C O N T E X T 1 4 9 : M A Y 2 0 1 7 23 COLIN KING How to deal with retrofit risks The challenge is to improve the thermal performance of historic buildings without a significant impact on their appearance and historic fabric, and at acceptable cost. Undertaking retrofit has for some time been a hot topic for the UK government. Some of the most difficult buildings to tackle were constructed prior to 1930. These have the difficult challenge of balancing cost and environmental impact. There are additional considerations relating to the aesthetic and cultural significance of the building and place, and its context as part of the built heritage. The current approach to retrofit starts with an assumption that thermal improvements can only be made by insulation. Some problems are systemic: failure or underperformance due to the process of decision making in selecting the most appropriate materials to be used for thermal upgrades to a building, or the method and decision making process for deciding which elements of the building are to be improved, such as the omission of insulating reveals, heads, eaves and other penetrations for a variety of reasons (such as increased cost) that can be attributed to extending roof lines and gables, isolating walls and fences to allow the placement of insulation behind their location, or relocation/temporary removal of incoming services to provide thermal continuity. Other problems relate to the assessment of materials and buildings. The current methods of assessing durability and condensation risk are undertaken using unrealistic parameters for risk.These limited methods of testing can provide a level of comfort and assurance which in reality may not be delivered, such as levels of high exposure to wind-driven rain, and high levels of internal and external relative humidity. Traditional buildings are the most challenging to improve thermally. Due to their use of lime mortar and porous construction they are the most susceptible to the effects of moisture ingress. Better quality control and accurate assessment are needed when choosing an improvement option.The decision to insulate must be taken with the greatest of care, with particular attention to the existing condition of the property, with a focus on fabric integrity and watertightness, and with a thorough investigation of the likelihood of introducing cold bridges into the building, and how these will be designed out in the installation process. It is accepted that there is a limit to the extent of changes that can be made to improve a building’s thermal performance without a significant impact on its appearance and on its historic fabric. Improving the sustainability of such buildings results in a tension between the competing demands for heritage preservation and the pressures to reduce the environmental impact of buildings in use. The problems caused by moisture in building structures are well known. There is a large body of literature addressing issues including rising and penetrating dampness and interstitial condensation. For regulatory purposes the key document is the BSCode of Practice for condensation in buildings, BS5250:2011, which is referenced in the three sets of regulations in the UK. Apart from a discussion on the principles of condensation risk, the main content of BS5250 is prescriptive guidance on how elements such as walls and roofs should be designed and constructed. Many, if not most, designers will refer to this guidance and not carry out any more complex assessments. If a more detailed analysis is necessary, BS5250 refers to a calculation procedure specified in ISO 13788, which uses a ‘Glaser’ procedure, taking account only of steady-state vapour diffusion. While this may be adequate for some structures such as flat roofs (and even here there are situations where it is inadequate), it does not represent the whole picture for constructions such as masonry walls where very large amounts of water are stored in the fabric. No account is taken of rain impacts and solar gain on the outside surface, liquid water movement, and the effect of moisture on the thermal and moisture transport properties of materials. A further standard, BS EN 15026:2007 Hygrothermal performance of building components and building elements: assessment of moisture transfer by numerical simulation, was developed to address these issues. This is becoming used in the UK through the German software WUFI (which can run calculations A standard detail applying external wall insulation at the party-wall line of a traditional terraced house
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