Interstitial condensation analysis and moisture control for office building walls in China

Accurate analysis of interstitial condensation for walls could influence the energy efficiency and durability of buildings. The Chinese standard adopts the classic 1-D steady-state vapor diffusion model to assess the risk of interstitial condensation in the heating period in severe cold and cold regions, where the indoor and outdoor boundary conditions could be approximated as a steady state. However, its applicability is subject to doubt in other scenarios where indoor and outdoor boundary conditions are highly dynamic. The transient hygrothermal model subsequently provides more accurate results but is also more complicated. This paper aims to investigate the advisable interstitial condensation calculation method for office buildings in various climate zones in China. The hygrothermal properties of four typical building materials were tested first. Next, the steady-state and transient calculation methods were compared. Finally the optimized vapor diffusion resistance factors of vapor barriers were proposed. Results show that the constant and variable material properties have a negligible effect on interstitial condensation analysis, with a maximum difference of 2.5% in relative humidity (RH). However, the steady-state and transient calculations have discrepant results, with the maximum difference in RH exceeding 10%. Wind-driven rain also plays an important role, with the maximum difference in RH exceeding 15%. Besides, the EPS internal thermal insulation composite system (ITICS) and rock wool external thermal insulation composite system (ETICS) require an additional vapor barrier in Shanghai and Guangzhou. Since the reduction of building’s sensible and latent heat consumption caused by the vapor barrier is limited, the vapor permeability resistance of the vapor barrier only needs to consider moisture control.