You may not know it to look at them, but your windows are working hard. While you can’t see or hear them doing anything, they’re always on the job. Every minute of every day, they’re busy influencing the amount of heat flowing in and out of your home.

High-performance windows are the ultimate multi-taskers. In addition to influencing heat flow, they connect us to the outdoors and bring in the daylight that improves our health and sense of wellbeing. On top of all this, high-performance windows help us save money by reducing energy consumption.

We’ve come to expect a lot from our windows. When it’s warm, we open them to provide us with cool, fresh air. When it’s cold, we can close them to provide thermal insulation. Understanding exactly how high-performance windows control heat gain and heat loss is integral to understanding how they deliver such consistent high-performance – without ever taking a break.

Heat Gain and Heat Loss

Heat gain is measured by the window’s Solar Heat Gain Coefficient (SHGC). The lower the SHGC, the less heat enters your home. Heat loss, on the other hand, is measured by the window’s U-factor. The lower the U-factor, the less heat escapes. Knowing how hot and cold air is transferred through windows helps you choose the right product to suit your individual tastes and preferences while achieving improved energy efficiency – the invisible benefit.

Factors Influencing Heat Flow

An awareness of some of the specific factors influencing heat flow can provide further insight when shopping for high-performance windows.

Radiation is the amount of heat that passes through the glazing. Radiation is responsible for about two-thirds of heat lost through a window. During the summer, glass transfers the outdoor heat into interior of your home, essentially wrestling with your air conditioner to maintain a comfortable temperature. During the winter, glass transfers the warm air inside your home to the outdoors, essentially wrestling with your heater to maintain a comfortable temperature.

Conduction is the amount of cold air passing through the edges and framing system of a window. The framing determines how well a window controls conduction. Aluminum, for example, is a good conductor, which means it facilitates the passage of cold air, reducing comfort and inhibiting energy savings. Vinyl and fiberglass, however, are two of the least conductive materials available for window frames, and both prevent the transfer of unwanted hot or cold air into your home.

Convection is when warm air touches the cold glass and transfers the heat to the coldest side of the glass. During the summer, the warmth is passed to the inside of your home, and in the winter the warmth is passed to the outside of your home. Too much air space between the panes increases convection while too little air space increases conduction.

Air Leakage is the amount of heat loss and gain that occurs by the infiltration of air through cracks in the window assembly.

Choosing Long-Term, High-Performance Windows

When shopping for windows, be sure to consult the NFRC energy performance rating label affixed to the product. This helps you estimate the kind of heat gain and heat loss you can expect while assessing your return on investment.

A basic awareness of thermal performance will help you choose the products that deliver consistent high-performance and make your home more comfortable while helping you save money on utility bills – even if you can’t actually see it happening.