Traditional stone walls in Scotland - validation of RdSAP U-value calculation methodology: research

Executive Summary

BRE has undertaken research to help Scottish Government assess the validity of the proposed RdSAP 10 U-value calculation methodology for traditional stone walls in Scotland. The research involved in-situ measurement of a sample of stone walls in properties across Scotland to determine their U-value. This was then compared with the calculated approach.

In-situ U-value measurements were carried out in accordance with BS ISO 9869-1:2014 ‘heat flow meter method’. Results for 14 stone walled properties are reported, broadly centred around Glasgow, Edinburgh, Perth and Aberdeen. seven of these were sandstone walls and the remaining seven were granite or whinstone walls. Measured results give U-values that take full account of the ‘as built, in service’ (ABIS) conditions of the wall, i.e. any influences related to workmanship or the service conditions (weather, etc) to which it has been subjected over time. The results have enabled comparison between these actual (measured) U-values and corresponding U-values calculated using the RdSAP 10 equations based on the measured wall thickness at each property.

In addition, the implications of any variation in wall U-value on the properties’ heating demand and Energy Performance Certificate (EPC) were also explored.

Most of the measured U-value results from this study are lower than the respective RdSAP 10 calculated values. The RdSAP 10 calculations themselves generally result in lower stone wall U-values than from the RdSAP 2012 calculations. Notwithstanding the fact that this project only consisted of a small sample, the results of this study suggests that the updated RdSAP 10 equations are somewhat more appropriate than those in RdSAP 2012.

When identifying properties to test, it was apparent that the majority of those with stone walls were internally lined (typically with lath and plaster or plasterboard on battens). Two of the walls ultimately tested were unlined, while the remainder were internally lined. The stone walls without an internal lining generally have higher measured U-value results than the lined walls. This is to be expected, since the lining introduces an air layer behind it that offers additional resistance to heat flow through the wall.

In general, the subsequent changes to key EPC outputs due to the different U-value calculation approaches were modest. However, a few exceptions showed more notable variation in the space heating demand in excess of 10%, and EPC score changes of -4 to +6. The variability does not correspond directly with the changes in wall U-values since there are differing ratios of walls and glazing to heated floor areas across the test properties. However, those with the greatest difference between the measured and calculated U-values also show the largest variations in space heating demand.

The EPC energy assessment outputs based on the measured U-values are closer to the outputs based on the RdSAP 10 U-values than to those presented by the RdSAP 2012 default approach. This again suggests, for the sample properties in this study, that the updated RdSAP 10 stone wall equations are generally more appropriate than those in RdSAP 2012.