficient of heat transmission. The U-value is measured by how much heat (Btu) flows through a certain area (a square foot) each hour for a certain temperature difference (°F), so it is measured in Btu/ft2-hr-°F. The U-value is the reciprocal of the R-value: the lower the U-value, the better the insulation value of the material. Many building and insulation products have their U-value indicated on their label. A U-value of 0.35 or less is recommended in cold climates. In warm climates a low U-value is helpful during hot days or whenever heating is needed, but it is less important than SHGC (Solar Heat Gain Coefficient).

What is R-Value?

R-value measures insulating power. The higher the R-value, the better the insulating power. To find the R-value you need, check with your local utility company. Recommended levels vary depending on where you live. The R-value is the inverse of the U-value. Remember: If you install more insulation than necessary, you’ll waste money. When having insulation installed by a contractor, be sure to discuss what R-value is best for your home. Ask retailers and home installers for a fact sheet on insulation before buying. The fact sheet, required by the Federal Trade Commission (FTC), tells you the type of insulation, its R-value, and the area it will cover. When the contractor installs your insulation, they must give you a contract or receipt showing the insulations R-value, coverage area, and thickness. If loose-fill insulation is installed, the number of bags used also must be included.

What is Solar Heat Gain Coefficient (SHGC)?

The Solar Heat Gain Coefficient is the fraction of incident solar radiation admitted through a window, both 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 SHGC of 0.40 or less is recommended in warm climates. In heating-dominated climates, a high SHGC increases passive solar gain for the heating, but reduces cooling season performance. A low SHGC improves cooling season performance, but reduces passive solar gain for heating.

What is Visible Light Transmittance (VLT)?

The visible Light transmittance (VLT) is an optical property that indicates the amount of visible light transmitted through the glass. VLT is expressed as a number between 0 and 1. The higher the VLT, the more daylight is transmitted. A high VLT is desirable to maximize daylight.

What is Air Leakage?

The air leakage rating (AL) is a measure of how much air leaks through the crack between the window sash and frame. The rating reflects the leakage from a window exposed to a 25-mile-per-hour wind, and is measured in cubic feet per minute per linear foot of sash crack. Heat loss and gain occur by infiltration through cracks in the window assembly. The lower the AL, the less air will pass through cracks in the window assembly. An air leakage rating (AL) of 0.30 cfm/sq. ft. or less is recommended.

What is an NFCR rating?

The National Fenestration Rating Council (NFRC) is a non-profit collaborative of manufacturers, builders, designers, government officials, utilities and consumers which provides unbiased energy performance ratings for window, doors and skylights (or “fenestration”). NFRC’s labels provide product-specific performance ratings for technical qualities such as U-factor (the rate of heat loss from your home through the window), and Solar Heat Gain Coefficient (how much heat your house gains from the sun as a result of the window’s performance). Look for lower U-factor and appropriate SHGC numbers for highest efficiency performance in your area of the country.

What is Energy Star?

Energy Star windows is a program designed to help consumers identify efficient windows, doors, and skylights. By choosing Energy Star window products, you can cut down your heating and cooling costs, and make your home more comfortable at the same time. Energy Star labeled windows are twice as efficient as the average window produced just ten years ago. Want to know more about Energy Star? Click here.