Deconstructing the Thermal Performance of a Window: How to achieve better performing façades

With the increased focus on providing daylight and views for building occupants and improved indoor environmental quality (e.g. thermal comfort, air quality), as well as reducing energy consumption, a significant tension has been created around the design of the building envelope.  While the use of highly glazed façades allows for sufficient daylight and views, they can also create thermal comfort issues for occupants near them, condensation issues because of cold interior surfaces (which can become breeding grounds for mould), and reduce the overall thermal performance of the façade (since walls are in general more insulating than windows).

However, with the right design using daylight harvesting and high performance fenestration, an envelope system can be designed which is more energy efficient than an opaque wall [1], and thermal comfort and condensation issues can be overcome.  Indeed, the US DOE has stated that the net zero envelope design is one that has fenestration with very low U-factor (0.1 btu/, dynamic solar control and integrated dimmable lighting control [2].  And they predict that the use of highly insulating windows alone can save 1 Quadrillion (1015) BTUs annually if installed the entire US commercial building stock.  Given that commercial buildings use about 20 Quads annually, this represents a 5% energy savings from just improving the U-factor alone.

Over the years, the industry has relied heavily on the increasing performance of low-e coatings to drive window U-factors (thermal transmittance) lower.  The center of glass U-factor is, however, only part of the story and to achieve the lowest U-factors needed to get to, or close to, net zero we need to look more broadly at the window as a system.  The full performance of the window is determined also by the frame and the edge of glass conductance, as well as aspects related to air leakage and installation.  In fact, depending of glass size, the center of glass performance can contribute to less than half of the U-factor of the window.

In order to help designers specify the appropriate fenestration components and performance, we will deconstruct the factors that make up the U-factor and condensation resistance of a window, and look at the sensitivity of these parameters to different edge of glass parameters such as frame, frame bite relative to spacer sight line, sealant height, spacer conductivity, etc.  We will also provide considerations relative to the edge of glass components and demonstrate the use of polyamide pressure plates for improved curtain-wall performance, and as a retrofit strategy.