26 C O N T E X T 1 4 9 : M A Y 2 0 1 7 and cob.The value of long-termmeasurements of moisture behaviour can not be stressed enough. Changes to the moisture profiles of walls can occur slowly. As the study progresses we have been able to identify both short- and longer-term trends that reach beyond the changes attributable to individual seasons or annual weather patterns; underlying trends which are impinged upon by the material characteristics of the walls themselves, including those of refurbishment. We find that a relatively thin, south-facing brick wall internally insulated with a small (40mm) quantity of woodfibre insulation is performing satisfactorily with regard to moisture risk.That is, while the wall becomes quite wet over the winter, it seems able to evaporate this moisture during the spring and summer so that over repeated annual cycles relative humidity (RH) within the wall is, on average, below an 80 per cent threshold. (Eighty per cent is the commonly used measure of RH risk above which mould growth and fungal decay can flourish, particularly on organic substrates.) In contrast, on a thicker north-west-facing granite wall internally insulated with 100mm of polyisocyanurate (PIR) insulation we find a rising trend of RH. Shortly after insulation RH in this wall began to increase. It is on average greater than 90 per cent, and continues to increase year on year, indicating that the wall is accumulating moisture. The cob wall is different again.This wall is externally insulated with a thick (50mm) layer of insulating lime render. As part of the process of applying this new render the wall was wetted down, resulting in very high measurements of RH through the section. Over the past five years we have watched as measurements of RH have gradually reduced in the centre of the wall as the structure rids itself of excess moisture. However, RH remains at 100 per cent in proximity to the new render, and there is a question as to whether this material is retarding the drying process. Valuable and interlinked lessons may be learned for retrofit from these two research studies.Applying insulation to the internal or external side of a solid wall will alter not only heat flow through it but also how moisture behaves within it.The extent of the change will depend on the material characteristics of the original wall, its relative permeability, porosity and moisture-carrying capacity. It will also depend on the location of the wall and its aspect, what kind of weather it is subject to, quantities of rain and particularly rain driven by prevailing winds and the amount of heat it receives from direct solar radiation. Crucially the quantity and type of insulation added to the wall will also have an impact. Thinking around the insulation of solid walls has become more informed over the past few years, partly as a result of research by the SPAB and others. However, too often retrofit decisions are still based solely on reducing heat losses without consideration of the effects this will have on moisture. The official (BS 6946) U-value calculation procedure suggests that solid walls lose far more heat than that measured from real walls. In retrofit situations this encourages the use of large quantities of insulation material. In addition, as large quantities seem to be required, the thickness of the insulation becomes problematic, steering people towards higher-performing, less thermally conductive materials, most of which are vapour closed. Often a measured in situ U-value shows a solid wall to have a heat loss of around two-thirds or half that Average relative humidity (RH) trend analysis of the three houses at Drewsteignton, Riddlecombe and Shrewsbury, 2012–2016, from the SPAB Building Performance Survey 2016 Report The granite house at Drewsteignton and (far right) the cob house at Riddlecombe.The SPAB research measured heat flows through the solid walls of these traditionally constructed buildings.
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