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

1. Introduction and background

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

The U-value or ‘thermal transmittance’ of a building element represents its rate of transfer of heat between the internal and external environment. Sufficient information is not usually available for existing wall constructions to accurately calculate thermal performance U-values and so their performance is typically inferred based on information gathered during an unintrusive building survey. Even when there is good confidence in the wall make-up, properties of ‘in-service’ materials can differ over time from original ‘design’ values (and indeed seasonally in some cases due to cycles of wall wetting and drying), in particular the moisture content of materials, which can affect thermal performance, leading to uncertainties/variability in wall U-values.

Previous investigations have been made into the in-situ U-values of existing solid walls to better understand (and therefore assign) their thermal performance. This includes a national investigation, carried out by BRE on behalf of UK Government^[1], into wall U-values by in-situ measurement (though this involved a relatively small sample of stone walls, with the majority of walls being brick). There have also been various investigations for Historic Environment Scotland^[2],[3], particularly focussing on the characteristics of Scottish traditionally constructed stone walls, as well as individual project/building-based analysis. However, due to the range of factors that can influence the thermal performance of solid walls (e.g. construction type, materials used, moisture content, etc), it is difficult to make accurate general assumptions regarding thermal performance.

The Standard Assessment Procedure (SAP) is used to assess new dwellings, and an adapted version requiring fewer data inputs is used for existing dwellings (Reduced Data: RdSAP), which uses assumptions based on age and other building parameters for some data inputs where it is not practical to unobtrusively obtain the same level of information detail that is expected for a newly built home.

Some stakeholders have expressed concerns that the way U-values are currently inferred in RdSAP 2012 does not well-represent traditional stone walls in Scotland.^[4] The Alembic Research Ltd Review of EPCs in Scotland recommended a systematic research programme to measure in-situ U-values for different types of existing stone walls in Scotland to assess the validity of the U-values calculated in RdSAP, and potentially propose revisions. The proposed methodology for the upcoming update to RdSAP (i.e. RdSAP 10) introduces a new algorithm for the calculation of U-values for stone walls compared to that currently used in RdSAP 2012.

The approach to assigning U-values for walls taken in RdSAP 2012 and proposed in RdSAP 10 is set out in Appendix A. The new RdSAP 10 approach generally results in a lower wall U-value than RdSAP 2012 for most stone wall types and thicknesses.

1.1 In-situ U-value measurement approach

BS ISO 9869-1:2014^[5] defines requirements for measuring in-situ U-values using heat flow meters. Such measurements give actual 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 from this assessment enable comparison between expected U-values (on the basis of calculation) and actual (measured) U-values.

The heat flux through a building element, such as a wall, is equal to the flow of heat (in watts) divided by its area (in m²). Hence, it is expressed in W/m². Under steady-state conditions, a U-value can be calculated by dividing the heat flux through a wall by the difference in temperature across the same wall. In practice, the situation is complicated by the need to consider fluctuating temperatures both internally and externally, and the effect of thermal storage within the wall. These issues are taken into account by monitoring over a period of time that allows the overall result to converge to a reasonably steady value. This is typically at least two weeks within the heating season when there is greater likelihood of a suitable temperature gradient between the inside and outside of the elements being measured).