Alden Kung, M.Eng. 2012
Supervisor: Dr. Rodrigo Mora
Windows are integral to building enclosures; they provide natural light to the indoors and allow views to the outdoors. There have been large improvements in glazing technology in the past 25 years but the improvements have been offset by the use of larger window‐to‐wall ratios (WWR) in modern buildings. Because windows are usually thermally weakest components in a building enclosure, increase in WWR results in higher heat loss. The use of window wall systems is common in modern high‐rise buildings which makes accurate calculation and assessment of window wall thermal performance important for
The method typically used for calculation of window and enclosure thermal performance is an area weighted average approach. Individual U‐values of window and wall components are typically determined using two‐dimensional heat transfer software and applied to area weighted average formulas to find effective thermal transmittance. This approach does not fully include complex three-dimensional heat flows through building assemblies and therefore, a new more accurate approach aims to replace the area weighted method.
The calculation of thermal transmittance using clear field, linear, and point transmittances was explored in ASHRAE’s project report 1365‐RP. The application of these transmittances was mainly for opaque walls and corresponding slab intersections through the walls. It offered a comprehensive and accurate method for calculating the thermal performance of building walls but further research into application of the concepts to windows was needed.
This report uses the main concepts from 1365‐RP and applies them specifically to window wall systems. An adapted transmittance method was required for calculating thermal performance of window wall enclosures because the concept of clear field in opaque walls is not entirely translatable to windows. The thermal transmittance values produced are applicable to a standard floor‐to‐floor height of 8’ – 7½” (2.629m). The mullions are counted as point transmittances because they are able to be counted individually while linear transmittance is determined without mullions and can be applied to a linear
Overall, the adapted transmittance method allows for accurate calculation of window wall systems with different slab edges and mullion configurations. An added bonus is the ability to determine percentage heat loss contributions through glazing, window frame, and opaque walls which can be helpful in the design of high performance enclosures.