A Numerical Investigation of Repeating Thermal Bridges from Metal Fasteners in Offsite Manufactured Timber Frame Closed Panels for Exterior Walls

Calculating repeating Thermal Bridges (TBs) in Timber Frame (TF) closed panels for exterior walls represents an opportunity to close the Performance Gap (PG) occuring in the Off-Site Manufacture (OSM) of Modern Method of Constructions (MMC). Despite the high precision reached by the OSM, neglecting repeating TBs from metal fasteners could add further uncertainty to the thermal transmittance of the construction, leading inexorably to real and predicted U-value differences. The paper discusses the TB calculation process for three dimensional (3D) Finite Element Analysis (FEA) models to assess the impact of fixings χ-values on the U-value of exterior wall TF MMC system. Five widely used fastener types have been analysed in 96-panel typologies differing for insulation materials, core and/or flanking insulation thickness, and reflective membranes. Results show that χ-values are reduced when the U-value decreases but, after accounting for the relative fastening density, the final U-value correction (resultant of all the fasteners) has a higher percentage detrimental impact on low U-value MMC systems such as those designed to meet near-to-zero and zero operational energy, ranging from 7 to 22%. Greater attention to repeating TBs should be paid with high performing constructions because high detrimental factors could be introduced by the metal connections.