Hygrothermal modeling and evaluation of freeze-thaw damage risk of masonry walls retrofitted with internal insulation

Abstract

For historical buildings with a worth-preserving appearance, internal wall insulation can be the only possible solution to improve the building energy efficiency. However, the application of an internal insulation layer changes significantly the hygrothermal performance of the building envelope. For masonry walls, such intervention may lead to freeze-thaw damage of the brickwork. In this study, a hygrothermal model is developed. The model takes into account moisture and heat transport in porous medium and tracks the occurrence of freezing and thawing in function of pore size distribution and as well as the ice content. Freezing and melting of water in porous medium is implemented based on the theory of freezing point depression, as freezing temperature of water in porous medium depends on pore size, i.e. water in the smaller pores freezes at temperatures lower than 0 °C. The numerical model results are compared with a porous medium freezing experiment and good agreement is found. Traditional hygrothermal assessment uses the number of zero crossings on a Celsius scale as the number of freeze-thaw cycles. We propose a method that uses the number of actual ice growth and melt cycles as an indicator more accurately accounting for the freeze-thaw process. In addition, we develop an index, called FTDR Index, to assess freeze-thaw damage risk. We perform simulations of uninsulated and internally retrofitted masonry walls using two Swiss climatic conditions. The study clearly shows increase of freeze-thaw cycles and ice content after internal retrofitting in both climates. Thus, FTDR Index increases after internal retrofitting.