C O N T E X T 1 4 9 : M A Y 2 0 1 7 27 estimated by a calculation. In these circumstances reducing the amount of insulation applied to a solid wall, and/or using a less high performing, more vapour open material will still result in meaningful reductions in heat loss. This is important from the point of view of the long-term performance of moisture within the wall. Insulation applied to the internal face of a solid wall means that less heat passes through the wall. The temperature through the wall section is lower and thus, during the colder winter months, dewpoint conditions can occur more frequently, leading to a possible increase in interstitial condensation. While a solid wall may be able to tolerate the removal of some heat input, this will need to be in proportion to its ability to evaporate moisture over an annual cycle in order to avoid moisture accumulating within the fabric. In reality this is something which is difficult to estimate and likely to be particularly significant for thick, shaded, exposed and more porous walls. By using smaller quantities of insulation we are more likely to maintain a balance between the desire to cut heat loss through the fabric and continuing to allow sufficient heat to pass through the wall to avoid moisture accumulation. If we abandon a race to the bottom for wall U-values on the grounds that these walls do not lose as much heat as we previously thought, then the use of smaller quantities of less-high-performing, more vapour-open materials, becomes more attractive. These materials are likely to be critical to the wall’s ability to move and lose water. The aforementioned brick wall is an example of a wall insulated internally with a small quantity of vapour-open material. Prior to insulation, this wall measured an in situ U-value of 1.48 W/m2K. Following the application of 40 mm of woodfibre board this U-value was reduced to 0.48W/m2K.This represents a significant improvement, being a 68 per cent reduction in heat loss. With regard to moisture, this wall is performing satisfactorily. On average it is below the critical 80 per cent RH threshold, indicating that moisture which accumulates within the fabric is able to evaporate away sufficiently over an annual cycle. However, this is not the case for the granite wall. Here we see average RH at levels well beyond the 80 per cent risk threshold.There has been a trend of increasing RH measured every year since the wall was insulated, which suggests that moisture is accumulating and that the internal wall insulation is a likely cause.The PIR insulation material is 100mm thick, vapour closed and sheathed in impermeable metallised foil. The in situ U-value prior to refurbishment was 1.20 W/m2K. Post-refurbishment it is now very low, 0.16W/m2K. Conditions within the wall have moved closer to dewpoint, increasing the likelihood of interstitial condensation. In addition, the insulation forms a barrier to the movement of water within the wall, preventing access to an evaporative surface. Moisture is accumulating as this treatment has increased opportunities for moisture formation within the wall fabric and reduced opportunities for evaporation.The cob wall is externally insulated with a relatively thick 50mm render.This again is a wall that demonstrates the importance of using materials that can allow excess moisture within a structure to evaporate. It is also a caution regarding the use of large quantities of water as part of refurbishment processes, and an as yet unfinished lesson in just how long drying processes can take. For many years the SPAB has advocated against the use of less permeable or vapour-closed materials on traditional or historic buildings.We now have measured evidence that these materials might prove damaging in some circumstances when used in the retrofitting of older solid wall buildings.With regards to saving energy and safe moisture profiles, we also have an example of an approach that appears to work.While the circumstances of each one of these walls may be different, it would be unwise to dismiss these case studies as isolated or extreme examples. Given the complexities of moisture performance in relation to both materials and building context, a precautionary approach should be adopted. This would discourage the use of excessive quantities of wall insulation and vapour-closed materials for solid walls. In relation to buildings of greatest significance, the measurement of U-values should be promoted in order to improve the careful specification of insulation quantities to avoid over-insulation and its associated moisture risks. Calculated U-values for solid walls should be treated with some scepticism. Where measurement would not be appropriate, tables of existing measured walls could be consulted (see references below). Proposals for changes to solid walls should be carefully scrutinised, with particular attention paid to moisture performance and this question uppermost: when the water gets in, how does it get out again? References and further reading SPAB Research Report 1:The U-value report 2010. Revised 2012 SPAB Research Report 2: SPAB Building Performance Survey 2011–16, interim findings Both documents are available as downloads from the SPAB website: www.spab.org.uk/advice/ energy-efficiency/. BS EN ISO 9869:2014 Thermal insulation: Building elements – In-situ measurement of thermal resistance and thermal transmittance BS EN ISO 6946:2007 Building components and building elements: Thermal resistance and thermal transmittance – Calculation method. Caroline Rye and Cameron Scott are co-founders of ArchiMetrics, a building performance research company. The brick house at Shrewsbury.The relatively thin, south-facing brick wall, internally insulated with a small quantity of woodfibre insulation, performed satisfactorily with regard to moisture risk.While the wall became quite wet over the winter, it seemed able to evaporate the moisture during the spring and summer.
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