Thermal analysis of above-grade wall assembly with low emissivity materials and furred airspace

Abstract

A 3D numerical model was developed to investigate the effect of foil emissivity on the effective thermal resistance of an above-grade wall assembly with foil bonded to wood fibreboard in a furred assembly having airspace next to the foil. This model solved simultaneously the energy equation in the various material layers, the surface-to-surface radiation equation in the furred airspace assembly, Navier–Stokes equation for the airspace, and Darcy and the Brinkman equations for the porous material layers. In this work, the furring was installed horizontally. In the first phase, the present model was benchmarked against the experimental data generated by a commercial laboratory for an above-grade wall assembly. The wall consists of a conventional wood frame structure sheathed with fibreboard and covered on the interior side with a low emissivity material bonded to wood fibreboard that is adjacent to a furred airspace assembly. The results showed that the predicted R-value was in good agreement with the measured one. After gaining confidence in the present model, it was used to predict the effective thermal resistance of the same above-mentioned wall but having Oriented Strand Board (OSB) sheathing in lieu of wood fibreboard sheathing. In the second phase, the model was used to quantify the contribution on the wall R-value by having a low foil emissivity. The results showed that a low foil emissivity of 0.04 can increase the R-value of this wall to as much as ∼9%. This is on-going research. The present model is being used to investigate the transient thermal response of foundation wall systems with furring installed horizontally and vertically, and subjected to different Canadian climate conditions.