Spandrel: The battle for the wall - July blog post

Twinsburg, Ohio

10 Jul 2020

blog by Helen Sanders, Ph.D

SOMA Towers in Bellevue, Washington, was designed by Bohlin Cywinski Jackson and used a unique window wall design in which the thermal performance was maximized by using full-height 10-foot spandrel panels (to maximize the center-of-panel relative to edge-of-panel performance), adjacent to large slab-to-slab glazed expanses.

Over the past few years, I have heard several people talk about spandrel as the weakest link in the curtainwall system. Initially I was confused, since with opaque insulation placed in the spandrel assembly, shouldn’t the spandrel thermal performance be better than the transparent areas of the curtainwall? As it turns out, it is not the comparison between spandrel and adjacent transparent areas that is important; it is the comparison of the spandrel with a typical opaque wall assembly. Here lies a large performance gap which, if unaddressed, could have serious long-term ramifications.

ASHRAE Standard 90.1 treats spandrel as a steel-framed wall. According to Tom Culp of Birch Point Consulting, “These walls are assumed to have steel stud framing 16-inches on center with fiberglass insulation in the cavity and continuous insulation across the face.” That means that in order to comply with the prescriptive U-factors in ASHRAE 90.1-2016, for example, spandrel must reach a U-factor of 0.064 btu/^oF.hr.ft²(R15.6*) in climate zone 4 (New York) and 0.055 btu/^oF.hr.ft² (R18) in climate zone 5 (Chicago). Even in warmer climate zones, such as Phoenix or Tampa, the requirement is 0.084 btu/^oF.hr.ft² (R12). Meeting any of these values is challenging for spandrel.

(*In this usage, the R-value quoted is the reciprocal of U-factor.)

According to Stéphane Hoffman, vice president and senior building science specialist at Morrison Hershfield, “Based on 3D thermal modeling, the best performance that can typically be achieved with insulation in the spandrel and continuous insulation in board is around R9 for a large 5-by-5-feet spandrel. A smaller, 3-by-5-feet spandrel can only get to R5, at best, as performance drops quickly as spandrel size reduces and the perimeter thermal bridging becomes more dominant over the center of panel.”...to continue reading click here.

(the full blog, as well as previous posts, are hosted on usglassmag.com)