Solid-wall<i>U</i>-values: heat flux measurements compared with standard assumptions

TLDR

The assumed U‑values of solid walls are a major source of uncertainty in estimating dwellings’ energy performance, with the typical UK value of 2.1 W m⁻² K⁻¹ used for stock‑level estimates and certification. The study aims to inform policy and guidance by incorporating empirical field data on solid‑wall U‑values. Re‑analysis of 58 walls using inverse parameter estimation yields a mean U‑value of 1.3 ± 0.4 W m⁻² K⁻¹, indicating that standard UK values are too high; adopting the revised value reduces estimated annual heating demand by 16% and alters EPC bands, highlighting the influence of component heterogeneity on domestic energy use.

Abstract

The assumed U-values of solid walls represent a significant source of uncertainty when estimating the energy performance of dwellings. The typical U-value for UK solid walls used for stock-level energy demand estimates and energy certification is 2.1 Wm−2 K−1. A re-analysis (based on 40 brick solid walls and 18 stone walls) using a lumped thermal mass and inverse parameter estimation technique gives a mean value of 1.3 ± 0.4 Wm−2 K−1 for both solid wall types. Among the many implications for policy, this suggests that standard UK solid-wall U-values may be inappropriate for energy certification or for evaluating the investment economics of solid-wall insulation. For stock-level energy modelling, changing the assumed U-value for solid walls reduces the estimated mean annual space heating demand by 16%, and causes a proportion of the stock to change Energy Performance Certification (EPC) band. The analysis shows that the diversity of energy use in domestic buildings may be as much influenced by heterogeneity in the physical characteristics of individual building components as it is by variation in occupant behaviour. Policy assessment and guidance material needs to acknowledge and account for this variation in physical building characteristics through regular grounding in empirical field data.