Abstract
In recent years, external airtightness membranes have become an option for timber frame wall systems, as they allow high levels of air- and wind-tightness with an easy installation and provide rainwater protection during construction. This opens up the option of removing the internal air and vapour control layer. However, the hygrothermal risks associated to this option could be higher than in conventional construction, because vapour transfer can occur from the indoor environment into the timber frame wall not just via diffusion but also advection. This can lead to moisture accumulation and mould growth risk within the wall structure. This paper presents a parametric study that aims at identifying the moisture risk when external airtightness membranes are installed on a timber frame wall in a temperate maritime climate. The parametric study considered the two-dimensional heat, air and moisture transfer within a timber frame wall. Parameters having higher influence on moisture risk were identified and should be considered when designing robust wall systems.
Highlights
The aim of installing airtightness membranes behind the plasterboard is to reduce the air flow between the indoor environment and the building fabric and the outflow of water vapour present in air
This paper presented a parametric study to identify the mould growth risk associated to material properties, when external airtightness membranes are installed on a timber frame wall
The wall build-up considered was made of an external airtightness membrane, Oriented Strand Board (OSB) as sheathing board, mineral wool as insulation, AVCL and plasterboard, and the analysis was performed for a temperate maritime climate
Summary
The aim of installing airtightness membranes behind the plasterboard is to reduce the air flow between the indoor environment and the building fabric and the outflow of water vapour present in air. Most airtightness membranes can help reducing vapour diffusion through the wall; these are called Air and Vapour Control Layers (AVCL). The extent of air flow control depends on the air permeability of the airtightness membrane, and – and more importantly – on the level/quality of sealing between membranes. The extent of vapour diffusion control mainly depends on the vapour diffusion resistance of the membrane. External airtightness membranes are recently being offered for framed walls as an easy-to-build way of ensuring greater airtightness (as there is no breaching of the membrane due to services), as well as providing wind-tightness and reducing construction moisture
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