Heritage Retrofit

24 BCD SPECIAL REPORT ON HERITAGE RETROFIT FIRST ANNUAL EDITION Annual Average Sat Margins Sensor 1 Sensor 2 Sensor 3 Sensor 4 SHREWSBURY 2011 6.46°C 6.41°C 5.12°C 3.96°C 2012–2013 6.34°C 5.08°C 4.30°C 3.08°C 2013–2014 6.33°C 5.00°C 4.08°C 3.45°C 2014–2015 6.85°C 5.16°C 4.20°C 4.24°C DREWSTEIGNTON 2011 5.30°C 4.82°C 3.53°C 2.38°C 2012–2013 5.60°C 2.23°C 1.53°C 0.57°C 2013–2014 6.90°C 1.97°C 1.14°C 0.49°C 2014–2015 7.09°C 1.58°C 0.67°C 0.59°C Table 1 Annual average saturation margins for interstitial sensors 2011–2015. Orange shading indicates increased margins, blue indicates decreased margins air is used to provide an indication of the moisture performance of the wall. The use of air as a proxy medium for moisture measurements has a number of advantages. As a quantity it provides an indication of dew point conditions within the wall (100% RH) and %RH is commonly used within fabric risk indices, 80 per cent being the threshold value often quoted for the formation of mould growth (see Further Information: DCLG and Altamirano- Medina). Unlike measurements of moisture made via electrical resistivity, it is unaffected by salt contamination and does not rely on assumptions regarding resistivity and moisture content, which is material-dependent and can therefore be hugely variable. In order to identify the fundamental drivers of hygrothermal performance within the walls, as opposed to just seasonal differences when the walls may become wet as a result of local weather conditions, long-term monitoring of fabric is necessary. By 2015 it was felt that sufficient evidence had been gathered to be able to describe, with some certainty, the reasons for the different performance of the walls in the BPS. Of the two internally insulated solid walls featured in this study, the first was a 345mm brick wall at Shrewsbury which was insulated with 40mm of woodfibre board and finished with 20mm of lime plaster. This wall does not incorporate any formal vapour control layer (VCL). The addition of a VCL is standard practice when adding internal insulation to solid walls to limit the movement of internal room vapour into the wall where cold fabric beyond the insulation might cause vapour to condense, but the practice has been called into question by conservation specialists where traditional solid walls are concerned. The other example chosen for the study was a 600mm granite wall at Drewsteignton in Devon. This wall had been internally insulated with 100mm of polyisocyanurate (PIR) board and, following manufacturers guidelines, an air gap, plasterboard and gypsum skim finish. In this construction the insulation is bound front and back with a metallised foil sheet which, being impermeable, performs the function of a VCL. Findings from the interstitial hygrothermal monitoring are examined across a number of bases. Vapour behaviour is examined as both relative and absolute humidity as well as in the form of dew point gradients which extend through the wall section. Dew point gradients are compared against the actual temperature gradients measured through the wall, the difference between the two being the drop in temperature required to create saturation conditions. This difference, which is described as the ‘saturation margin’ and is measured in °C, provides another indicator of risk for the wall in terms of how close the air is to saturation, for what duration and at which times of year. In 2012, following insulation, the saturation margins measured in both walls narrowed, something that might be expected for internally insulated walls as temperatures reduce on the cold side of the insulation. However, it is long- term trends that are most of interest and here we see a difference between the walls. Saturation margins continue to narrow year on year in the granite wall at Drewsteignton, indicating a wall moving closer to permanent saturation of the air within parts of its structure. The other internally insulated wall, at Shrewsbury, appears more stable with wider margins and little year on year change in these following insulation. Another way to examine moisture behaviour in the walls is to study their RH profiles. In particular, RH behaviour in the central part of the Monitoring equipment installed on the internally insulated granite wall at Drewsteignton Figure 1 Wall sections showing build-ups and locations of sensors for walls at Shrewsbury and Drewsteignton