How Windows Make (or Break) Wall Performance

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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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