The humble curtain wall spandrel, often unexciting and without receiving much attention, can be a dynamic and complicated element of curtain wall design that requires focus beyond simply stuffing insulation to meet local thermal performance requirements.
Traditionally a relatively simple construction of insulation sandwiched between an external and internal finish separated by a pressure sealed air gap, performance aspirations are normally set by budgets, climate and the compliance strategy of the design team.
Commonly occupying a relatively small percentage of the overall curtain wall area, spandrels are seeing an increased focus as they often represent of a major source of thermal bridging or heat loss (as illustrated below, but for the vision area) in mild to colder climates.
As a result, design teams are increasingly requesting that this thermal bridging be accurately accounted for in sizing heating systems and energy modelling while main contractors focus on defects and liability.
Today, it is common practise in Australia to measure the thermal performance of a spandrel by its resistance (or R-value - m2·K/W) through the centre of the unit only. While making life simpler for the construction procurement process, when we account for thermal bridging, we also need to assess the performance of the frame to provide us with a Total R-value (including frame) for better construction representation. Let’s look at an example:
Let's assume we need to meet a typical Melbourne performance aspiration of an R-value of 2.8 (fairly straightforward with plenty of design options) but we want to include a typical frame (which makes up a minimum of 30 per cent of the overall area) on a 1.5 square metre south facing spandrel panel.
In this scenario, our performance aspiration of R 2.8 is blown out of the water, with the true design Total R-value (including frame) somewhere between 0.5 and 1.0 (depending on frame and insulation quality), representing an additional 65 per cent of heat loss, or more that has not been accounted for within out energy models.
The humble curtain wall spandrel design now needs a rethink. With all markets now moving to the verification of the built form against energy modelled benchmarks, it is time to push for a greater understanding of the impact of thermal bridging not just in vision areas, but opaque too.
Going beyond the centre of the spandrel, what is the actual Total R-value (including frame) we should be aiming for and why? Once we get this right, alongside with the air tightness of the building as a whole, it is only then we truly engage in those conversations to downsizing our services to design the smarter, greener buildings of tomorrow.