A Strain-Normalized Serviceability Limit State for the Secondary Seal of Insulating Glass Units with Rigid Box Spacers: Optimizing Serviceability of IGU Seals
Insulating glass unit (IGU) geometries, cavity dimensions, installations, and edge seal forces can vary considerably from those qualified with laboratory test specimens for durability. The life span of an IGU is often limited by the design and fabrication quality of the edge seal system. The durability of IGU edge seal systems in North America is evaluated with standardized laboratory weathering tests for the established IGU specimen dimensions and glass compositions specified in ASTM E2190. For edge seal design of structurally glazed dual-sealed IGUs, the secondary seal contact width for a selected edge seal system is based on the minimum dimension qualified in the ASTM E2190 test specimen and the calculated capacity to resist wind load per ASTM C1249. The contact width of the secondary seal is currently evaluated for capacity to resist wind load without an application-specific review of edge seal durability—that is, there is no method to ensure that predicted environmental forces, such as cavity loads, do not cause excessive strain in the primary sealant that would accelerate moisture vapor transmission and gas loss substantially above that experienced by the IGU laboratory test specimen.
A serviceability limit state is proposed for design of the secondary seal of dual-sealed IGUs with a rigid box spacer to achieve a comparable polyisobutylene (PIB) strain to that experienced by IGUs when tested according to ASTM E2190 (strain-normalized performance). This paper describes an evaluation of IGU edge seal structural mechanics with finite element analysis. On the basis of our results, we suggest that service-level design capacity is a function of PIB strain (rather than structural sealant stress) for long-term cavity loads and is the governing limit state for IGU edge seal design. This is an initial step in the development of an analytical method to engineer IGU edge seals for serviceability.