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The greatest difference between buildings of traditional and non-traditional construction design lies not so much in the building materials used, but in the way they are used to construct the external envelope of a building – in the way they are assembled. One may say that the defining approach of traditional construction is the management of moisture and, in contrast, one can say that modern construction systems generally (but not always) depend on blocking, not managing, moisture. The deterioration and decay that occurs in buildings almost always involves moisture. Three relevant moisture sources are identified in this report: indoor vapour, rain water (including wind driven rain) and ground water. Since heat, water vapour and liquid water are all driven by different forces, they can ‘move’ in different directions at different times within the same wall. However, because the forces driving these transport phenomena are coupled, retrofit strategies to address one issue may have unexpected effects on another. Retrofitting building elements to improve thermal performance can carry an unestablished level of risk of moisture-related damage occurring, to the detriment of existing and newly installed portions of the building fabric as well as for occupants. The report discusses two available methodologies for conducting hygrothermal risk assessments: steady-state condensation risk assessments, using the Glaser method, and transient hygrothermal performance analysis, using numerical simulation. In Ireland and the UK, construction guidance and practice are still heavily influenced by the diffusion paradigm, a reductionist but deeply held view that vapour diffusion is the only relevant moisture transport mechanism in building fabric and the use of a vapour barrier to control it is always best practice. The case study in this report has demonstrated, by comparing Glaser method and numerical simulation assessments (using BuildDesk U and WUFI software respectively), that (a) their results can be diammetrically opposed, (b) moisture transport in solid, unrendered stone walls is predominantly in the form of liquid migrating through the materials’ capillaries, due to capillary action and surface diffusion. The report demonstrates that choosing the most appropriate risk assessment method is important to ensure that retrofits are durable, sustainable and create healthy environments.
Little, J., Ferraro, C. & Arregi, B. (2015). Assessing Risks in Insulation Retrofits Using Hygrothermal Software Tools. Affiliation: Historic Environment Scotland, Scotland, Report number: Technical Paper 15 Affiliation: Historic Environment Scotland, Scotland. doi:10.13140/RG.2.1.2493.3844