What is the proper airspace for 3mm 3mm double pane windows?

When it comes to double pane windows, everyone knows there is spacing between the glass panes. But is there a reason the space is as wide as it is? The Window Geeks are here to answer that question!

The common understanding is that a window’s thermal performance is achieved through the use of multiple layers of glass. The reality is, this concept is based on a 40-year-old idea of what a window is supposed to be. When you look back, single pane windows were the most common ones found in the 70s. As we progressed, we started to realize that double pane windows were more efficient and they did a much better job at reducing surface condensation on the glass.

It’s worth noting that there were some double pane windows used in the 70s, such as the one pictured below. These windows are on a cabin in a BC region where the temperature can get down to -20 degrees Celsius in the winter. They were around back then – they just weren’t the most prevalent type of window as they are nowadays. The 80s changed things. What we tend to see with windows from that decade is a double pane window, but with a mere ¼” air space. How did this sized spacing come about? It’s because the window frame that was typically used then was originally intended for single pane windows, and it required a slight modification to make the two-pane design work.

It wasn’t until larger aluminum frame windows came in that we started to see proper double pane windows with a ½” air space between the glass. Why ½”? That is considered the optimum width to reduce conduction, but it also reduces convection currents. In this scenario, we’re talking about 3mm 3mm glass as the optimum width will change as glass thickness changes. 

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