Influence of omitting the vapor barrier in light timber frame walls with bio-based insulation on the risk of mold growth and indoor climate: A literature review

This paper aligns with the rising interest in improving indoor comfort and health of occupants by moisture buffering with hygroscopic materials and the use of bio-based materials with low embodied emissions. The study focuses on literature on light timber frame walls (LTFW) with bio-based insulation and hempcrete walls, both without a vapor barrier. The aim is to highlight the consequences of omitting the vapor barriers in such wall assemblies and to ensure their durability. This review examines two main questions. Firstly, is there a risk of mold growth in LTFW with bio-based insulation and no vapor barrier? Secondly, are there possible links between these building envelopes and the indoor climate and its potential effects on occupants? The findings show that the risk of mold growth in vapor-open walls depends on the external climate, user behavior and applied materials. It is influenced by several factors such as vapor pressure difference, water vapor resistance ratio, exterior finishing, presence of a ventilated cavity, and the hygroscopic properties of insulation materials. Vapor-open walls exhibit a higher risk of mold growth between insulation and exterior lining compared to walls with vapor barriers. Additionally, there is a lack of field studies that visually examine mold growth in vapor-open buildings. While simulations suggest that vapor-open buildings with hygroscopic insulation can buffer interior relative humidity fluctuations, supporting field studies are limited. Further research and field studies are needed to improve the understanding of vapor-open or permeable building and its implications on mold growth as well as on the indoor climate and effects on occupants.