An investigation into the insitu "U" values of wall construction

Abstract

The results of an insitu investigation into the thermal performance of a range of wall constructions are presented. The investigations were mainly undertaken to establish the uncertainty introduced by the measurement, the variability in the performance of the wall due to factors such as workmanship and the relevance of design values in practice. This study provides a useful addition to the otherwise sparse literature of actual measured heat flow and temperature data available from large scale field surveys in UK. A review of methods and equipment used to recover the thermal transmittance value of wall constructions is presented. The Heat Flow Sensor measurement method was found to be most suitable for the needs of the study and was evaluated both theoretically and experimentally. An experimental design approach was devised which enabled the separation of the variability involved in the measurement process and the variability involved in the performance of the wall to be obtained. The four generic wall types sampled included a representative range of existing and new build constructions. The indications are that the wall constructions investigated broadly perform as expected. However, for certain wall types there were significant differences between measured and standard design calculation values. This was because the appropriate theoretical model was not applied in order to establish the transmittance of the wall construction at the design stage. It would appear that the overall error in the measurement process, which is a combination of both the systematic and the random error, was typically of the order of +/-11%, whereas the variability in the wall performance was seen to vary as a function of the wall type with the resulting values ranging between 4% and 39.5%. The differences in the observed performance of the wall may be potentially attributed to 4 major causes, namely: (a) dimensional tolerances and material properties, (b) changes in material properties, (c) the wall as part of the construction and (d) workmanship. The average wall performance in some circumstances can be estimated satisfactorily by using a one dimensional model where a relatively homogeneous wall construction is assumed. While the area weighted one dimensional model gives a reasonable estimate of the average wall performance by taking into account the cold bridging of the mortar joints, a more complete understanding of the wall performance can only be achieved by the use of a three dimensional model.