Prof Richard Fitton R.Fitton@salford.ac.uk
Professor of Building Performance
Benjamin Roberts
Mohamed Dgali
Samantha Organ
Mr Grant Henshaw G.P.Henshaw@salford.ac.uk
Energy House Labs Research Assistant
Prof William Swan w.c.swan@salford.ac.uk
Director of Energy House and Director of Sustainability
Abstract
A comparison of SAP assumptions vs measured U-value and airtightness in pre-1919 dwellings. Abstract This paper examines the measured airtightness and wall U-values of nine homes that represent a cross section of heritage properties owned by the National Trust. The performance gap has been well studied in the UK, but less so in the heritage sector, especially studies that consider both U-values and airtightness. This work, albeit a limited case study approach, highlights a discrepancy between the values found in succesive versions of SAP and RdSAP. With upcoming versions of RdSAP allowing assessors to make changes where these values have been measured, then this issue becomes more crucial. Findings point to discrepancies of up to 47% for U-values, and 49% for airtightness. This leads to energy demand and running cost variances of up to 24%. Highlights • It was found that U-values predicted in RdSAP/SAP models can make the dwelling appear to perform worse than when measured. Upcoming changes to SAP may help to resolve this, but have yet been enabled. • Airtightness in the sampled dwellings have a large variance. However, in the majority of cases the model assumptions leads to a worse prediction of performance than is measured in-situ. • For these reasons the authors suggest that that where older dwellings such as those constructed pre-1919, then airtightness measurements should be carried out to inform the models. U-value measurement can be costly and time consuming, however, if carried out to a recognised standard then it can have a signifcant impact on the result of an EPC. • With the possible introduction of Minimum Energy Efficiency Standards (MEES) for homes at a higher band of C, then the measurement of U-values and airtightness could have greater signficance.
