The Building Conservation Directory 2023

136 T H E B U I L D I N G C O N S E R VAT I O N D I R E C T O R Y 2 0 2 3 | C E L E B R AT I N G 3 0 Y E A R S C AT H E D R A L C O M M U N I C AT I O N S alternative options to improve thermal comfort should be reviewed. From the outset, the author advises a cautious approach when considering an insulation retrofit on a solid-built traditional building. Alternative options other than insulating the external envelope of a property can improve a property’s internal level of comfort, such as draughtproofing and the introduction of radiant breaks. After all, a draughty room is likely to feel cold even at 21°C. Utilising window shutters, introducing secondary glazing and fitting heavy curtains can appreciably improve thermal comfort simply by reducing the draught. Traditionally, wall hangings and tapestries were used as a layer of removable insulation which helped as a thermal break to enhance comfort. (We might also now consider summer cooling to prevent overheating in a historic property as the planet warms up.) Notwithstanding that, insulation retrofits to improve the thermal performance of buildings’ external envelopes are gaining momentum. ASSESS THE CONDITION Before any decision on retrofitting is made, an understanding of the condition and the internal environment of the building is critical to its success. If a property already has significant dampness which has not been addressed, then it is likely the retrofit will fail. When reviewing a structure, we need to look at: The outer envelope – for the integrity of its fabric, for evidence of wind-driven rain penetration or penetrating dampness at ground level, mainly where there are high ground levels, and for signs of leaking gutters. The centre of the wall – for evidence of interstitial condensation or for voids in the wall core, which may hold water in reservoirs. The inner envelope – for signs of surface condensation, which are widely underestimated or misdiagnosed as rising or penetrating dampness. Indoor environment – for internal humidity/air quality. Embedded timber will reach an equilibrium moisture content and can be a good indicator of the wall moisture content. Additionally, where timber decay occurs, it can often indicate a long-term pattern of moisture penetration. Building professionals with expertise in building conservation should be consulted well before any retrofit is considered, to understand the building’s condition. Specialist surveyors use building pathology to avoid conjectural diagnosis. The evidence is gathered using non-destructive techniques such as atmospheric data loggers, thermal imaging and drone technology wherever possible. Some specialist techniques, such as decay-detecting micro-drilling and gravimetric sampling, which may be considered minimally destructive, are also invaluable tools. This approach to analysis can eliminate post-retrofit problems, particularly regarding timber decay. EXTERNAL & INTERNAL WALL INSULATION External wall insulation (EWI) has several advantages over internal wall insulation. The main advantage being that the high thermal mass of solid masonry is kept on the warm side of the insulation. This method reduces the risk of condensation caused by thermal bridges that arise where interior walls, floors and other structures are attached to the cold mass of the exterior wall. In addition, there is minimal impact on internal finishes and no reduction in room sizes. However, viewed from the outside, the aesthetics of a building are altered by installing EWI, even if it is already rendered. Particular problems may arise at the junction of neighbouring buildings when retrofitting a terrace or semi-detached property and when detailing architectural features. As a result, EWI will not be appropriate in many cases. Nevertheless, where it is appropriate, a suitable and energy-efficient solution is possible. Installing an internal wall insulation (IWI) system changes the temperature conditions of the existing construction even more. When insulation is applied to the interior faces of the external walls, the wall temperature level decreases, leading to a reduction of the drying potential. Installing IWI has risks associated with thermal bridging and poor detailing which require a coherent methodology. This system can alter the internal space, put historic fabric such as cornices and window linings at risk of loss, and physically reduce the interior space. However, mitigating the loss of original fabric against keeping a building viable or in use may deem this approach suitable for more buildings. Where IWI is appropriate, it can be successfully installed using the proper detailing, taking a holistic approach to the building. MOISTURE Excess moisture is one of the most harmful agents of decay in our built environment. Traditional buildings manage moisture differently from modern ones, as they allow moisture absorption, evaporation and movement within the building fabric. Their materials enable a building to ‘breathe’ by utilising vapour permeability, hygroscopicity and capillarity. Subsequently, any retrofit interventions of traditional buildings The result of an atmospheric data logger used to understand the building environment before retrofitting it with insulation A wall supporting a timber floor on the left before internal wall insulation is applied and on the right after it is insulated and the embedded timber is in the cold zone (© Saint-Gobain Rigips GmbH)