Heritage Retrofit

26 BCD SPECIAL REPORT ON HERITAGE RETROFIT FIRST ANNUAL EDITION these U-values have been measured rather than derived from the standard calculating method, which has been shown to have limitations when used to estimate heat loss for solid walls (see Further Information: Baker and BRE). The wall at Drewsteignton is quite different, being a north-west-facing, 600mm thick granite construction. In this wall we find higher moisture levels (in terms of both %RH and AH g/m 3 ) and narrower saturation margins. We also find, over the past three years, a trend of rising RH in the centre of the wall which, year on year, moves this part of the wall closer to saturation conditions. As this trend has continued over a number of years, we conclude that the high RH within the wall is not solely a response to atmospheric conditions but is also a function of certain qualities of the construction that might limit or inhibit drying. This may be, in part, down to the heavyweight nature of the wall and its aspect, but vapour profiles have climbed since the wall was insulated and have not returned to pre-insulation levels. This suggests that the insulation itself may be having some impact on the wall’s performance, although it is not clear whether this is primarily due to its thickness or its impermeability. The wall at Drewsteignton has been insulated with a greater quantity of more thermally resistive insulation and this reduced the measured in situ U-value from 1.20 W/m2K to 0.16 W/m2K. This ensures that less heat passes into the cold side of the masonry during the winter period, thus saturation margins are lower and air is more likely to become saturated and remain saturated for longer periods, limiting the wall’s ability to dry. Furthermore, the foil-facing of the PIR board acts as a barrier to the movement of moisture from the core of the wall, which can no longer access the potential evaporative surface of the interior wall face. In conclusion, we find that the performance of these walls is in part conditioned by their individual material components, including changes made to the fabric to improve energy efficiency. Interstitial condensation has been a particular concern, yet the internally insulated brick wall at Shrewsbury, which uses a limited quantity of insulation and does not incorporate a VCL, has stable vapour responses that operate within safe limits. In contrast, at Drewsteignton, where insulation has reduced the U-value of the wall to a fraction of its previous heat loss and a VCL limits the movement of vapour towards the internal side of the wall, vapour conditions are deteriorating. The measurements from the BPS tell us that, rather than internally generated moisture, the influence of the external environment in combination with the individual circumstances of the walls – their materials, aspect and condition – has the greatest impact on their moisture performance. These walls are solid, there is no capillary break in the form of a cavity or damp-proof course to prevent moisture, particularly wind-driven rain, penetrating deep into the core of the wall. Many solid walls are thick, built with heavyweight materials and they can be shaded and/or sheltered. This means that the ability of heat and air movement to dry these walls may be limited. While this may not, prior to insulation, create a moisture problem in the wall, the method by which a wall is retrofitted must take into account all the factors which might impinge upon its performance. The decision as to what type and what thickness of wall insulation might be suitable for a solid wall cannot be answered by looking at heat loss reduction alone. Those charged with improving the energy profiles of these buildings must view the building as a whole, looking at how it may perform in its specific context including individual wall aspects and what the effect of its constituent materials, condition and finishes may be. The wall at Drewsteignton shows that the use of a relatively large quantity of higher performing close-cell insulation, incorporating an impermeable VCL, can result in a risky vapour profile. This is not to say that the application of similar material to the internal wall at Shrewsbury would have produced the same results. Indeed, at this location the wall’s performance may have been more satisfactory as this wall is able to dry more readily. However, the BPS shows how complex and multifactorial the hygrothermal performance of walls can be. It is an interplay between materials, condition and context, and the exact effect of all these upon the long-term performance of the building may remain unknown or difficult to predict. Given this uncertainty, we need to acknowledge the limits of our understanding and adopt a precautionary principle. This would ensure that elements are not deprived of all internally generated heat by excessive amounts of internal insulation because it may be that it is the contribution of this heat, in combination with external solar radiation, that allows the wall to moderate its moisture load over time. In addition, materials that are vapour-open and capillary-active and thus have some ability to move moisture through the structure to surfaces from which it can evaporate are also more likely to be a safer option for the insulation of a solid wall. This study demonstrates that it is possible to make positive changes to the energy efficiency of solid walls through the application of insulation but that an approach that favours limited improvements to heat loss and materials that promote moisture movement may introduce less risk than alternative strategies. Further Information H Altamirano-Medina et al, ‘Guidelines to Avoid Mould Growth in Buildings’, Advances in Building Energy Research , Vol 3:1, 2009 P Baker, Technical Paper 10: U-values and Traditional Buildings , Historic Scotland, Edinburgh, 2011 (http://bc-url.com/trad-buildings) Building Research Establishment, In-situ Measurements of Wall U-values in English Housing , BRE/Department for Energy and Climate Change, Watford, 2014 (http://bc-url.com/wall-uvalues ) Department for Communities and Local Government, Approved document F: Ventilation , NBS, London, 2010 CAROLINE RYE and CAMERON SCOTT founded the research company ArchiMetrics Ltd in 2011, with the aim of increasing the understanding of building performance via the measurement of real buildings. ArchiMetrics carries out research using bespoke methods of measurement and analysis to ensure an integrated and thorough approach, and the partnership specialises in older or non- standard buildings throughout the UK. Monitoring a south-facing brick wall at Shrewsbury