Heritage Retrofit

32 BCD SPECIAL REPORT ON HERITAGE RETROFIT FIRST ANNUAL EDITION THE LIME CYCLE as effectively as hydraulic lime but then delivers the flexibility that is essential for the conservation and repair of old buildings. In one experiment, a piece of carpet was skimmed with a plaster made from this lime and allowed to set. It did so overnight. It was possible to dent the plaster with a thumb without cracking it and even to bend the piece of carpet without damaging the plaster. The remaining mix from the experiment was left in a tub. This same batch is now a hard lump and the carpet plaster is still intact – 2½ years of poking and prodding, later. DEVELOPING A LIME-BASED INSULATION PRODUCT In 2014 development of an insulation system based on a non-hydraulic setting mortar was awarded funding under a Small Business Research Initiative (SBRI) established by The City of Cardiff Council, Cadw, Innovate UK (formerly Technology Strategy Board) and Low Carbon Trust with principal funding from the Welsh Government. The aim of the scheme was to assist the development of ‘innovative measures that will improve the energy performance of traditional and historic buildings’. The first step was to prepare sample insulation panels (left) with a thickness of 62mm (2½"). These contained Vivus lime, mineralised wood chips and various permeable aggregates selected to ensure a very high degree of vapour transfer, ideally suited to traditional solid wall construction. Independent testing confirmed thermal conductivity of 0.1 W/mK in the least efficient sample of Vivus render, in order to establish a base-line, compared to 0.5 W/mK typically found in conventional sand/ cement renders. (Thermal conductivity is measured in watts per square metre of surface area for a temperature gradient of one kelvin for every metre thickness – W/mK.) The tester also confirmed that in his view even better results could be expected if different aggregate materials and thicknesses were used. Currently the insulation systems in common use are all impermeable and are not readily compatible with older building walls. The research confirmed that a suitable insulation panel would be a useful tool in the retrofit armoury. Testing and experimentation is on- going to determine exactly how thick the panel or how deep the insulation needs to be, to provide adequate insulation and to buffer humidity, but without being too deep to apply to older walls with existing architectural features. Findings are expected during the course of 2017. Other materials such as plasters and renders were also developed using the same quick setting non-hydraulic lime and a similar range of aggregates. During workshop trials these were shown to be successful in their ability to set and, once dry, in their ability to absorb and readily release humidity. Conventional lime sand mortars tend to have a much higher degree of capillarity due to the impervious nature of the mineral aggregate, drawing moisture in and retaining it for longer, off- setting some of the benefits of the lime. The advantage of a premixed product using carefully selected aggregates is that the resulting render, plaster or insulation panel is able to work in a diffusive manner, without capillarity. The plasters and renders also work in conjunction with the insulation material to create a holistic approach to insulating and finishing historic buildings. The materials are all compatible with those found in older buildings. The panels are best fitted to either internal or external faces of exterior walls by being solid bedded onto the surface using the non-hydraulic setting lime mortar. The reasoning is that the panel will then become an integral part of the wall, thus ensuring the original design is maintained, promoting seamless humidity extraction through the structure. This simple approach contrasts with many modern retrofit solutions which include air-gaps, capillarity and impervious layers. MANUFACTURING AND TESTING Following successful completion of the insulation tests, the product was approved for a second phase of SBRI funding. £142,000 was awarded for developing commercial production, for developing variations in the setting time, and for demonstrating the products in a ‘whole house project’. The facilities of a manufacturing company in Derbyshire were used to test production of the material in normal commercial mixing and blending apparatus, and to benchmark a manufacturing process and ability. Due to the high temperature of the chemical reaction (approaching 200°C during slaking), it very quickly became obvious that specialist machinery would need to be developed in order to manufacture the binders if they were to ever reach the market. Nevertheless, enough materials were produced for demonstrating the product. The first successful prototype machine is now in operation, with basic materials being produced in autumn 2016. The house chosen for the ‘whole house’ demonstration was Mill Cottage, in Pontcanna, Cardiff, which was saturated and rotting before the works began. The house is of 18th-century origin with 19th-century rebuilds. As with many One of the pre-production insulation panels tested, 62mm (2½ " ) thick FIGURE 1 CARBONATION Carbon dioxide (CO 2  ) slowly absorbed from the air causing the lime to set FIRING At high temperature, crushed chalk or limestone (calcium carbonate) turns to quicklime and carbon dioxide SLAKING Water is added to the quicklime producing a violent reaction generating heat and steam (if the quicklime is mixed with aggregate before slaking, this is said to be a ‘hot mixed’ mortar) Calcium hydroxide (CaOH) ‘Lime’ – often referred to as air lime, non-hydraulic lime, lime putty, or if dried, as ‘hydrated’ lime Calcium carbonate (CaCo 4 ) eg limestone or chalk Calcium oxide (CaO) ‘Quicklime’