The presence of liquid water within or on a building element due to condensing water vapour. This can lead to fabric decay and/or mould growth that are hidden. It can occur through vapour diffusion or air leakage into structure where warm moisture meets colder areas/surfaces. It can occur in walls, roofs or floors through incorrect installation of insulation systems, air leakage, thermal bridging and reduced ventilation.

Suggested actions

Before Implementation

Carry out a thorough assessment of context, moisture loads, fabric types and condition of building. Understand the restrictions imposed by the exposure for the different orientations of the building (e.g. internal insulation is not suitable in some orientations in certain locations). Consider that different solutions might apply for different orientations (i.e. a combination of insulation measures may need to be considered). Ensure mechanisms for managing moisture within existing fabric are thoroughly understood. Monitor fabric elements (e.g. walls) prior to refurbishment to establish interstitial hygrothermal (heat and moisture) behaviour, including monitoring conditions within voids before refurbishment in order to assess humidity levels and consequences of any temperature reduction. Carry out condensation risk calculations to EN15026 (WUFI, Delphin modelling) informed by the in situ interstitial hygrothermal monitoring. Modify design according to simulation and monitoring results to avoid material moisture contents in excess of 15% and/or RH above 80%. NOTE: Meaningful interstitial hygrothermal modelling requires detailed modelling of wall construction components including the specific material properties for the exact construction materials, specific and detailed weather data and expert software operation. Undertake basic Glaser Analysis (BS 13788) to assess some risks posed by diffusion of internally generated water vapour. Avoid target U-values below 0.4 W/m2K. Use of Air Pressure Test to establish existing rate of air exchange.

During implementation

Ensure the technical properties of any proposed new materials particularly in relation to moisture (vapour permeability, hygroscopicity and capillarity) and behaviour of these in relation to existing fabric are clearly understood. Choose design solutions that are coherent with the properties of the existing fabric. Avoid non breathable highly vapour resistant insulation materials and barriers.

After implementation

Monitor fabric post refurbishment particularly at high risk areas, i.e. joist ends, consider inclusion of an alarm element to provide notification of extended periods of dewpoint within fabric and report on findings. On-going monitoring to assess condition of voids following refurbishment and to inform and necessary mitigation e.g. installation of mechanical ventilation. Regular visual inspection, if possible to check for signs of decay. Continuing monitoring and reporting to safeguard fabric