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Unlike cars, houses do not have regular checks and their emissions tested so actual performance data is often limited to calculations and thermal modelling

Calculated steady-state U-values are based upon information provided by material suppliers and allow for quick and easy comparisons to be made between various materials and building elements. However, as the simple calculation fails to consider: thermal storage; build quality; moisture content; fluctuations in temperature and humidity, exposure to sun, wind, and rain, it may be best to only use calculated U-values as a rough guide.

Due to the lack of post-occupancy assessments and actual in-situ performance measurement, U-values may vary greatly from calculated (Zero Carbon Hub, 2010). The Standard Assessment Procedure (SAP) building survey is currently under review as the ratings for solid wall homes are based upon U-values that have been repeatedly shown through in-situ U-value testing to be overestimating heat loss through solid brick walls which had led to recommendations which suggest revising the current U-value of 2.1 down to 1.7 W/m²K (Department of Energy and Climate Change, 2014a) (Rye and Scott, 2012). In contrast to this insulated cavity walls have been shown to have measured values above the standard values used in the SAP calculations, further undermining the reliability of calculated values and complicating any potential national insulation programme (Department of Energy and Climate Change, 2014).

As a result previously estimated energy performance, heat loss, GHG emissions and reductions along with any potential financial saving will be inaccurate guesses at best (Li et al., 2015). The thermal performance of buildings may be further misled where infiltration occurs either through occupant use or poor construction quality (Guerra-Santin et al., 2013).

The performance gap