Volume 39, Issue 2 (March 2011)
Fenestration Condensation Resistance: Computer Simulation and In Situ Performance
The purpose of this paper is to examine condensation rating procedures, specifically the procedures as defined by the National Fenestration Rating Council (NFRC), and to compare the standardized procedure with in situ performance data. The American Architectural Manufacturers Association (AAMA) was the first to develop standardized methods to compare fenestration system performance with respect to the formation of condensation. Following this, the NFRC developed its own procedure for developing condensation resistance values. The AAMA test method involves laboratory testing, while the NFRC method involves computer simulation. Both methods provide a rating system to enable relative ease in performance comparison between similar fenestration products. The ratings are based on standardized conditions (i.e., fenestration size and shape, fenestration surround, and environmental conditions) that do not account for the in situ conditions and therefore may not accurately predict actual performance. Recent literature has discussed the limitations of condensation resistance ratings relative to as-built conditions that can impact the formation of condensation on a fenestration system. For example, (1) installing spandrel panel insulation, although necessary to improve the thermal performance of a curtain wall, can lower surface temperatures of adjacent framing members; (2) existing interior blinds or drapery act as an insulator, which can cause the framing members to be colder; and (3) if the fenestration’s thermal break is not aligned with the wall’s insulation, the framing members can be subject to colder temperatures. This paper will focus on the NFRC rating procedure and examine a case study to demonstrate the differences between NFRC ratings, simulated surface temperatures using THERM and WINDOW computer software, and in situ measurements of a high-rise office building curtain wall. The case study will provide a representative illustration of how actual conditions can impact the fenestration product’s in situ condensation resistance and provide insight on the condensation resistance of a representative curtain wall system.