Water vapor transport to material surfaces-Simplified analytical expressions for non-linear material properties

Carl-Eric Hagentoft,R Černý,V Kočí,J Kočí

doi:10.1051/matecconf/201928202002

Abstract

The water vapour transfer between the indoor air and material surfaces is of importance for the moisture balance of the room. It can also be important for the moisture content and durability of the material surface layer such as artefacts in churches and historical buildings. For most building materials the penetration depth due to short time fluctuations, such as diurnal ones, is very limited. For these cases the assumption of semi-infinite analysis gives accurate results even for a rather thin material layer. In the paper, the moisture profile and surface moisture uptake are modelled in detail for isothermal cases and strongly non-linear material properties for the sorption isotherm and vapor permeability. An approximative formula is given for a quite accurate estimate of the moisture up take for a demonstration case with a strongly non-linear material.

Highlights

The water vapour transfer between the indoor air and material surfaces is of importance for the moisture balance of the room
If the changes in RH are significant, or frequent enough, permanent deformation or damage may occur. This problem coupled to wooden artefacts in churches is analyzed in (Melin et al 2018)
* Corresponding author: carl-eric.hagentoft@chalmers.se https://doi.org/10.10 51/matecconf /201928202 002 to study semi-infinite domains and still get the same moisture profile in the surface of the material as for materials with a thickness of say 5 times the penetration depth. In this article both the moisture profile in semi-infinite materials with strongly nonlinear properties and the moisture uptake at the surface is investigated for step-changes in the ambient climate during isothermal conditions

Summary

Problem formulation

Hygroscopic organic materials, such as wood, are susceptive to changes in the ambient climate. This moisture interaction between the room and the surfaces can, especially for fast changes, influence the peak values of the moisture content of the indoor air It is shown (Hagentoft, 2001) that the penetration depth of the propagating moisture change in a material after step change at the surface is very small. Https://doi.org/10.10 51/matecconf /201928202 002 to study semi-infinite domains and still get the same moisture profile in the surface of the material as for materials with a thickness of say 5 times the penetration depth In this article both the moisture profile in semi-infinite materials with strongly nonlinear properties and the moisture uptake at the surface is investigated for step-changes in the ambient climate during isothermal conditions. In the study the effect of surface resistances and hysteresis is neglected

The mathematical problem

Numerical solution technique

Example

Conclusions