Experimental field-trial design to investigate the effects of a defective internal vapour layer on timber frame wall constructions including initial findings

Abstract

Hygrothermal analysis in multi-layered building components is becoming common practice. Low energy design criteria demands an increase in thermal and airtightness requirements resulting in more complicated building envelope designs to accommodate the necessary insulation and airtightness layers. Furthermore, in many cases materials are being chosen based solely on their thermal characteristics without fully considering other properties and this may lead to unintentional interstitial moisture-related problems. Much progress has been made in developing tools for undertaking hygrothermal simulations; however, there are on-going questions regarding how best to model imperfections and defects accurately using these software packages. Results of simulation models carried out in accordance with the new WTA guideline have been reported in literature as encouraging and confirming practical experience. Further verification of these simplified methods is therefore essential, including investigations of the relationship between model assumptions and typical defects in different construction types. Therefore, there is a need for specific field experiments and laboratory tests which gather the data necessary to validate and/or calibrate these models under a wider range of constructions types, defect types and climates. This paper describes the experimental design and fabrication of a full-scale timber frame test house that has been developed to assess the impact of a common defect in the internal vapour control/airtightness barrier, along with initial data results and findings. The data obtained will be used to validate existing commercial hygrothermal models and investigate different parameters and methods for modelling these vapour barrier defects.