Importance of Durability Assessment of Emerging Energy Efficient Windows

Fenestration systems play a key role in determining the energy demand of a building. A recent report from the US-DOE and the national laboratories1 shows that total US energy consumption is around 100 Quads, 35 of which are consumed by the commercial and residential building sector, and HVAC alone accounts for 12. Considering the 30% energy load due to windows in a building, nationwide energy use by windows is estimated to be around 3.6 Quads. The US-DOE is encouraging industry to develop R-10+ glazing, a jump from current R-4 glazing units in the market, which will result in energy savings of approximately 67%, ~2.4 Quads nationwide. The technologies being considered to achieve this target are vacuum insulated glass (VIG), aerogel glazing, and triple/quad glazing (where intermediate panes are thin glass). Highly insulating spacers are also being developed to reduce thermal losses in the edge of glass region, which can significantly reduce the overall thermal resistance of the glazing unit. Failure of these new high-performance technologies in the field before their specified life cycle could result in severe energy loss, increasing building energy use and negating the US-DOE objective of reduced energy consumption. The current insulating glass unit (IGU) durability testing standard is ASTM E-2190, which calls for an IGU unit of 20” by 14”. This standard may not be adequate for testing the durability performance of some of the mentioned emerging technologies. As such, NREL has developed a guideline for the enhanced durability evaluation of advanced glazing products2. This method evaluates glass units for seal failure, optical degradation, and thermal transmittance by exposing it to rigorous environmental exposure that accounts for actual field conditions. The guideline also looks at larger size IGUs, especially for VIG, to account for bending and shear stress failures. This paper presents the findings from testing IGU units using both the current ASTM E-2190 and NREL's Enhanced Guideline with recommendations to use the latter for reliable durability performance characterization.