Several studies show that the designed performance value of buildings may be dissimilar to the actual energy performance value. A large part of this performance criterion is due to the thermal resistance of the elements in the building envelope. The most frequently utilised technique for calculating the thermal performance of a real building is the co-heating test. It is an experimental method used to specify the heat loss coefficient (HLC) of an as-built of building. This determination emerges by determining and plotting the daily heat input against the indoor and outdoor temperature difference of the building. Also, it is a quasi-steady state method that can be calculated correctly for the whole dwelling HLC. This article covers the evaluation of the thermal insulation performance of residential buildings in the UK by using the co-heating test method. The aim of this study is to describe improvements in U-values of building envelopes at pre- and post-retrofit cases in the UK dwellings. The assessments are done over a wide range of building types with different structural and constructional features.