Good question, glad you asked.
There’s a lot of confusion out there about heat and energy terms, particularly when it comes to understand the science terms behind energy efficiency in windows. Window experts like to throw these terms out there, and they do sound the same. They’re not.
Never fear – we won’t let you take the heat. Welcome you back to high school physics – take your seat. And no chewing gum.
Here are the terms: air infiltration, conduction, convection, and radiation (radiant heat loss).
Also known as air leakage, air infiltration is the unintentional outside air coming into a building. With windows (unless you have the window open), this is the amount of air that leaks between the window sash and the window frame.
By the way, the leakage of room air out of a building called exfiltration.
Conduction’s definition is that is the transfer of heat through a material. In a window’s case, conduction is the direct transfer of heat through a window, most typically from the inside to the outside in the colder months, and from the outside to the inside in the warmer months. Conduction is experienced when you touch the material.
Convection is the transfer of heat from particle to particle, and it is the transfer of heat by the movement of fluids such as air or water. Everyone knows that hot air rises and cool air sinks. In the case of windows: you can experience convection when a chilly, circulatory draft occurs around the window when the warm inside air meets a cold window.
IR Radiation (Infrared Radiation)
Radiation is energy transmitted directly through space. The movement of heat through infrared energy through the glass panes. This energy requires direct line of sight. You experience radiation when you stand by a window in the summer and you feel warm, until you block the sun in some way (like a curtain). The sun’s radiation heats the space excessively and adds significantly to air conditioning costs in summer and southern climates.
External Reports of Windows and Heat Loss
Here’s a good educational and visual resource about windows and their effects on a building’s heat transfer/heat loss, from Penn State University’s Dr. Sarma Pisupati, Associate Professor of Energy and Mineral Engineering, College of Earth and Mineral Sciences: Windows and Heat Loss.
In it, he reports (bold highlights are ours):
“Windows typically occupy about 15 to 20 percent of the surface area of the walls. Windows not only add aesthetic looks and often a very important aspect of a home, but also a very significant component of home heating and cooling costs. Windows lose more heat per square foot of area in winter and gain more heat in summer than any other surface in the home.”
He goes on to describe the general costs:
[In a single year], “… the energy used to offset unwanted heat losses and gains through windows in residential and commercial buildings cost the United States $20 billion (one-fourth of all the energy used for space heating and cooling). However, when properly selected and installed, windows can help minimize a home’s heating, cooling, and lighting costs.”
What to do? Dr. Pisupati states:
“Although energy is spent to heat the air in the room, windows can make the temperatures uncomfortable. However, by making the windows efficient, a significant amount of the energy and money can be saved”.
How much money can be saved? His research states that the difference between a single pane window and even just a good double paned window can save close to $400 a month in places like Boston, MA. Even more, triple pane windows – what window insulating panels called Advanced Energy Panels can equate to – save another $30 a month. Imagine saving over $5,000 a year on energy costs. By the way, before you go out and spend the extra dollars on triple pane windows, check this article out.
But the article isn’t just about heat loss from poorly insulated windows in colder northern environments. He also Phoenix, AZ as an example of cooling costs with different types of windows. Again, you’ll experience massive savings when you have better window insulation.