Hygrothermal characterisation of in-situ solid brick walls and the impacts of internal wall insulation
Naomi Grint, University College London
Building fabric upgrades are an essential part of the UK plan to reduce energy demand from existing buildings, and solid wall insulation is among the most effective upgrade strategies. Unfortunately there is uncertainty surrounding moisture risk arising from the modification of solid brick walls. Hygrothermal simulations can be used to predict moisture risk, but these rely on appropriate assumptions regarding the character of the existing wall and the surrounding environment.
Firstly, the need for long term, high resolution, in-situ datasets is addressed by measuring in-wall humidity and temperature in three walls of in-use buildings over at least two years. This includes at least one year of data before and after the installation of internal wall insulation from two walls, as well as corresponding data from an uninsulated control wall over the same time period.
Secondly, the datasets are used to inform sensitivity and uncertainty analyses on a DELPHIN model to indicate the impacts of material property uncertainty on moisture accumulation. Sensitivity analyses indicate that the relative humidity in the wall is influenced by uncertainty in material properties, particularly liquid transport and storage functions.
Thirdly, the datasets are used in a Bayesian optimisation procedure, in order to infer the heat and moisture, transfer and storage properties of the walls under study, which are represented by simplified hygrothermal equations informed by the Sensitivity analyses. This is implemented using UCL’s in-house Bayesian optimisation software, developed for the estimation of thermophysical properties of building elements (by Phil Biddulph and Virginia Gori) and currently being extended for the analysis of whole building heat loss (by Frances Hollick).
This project is a response to the requirement for a greater understanding of the hygrothermal character of brick walls in the UK, with and without internal wall insulation. It is part of a wider effort to reduce the unintended consequences associated with retrofit in the long term. In the short term it provides evidence of the impacts of our lack of knowledge about the in-situ material properties of solid brick walls, and explores methods to estimate dynamic hygrothermal properties in-situ, potentially providing an alternative means of assessing moisture risk in walls.