| Resumo: | Degradation in walls of Historical Building with rising damp is a complex problem to solve, due to the thickness and heterogeneity of those walls. The traditionally treatment techniques used (such as watertight barriers, injection of hydrofuge products, etc.) show, sometimes, to be ineffective, justifying the need to find a new approach. Experimental studies validate the effectiveness of a new treatment technique applied to the walls of old buildings wall base ventilation system. Building Physics Laboratory of the Faculty of Engineering, University of Porto is developing a model of this technique. The sizing of the treatment system is based on knowledge of the characteristics of the wall, of the geometry of the ventilation system and of the building being dealt with. In this paper it is described the moisture transfer process between the moving air flux, inside the system, and the wall. A mathematical solution of a partial differential equation describing moisture conservation gave the concentration field near the wall surface and the mass transfer flux was integrated to give values of the Sherwood number as a function of Peclet number. Experimental results were used to validate the mathematical solution and the values obtained are practically the same. It is possible to get a relation between the flux of evaporation inside the system and some sizing parameters which is crucial to control the damp front level
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