Durability = Sustainability: Ensuring Long Term Thermal Performance

by Helen Sanders

Energy efficiency is usually the first thing discussed when it comes to sustainable design. What’s often forgotten is durability. But what good is energy efficiency in a building if it doesn’t last?

Consider glazing systems: The insulating glass unit’s (IGU’s) lifetime, along with accompanying window gaskets and seals, is generally the determining factor in the lifetime of an aluminum window and curtainwall. According to Graham Dodd from Arup, the lifetime of an IGU currently is expected to be 25 to 40 years¹, which is quite a large range. Given the critical nature of fenestration in the building envelope, the longer the lifetime of the IGU, the more sustainable the building itself is likely to be.

OVERALL PERFORMANCE

There is a lot of pressure to improve the energy performance of fenestration through using inert gases, triple- or quad-pane configurations and warm-edge spacers with incrementally-improved thermal performance. When making decisions to change the design (or processing) of an IGU, the first question to ask is: Will this change negatively
impact the IGU’s durability and potential lifetime expectations? There are several insights that can be gained from IG testing and certification.

Desiccant absorption capacity of the IGU is a good place to start. The Insulating Glass Certification Council’s (IGCC) IG Certification Program requires manufacturers to declare the absorption capacity per foot (ACPF) of the edge spacer. The value depends on the amount of desiccant that is physically incorporated into the spacer and its percentage absorption capacity by weight. All other things being equal— moisture ingress rate, unit size, etc.— the more desiccant capacity there is in the edge seal, the better it will do in durability testing and the longer the IGU should last. ACPF gives a value that can be used to provide a first level comparison between different edge seal systems, including between those with loose and integrated desiccant.

IGU seal testing is another way to assess the relative durability of different IGU designs. Because the ASTM E2190 IGU durability performance specification only gives a pass or fail target for dew-point (<-40°F) after weathering, it does not provide insights as to the relative margin by which IGUs pass the durability test. However, some manufacturers will put the same set of IGUs through the ASTM E2188 15-week weathering protocol multiple times to assess time-to-failure as a way of comparing the performance of new edge seal designs or processes to a previous benchmark.

KEEP UPTAKES LOW

Another way of assessing relative durability performance is to measure the moisture uptake of the desiccant in the IGUs after testing and compare it to the moisture content of desiccant in an un-weathered IGU made at the same time. The lower the uptake, the better the IG seal is at keeping water out. Improving durability is as important as improving thermal performance. The long-term thermal performance of an IGU is only as good as its durability, and the sustainability of a building is only as good as the durability of its glazing.

¹G. Dodd, Design Freedom and environmental performance beyond the constraints of the
hermetically sealed insulating glass unit, Proceedings of GlassCon Global 2018, p101.

Helen Sanders is in strategic business development for Technoform
North America Inc. in Twinsburg, Ohio. Read her blog each month at
usglassmag.com/insights.

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