Heritage Retrofit

BCD SPECIAL REPORT ON HERITAGE RETROFIT FIRST ANNUAL EDITION 25 walls, at sensors 2 and 3, is indicative of its underlying performance because this part of the wall is less influenced by both the wetting and drying influences of external and internal environments. In Figure 3 it can be seen that since 2012 measurements of RH at sensors 2 and 3 within the wall at Drewsteignton rise immediately following the application of insulation and continue to rise over three years of measurements. There are periods of rising RH seen in the traces from sensors in the Shrewsbury wall but these are also seen to fall at certain times of the year indicating periods when the air in the wall is able to dry via evaporation. Importantly, at Shrewsbury, there are also times when RH quantities fall below those initially measured immediately after the wall was insulated, something not seen at Drewsteignton. In terms of risk, the quantities of RH measured at Drewsteignton exceed 80% from March 2012 onwards and would suggest that the wall, or perhaps more accurately certain materials such as timbers which are embedded in the wall, may be at risk of mould growth. The majority of the measurements for the wall at Shrewsbury fall below 80%. The reasons for the differences in moisture behaviour between the two walls originate in their very different constructions. It is possible, however, to extrapolate from this certain qualities that determine moisture behaviour and apply this learning to solid walls more generally. The wall at Shrewsbury is south- facing and, compared to that at Drewsteignton, quite thin. The pointing is in poor condition and the brick is quite porous and permeable. It has also been insulated with a relatively small quantity of a vapour-open, capillary-active and hygroscopic material with no formal VCL. Of the walls under study, it is the driest in terms of both relative and absolute humidity (%RH and AH g/m 3 ) and it has the widest saturation margins. Vapour responses in this wall are very dynamic and at times quite extreme. This is due to the nature and orientation of the construction. The external side of the wall quickly becomes wet during periods of driving rain and this moisture can easily penetrate towards the centre of the wall. However, the wall also dries out rapidly due to heat from direct (and diffuse) solar radiation and plentiful air exchange through the substrate. It is noticeable that, despite this volatility, overall the wall operates below the 80% RH threshold for mould growth. It is also possible that the quantity of insulation installed (40mm), which reduced the measured in situ U-value from 1.48 W/m 2 K to 0.48 W/m 2 K, ensures that while the passage of heat through the wall is reduced, sufficient heat still travels from interior to exterior during colder winter periods to provide a safe margin between the measured air temperature and the dew point temperature. It is important to note that Figure 2 Hygrothermal section showing temperature and dew point gradients for the wall at Drewsteignton Figure 3 RH trends for the walls at Drewsteignton (solid) and Shrewsbury (dotted), 2011-2015