An increased interest in organic building materials is observed in the construction industry, as their use can reduce greenhouse gas emissions. At the same time, there can be observed an increasing interest, especially among architects, for the constructions without the use of plastic - with organic materials only.
There may be various reasons for choosing a diffusion-open lightweight exterior construction without a traditional plastic-based vapor barrier, e.g.: expensive and difficult work process, including the tightening of penetrations in the vapor barrier. Diffusion-open construction also has the potential to improve the indoor climate. However, the risk of condensation and mold should be assessed as the organic materials are very sensitive to moisture.
The purpose of this report is to assess the moisture-robustness of an exterior wall constructed without the use of traditional plastic vapor barrier. The literature study examines which material parameters have the biggest impact on the moisture conditions in a light exterior wall. The focus is on the insulation material and its properties.
In this report, hygrothermal simulations of lightweight exterior walls without the use of vapor barrier have been performed to investigate the significance of variations in material properties, e.g. equilibrium moisture content and vapor diffusion resistance, and to set the requirements for insulation layer in these constructions in the internal humidity classes 1-3. The moisture-robustness of the construction is assessed based on the risk of mold growth.
The report concludes, that the moisture capacity of commercial organic insulation materials does not have a significant impact on the overall moisture performance of the wall, on the contrary to what has been suggested in the previous studies. Simulations showed that for insulation layer, at least 1 of the following requirements must be met, to ensure moisture-robust lightweight exterior walls without a traditional vapor barrier (PE foil):
• High diffusion resistance of the insulation (occurs in some commercial organic insulation materials e.g. cork, hempcrete, high density wood-fiber insulation)
• High moisture capacity of the insulation in RF intervals between 60 and 90% (materials that can be developed).
Moreover, initial simulations of wind barrier layer based on literature studies conclude, that high moisture capacity of insulation layer in combination with high thermal resistance and high permeability of the wind barrier can have a positive effect on the moisture conditions in a light exterior wall, if a vapor barrier is present.