C O N T E X T 1 4 9 : M A Y 2 0 1 7 25 CAROLINE RYE and CAMERON SCOTT Understanding the performance of solid walls If we are to make effective changes that will not cause harm to buildings or their occupants, we need to ask: when the water gets in, how does it get out again? Our ability to make changes to solid walls has probably never been greater and our motivation, driven by the desire to save energy, has never been stronger.We can now effect radical change in structures that have, in some cases, stood for hundreds of years. Many of the means by which we may make these alterations are unprecedented. An understanding of how solid walls perform is essential if we are to make genuinely effective changes that will not cause harm to buildings or their occupants. Towards the end of the noughties, in response to growing public debate concerning climate change and carbon emissions, the Society for the Protection of Ancient Buildings (SPAB) saw an increase in enquiries seeking advice on improving the energy efficiency of older buildings. In the absence of official guidance specifically for these buildings, the society decided to undertake its own programme of research to underpin its advice and information services. Central to this research was the need to understand how both heat and moisture performed in solid walls. The SPAB U-value research measured heat flows through 77 solid walls of traditional construction in historic buildings. This work was carried out between 2009 and 2012 byArchiMetrics,with guidance fromPaul Baker of Glasgow Caledonian University. Following the method set out in BS EN ISO 9869, it used a heat flux monitor in combination with measurements of internal and external temperature, taken over time, to provide an in situ U-value (an estimate of heat loss) for each of the walls.The types of walls measured were deliberately diverse, including different examples of stone, cob, brick and timber-frame constructions. Some measurements of more modern materials were taken where these materials had been retrofitted. Some of the walls had historic or more modern dry-linings.As well as surveying the range of heat losses from these different wall types, the study undertook a comparison of these in situ U-values with those produced via the standard U-value calculating method BS EN ISO 6946 – with startling results.The research found that in 77 per cent of cases the calculated U-value overestimated the heat loss of the solid walls in comparison with the heat loss that had been measured in situ. While undertaking the U-value research it became clear that issues of moisture within solid walls were closely linked to those of heat loss and were of equal, if not greater, importance. In order to answer questions about the behaviour of moisture in solid walls, ArchiMetrics developed a method called interstitial hygrothermal gradient monitoring (IHGM), by which temperature and relative humidity could be measured through wall sections. Monitoring equipment was installed in solid walls that were to undergo insulation. Later these measurements were joined by others to determine the moisture content of materials through the wall sections. This continuing work is part of the SPAB Building Performance Survey, a project which has measured not only heat loss and moisture in walls, but also the effect of other changes within buildings before and after refurbishment. The Building Performance Survey, started in 2011, measures moisture in three walls made of brick, granite BuildDesk and in-situ U-value comparison, 2012, from the SPAB Building U-value report. BuildDesk is a software package for calculating U-values, widely used in the UK building industry.
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