The R-Value of Windows

by Randy Williams from Northern Built

Heat is a funny thing.

It wants to move from someplace warm to someplace cold, (the opposite of being a snowbird) and it’ll continue to move until it has reached equilibrium.  In the heat of the summer or cold of winter, that means if we can’t slow the movement of heat, the temperature outside will be the same as the temperature inside.  This might be 110°F or -25°F, neither is appealing.  Of course, we heat and cool our buildings to maintain comfort, but these systems work best when we’re able to slow this constant march of heat leaving or entering the building, we do this with insulation.

What do you think of when I say the word insulation? 

I think of fluffy stuff in my walls and attic, but there are many other products in a building that act as insulation.  Windows are designed to give us a view of the outdoors, this allows light from outside into the home (which may lead to some solar heat gain).  They may also provide ventilation and offer a means to exit the building in an emergency.  Modern windows are also designed with energy efficiency in mind, that “part of the insulation package” slows or resists the movement of heat.  This resistance to heat flow is commonly expressed as R-value.

R-value for Windows Explained

R-value makes sense, the higher the number, the slower heat will move, which is good.  Let’s use a wall as an example, the center of an exterior wall will most likely have some level of insulation present, let’s say it’s a newly built 2 x 4 wall with R-13 fiberglass insulation.  If we decide to change this wall to a 2 x 6 wall, we can now use R-20 fiberglass insulation, increasing the walls’ resistance to heat flow.  The heating or air conditioning you just paid for will last longer with R-20 than if the wall were built as a 2 x 4 with R-13 insulation.  The higher the R-value, the less costly to operate and the more comfortable the structure will be.

Why Windows are Different

I have yet to see a window sticker, which is required to be on every new window, to list the R-value of the window.  There are lots of other values printed on the sticker, just not the R-value. 

Why is that? Windows are an assembly of many parts and pieces that combine to make the window, including the glazing or glass, sash (the part that secures the glass, the sash may operate or be fixed), and frame (the part that secures the sash and attaches to the building’s framing).  There are other important window components that we will discuss in future blog posts.  Each of these parts and pieces can be made from different materials and have different resistance to the movement of heat.  All these different values for the various pieces are averaged in what is called the U-factor.

Alpen’s Zenith windows are made from fiberglass and designed to meet higher energy efficiency standards and come in multiple glass options to fit your budget.

What’s U-factor and How Does it Differ from R-value? 

As I mentioned earlier, R-value is a measurement of heat resistance, U-factor on the other hand is the rate or speed of heat loss (or gain) through the window, not just the glass, but the entire window assembly. 

Remember how I said R-value was easy, the bigger the number, the better the performance, with U-factor, the lower the number, the better performance, it’s the opposite.  The U-factor is also the way all windows will express a window’s performance, (solar heat gain coefficient and air leakage rates are two other performance metrics that may be listed). 

The photo of the window sticker indicates that particular window’s U-factor is U-0.16.  By comparison, a common window in my market might have a rating of U-0.30.  Unless you’ve studied window performance or understand U-factor, those numbers probably don’t have a lot of meaning. 

The Math Behind R-value

In the beginning of my career, they didn’t for me anyways.  I learned to convert the U-factor into an R-value to help understand the performance.  It’s a very simple math equation, R-value = 1/U-factor.  Using the photo of the Alpen window sticker as an example, R-value = 1/0.16, or R-6.25.  That’s a really good R-value for a window.  Performing the same math for the common window in my market, we get an R-3.33, a code minimum performing window, which window would you rather own? 

On a side note, I’m a big fan of Matt Risinger and the Build Show Network. Matt recently toured a very cool development in Bozeman, MT featuring Alpen’s Zenith Series of windows. In the video, Matt and Aaron from 406 Windows talk about the different window gas options and the U-factor of Alpen’s ZR-7 window and give you a sense of what you can achieve with high-performance windows.

But back to the science of it all. I’m going to give you one more equation, this time converting R-value back to U-factor.  U-factor=1/R-value.  Two building science equations that are easy to remember.  If you decide to venture down the building science rabbit hole, you’ll find yourself using these two equations often, and even if you don’t choose the rabbit hole, you now know how to read and understand at least part of those pesky window stickers, you’re welcome.

Are you looking to build a more energy-efficient home or upgrade your existing windows?

Learn more about Zenith Windows here and if you’re an architect be sure to check out our Architectural Design Manual to get all the specs you need to design your next project.