Heritage Retrofit

BCD SPECIAL REPORT ON HERITAGE RETROFIT FIRST ANNUAL EDITION 23 INTERNALLY INSULATED SOLID WALLS The SPAB building performance survey CAROLINE RYE and CAMERON SCOTT T HE SOCIETY for the Protection of Ancient Buildings Building Performance Survey (SPAB BPS) was first established in 2011 to address the dearth of information on energy efficiency and traditional buildings. In particular, there was an absence of measured evidence showing how traditional buildings performed before alteration, and a lack of understanding as to what constituted effective and risk-free energy saving interventions. Of specific concern was the potential for damage to fabric and occupants’ wellbeing over the long-term as a result of the application of insulation and reductions in ventilation/ air infiltration in older buildings. The BPS measured various aspects of performance in solid-wall, traditionally constructed properties before and after energy efficiency retrofitting. The survey looked at fabric heat loss, air leakage, indoor air quality, wall moisture behaviour, room comfort and fabric risk conditions in seven houses. A central part of the study looked at the impact of insulation on solid walls. Measurements of four of the buildings were made again after refurbishment, and the analyses of three are ongoing, with findings published annually online at www.spab.org.uk/advice/energy- efficiency. One wall in each of the three buildings chosen – two internally insulated and one an externally insulated cob wall – were subject to extended interstitial hygrothermal monitoring. In particular, the internal insulation of a wall is seen as a risk because fabric on the external side of the wall, outside the insulating layer, no longer benefits from the heat inside the building and in the winter months becomes cooler. The effect of this is to lower the dew point, meaning the air within the wall may more frequently reach saturation – 100% relative humidity (RH) – leading to condensation. High levels of fabric moisture could give rise to uncomfortable living conditions and increased heat loss. They could also have serious consequences in the form of mould growth and rot, which can be harmful both to human health and to the structural integrity of the building. Over the past four years, as part of the BPS, moisture profiles (in the form of vapour, measured as RH) and temperature profiles have been monitored continuously at four points through and either side of insulated solid walls. (This element of the BPS work was extended in 2014 due to a grant provided by English Heritage.) This method of moisture monitoring, which relies on high quality instrumentation and careful installation, has been developed specifically for this purpose. The measurement of water vapour in North-west facing granite wall in Drewsteignton, Devon: one of two internally insulated solid walls featured in the Building Performance Survey, this 600mm granite wall had been internally insulated with 100mm of polyisocyanurate board with an air gap and a plasterboard and gypsum skim finish.