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 article reviews existing literature on light timber frame walls (LTFW) with bio-based insulation and hempcrete walls constructed without vapor barriers, focusing on the implications for mold growth risk and indoor climate effects. The majority of studies, primarily from Nordic climates, indicate that vapor-open wall assemblies—those allowing moisture diffusion due to the absence of vapor barriers—exhibit a higher risk of mold growth, especially between the insulation and exterior lining, influenced by factors such as vapor pressure differences, vapor resistance ratios, exterior finishing, ventilated cavities, and hygroscopic properties of insulation materials. While vapor-open walls with hygroscopic bio-based insulation can buffer indoor relative humidity fluctuations, supporting simulations exist but field studies are limited and show smaller practical effects; no direct research was found linking these wall assemblies to occupant health or comfort outcomes. The review highlights gaps including a lack of consistent definitions for vapor-open construction, insufficient long-term field studies with visual mold assessments, and unclear thresholds for vapor diffusion coefficients to prevent mold. It recommends project-specific hygrothermal simulations and further research on real-life mold risk, durability, and indoor climate buffering by vapor-open bio-based wall assemblies.