Exterior Wood-Frame Walls—Wind–Vapour Barrier Ratio in Denmark

Martin Morelli,Torben Valdbjørn Rasmussen,Marcus Therkelsen

doi:10.3390/buildings11100428

Abstract

Wood-frame walls in cold climates are traditional constructed with a vapour barrier that also constitutes the air-tightness layer. Polyethylene foil as a vapour barrier is likely used; however, other building materials can be used to obtain correspondingly sufficient properties. 1D hygrothermal simulations were conducted for a wood-frame structure to investigate the wind–vapour barrier ratio, and if the vapour barrier of polyethylene foil could be omitted and replaced by other materials. The results were postprocessed using the VTT mould model. The results showed how wood-frame walls can be designed with respect to internal humidity class and diffusion resistance divided into three categories: no risk for mould growth, needs further investigation, and is not performing well as the risk for mould growth is present. For internal humidity classes 1–3, the ratio between wind and vapour barrier must be about 1:5, and 1:10 for classes 4 and 5 to be on the safe side. Simulations were performed for the climate of Lund, Sweden, which were used to simulate climate in Denmark too. Nevertheless, the results are related to climate data and, thus, the location.

Highlights

In view of increasing climate changes, the building sector emphasises sustainability by using less energy-intensive materials to decrease the emission of carbon dioxide
The aim of this study is to investigate how the ratio between the water vapour diffusion resistance (Zp-value) of the wind barrier and the vapour barrier influences the performance of an exterior wood-frame wall
The different configurations of the wood-frame wall with different boundary conditions comprised a thorough assessment to reveal which configurations of wind–vapour barrier the wall were well-functioning

Summary

Introduction

In view of increasing climate changes, the building sector emphasises sustainability by using less energy-intensive materials to decrease the emission of carbon dioxide. Cold climate buildings have an outgoing vapour pressure, and the vapour barrier is installed in the building envelope towards the interior to protect the thermal insulation against vapour diffusion and exfiltration of warm humid air. Some people prefer to construct building envelope structures without vapour barriers of PE-foil, and to secure the thermal insulation of the building envelope by using, for example, board materials. It is well-known that moisture transport by diffusion is much smaller than by convection. Prior studies [2,3] showed that ceilings that are sufficient airtight at the interior side can be constructed without a vapour barrier layer

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Conclusion