Solid-wall U-values: heat flux measurements compared with standard assumptions

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  • 5 February 2015 Create Date
  • 25 September 2019 Last Updated

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 Wm22 K21
. 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 Wm22 K21 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 stocklevel
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.

Solid wall U values - heat flux vs standard assumptions