How Windows Make (or Break) Wall Performance, Illustrated

The thermal performance of your building’s walls is only as strong as the weakest link, and windows are too often that weak link. Even if you’ve gone to great lengths to build a super-insulated high performance wall, if you poke that wall full of holes, then plug those walls with mediocre windows, you will get a poorly-performing wall.

Bad windows will ruin the best wall assemblies.

Conversely, good windows can make run-of-the-mill wall assemblies good, and good wall assemblies great. To illustrate just how dramatic the role of windows can be in a building, we worked with building science expert Skylar Swinford to develop a series of scenarios for an exterior wall in a typical multifamily building unit. In each of the scenarios, illustrated below, we compare wall assemblies of varying levels of insulation, and the relative energy performance impact of installing (conventional) R-3 windows versus (high performance) R-7 windows. This overall impact is measured and compared in the "Effective Wall R-Value" bar graph at the center of each scenario illustration, which shows the actual R-value of the entire wall-and-window assembly.

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Good Windows First

Scenario 1 shows that unless you are using good windows, increasing insulation won’t get you far in improving wall performance. On the left we see an R-23 wall assembly (2x6 wall with cavity insulation plus ZIP-R6 exterior insulation), but with mediocre, R-3 windows installed. On the right we see a code-minimum R-17 wall, but with high performance, R-7 windows installed. Because the code minimum wall uses these high performance windows, it actually outperforms the wall with the added exterior insulation; its effective R-Value is 14.8, versus 13.8 for the thicker wall with inferior windows.

Notice that Scenario 1 assumes a low window-to-wall ratio (Window:Wall or WWR) of just 10%­—fairly miserly glazing. The relative benefit of the R-7 windows only increases as the WWR grows. Scenario 2 shows that with a WWR of 35%, the code minimum wall assembly with R-7 windows outperforms the wall with extra insulation but R-3 windows by nearly double, 11.3 versus 6.9 in effective R-value.

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Better Windows Make Bigger Windows Possible

Scenario 3 compares two identical wall assemblies (both using the R-23 assembly with the exterior insulation) and, holding the effective R-value of the two walls equal, explores how much bigger the R-7 windows can be compared to the R-3 windows. The result: the R-7 windows can be three times bigger than the R-3 windows and still achieve identical effective wall R-value.

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Good Windows + More Insulation = Really Good Performance

As Scenario 4 shows, when you take a better-than-code wall (again, the R-23 assembly with the exterior insulation) and add decent windows, you leverage the best performance results. The R-7 windows garner an effective R-value of 15.8 for the wall, while the R-3 windows drop that to just 9.9.

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Only Insulation Heroics Can Make Up for Bad Windows 

Scenario 5 really drives home how much more consequential window performance can be on the thermal performance of your building than insulation, even at a modest 20% window to wall ratio. In order for the wall on the left, with its R-3 windows, to perform as well as the 2x6 wall on the right with R-7 windows, you would have to build an insulated wall assembly roughly two feet thick.

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R-7 Windows Are "Run of the Mill" for Zola

While the performance impacts of R-7 windows are remarkable, we see these impacts everyday at Zola. For us, R-7 is run of the mill. We have lots of window lines at that performance level or higher, available in any of our frame materials. At the highest end of the spectrum, our Zola Arctic line performs at an industry-leading R-11 and is currently being applied to a research station in Antarctica. But closer to home, for the typical multifamily building or budget-conscious single family home in North America, we have several uPVC options that deliver R-7 and R-8 performance at a price point that is competitive with much lower-performing domestic windows.

Regardless of your building type, high-R windows have tremendous power to optimize performance, maximize glazing flexibility, and either avoid investments in thick walls or leverage those investments to achieve Passive House-levels of performance.

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[Featured photo above is of Accord Passive House.]