Jesper Arfvidsson

Isothermal moisture flow in building materials:: modelling, measurements and calculations based on Kirchhoff’s potential

Abstract The modelling of non-linear, isothermal moisture flow in porous media without hysteresis is considered. Different formulations based on different potentials for the Fickian moisture flow are compared. Kirchhoff’s flow potential, i.e. the integral of any state-dependent moisture flow coefficient, is introduced. The problem with a highly variable flow coefficient vanishes. The theoretical, numerical and computational basis for isothermal moisture transport in building materials using Kirchhoff potentials is described. It is shown that the use of Kirchhoff’s potential has clear advantages compared to models using a moisture flow coefficient. There is a conceptual simplicity. Only the relation between moisture content and Kirchhoff’s potential is used in the internal process, while the relation between Kirchhoff’s potential and any intensive variable is needed at boundaries. Precisely these relations are obtained in measurements, while the flow coefficients require a troublesome derivation. The use of Kirchhoff’s potential simplifies considerably the numerical calculation, since flow coefficients and ensuing interpolation problems are avoided. Complete sets of data for moisture flow and equilibrium are given for five common building materials. In two examples, numerical calculations are compared to experimentally determined moisture distributions.

A Method for Including Moisture Safety in the Building Process

Many buildings, both new and old, suffer from moisture-related problems, with negative consequences on health, costs for rebuilding and lost confidence in the building industry. These problems could have been avoided if moisture issues had been focused on and dealt with throughout the building process. A method for including moisture safety in the building process has therefore been developed. The purpose of the method is to help all those involved to work with moisture safety activities and to document them in a structured way. The method includes a number of routines, templates and checklists for clients to formulate requirements for moisture safety and to follow up and document the measures employed by different participants. There are also tools for architects and design engineers, such as lists of references to literature, check lists and design examples to use for dry building design. For contractors, a number of routines have been developed for moisture control during construction. The method has been applied to a number of building projects. Based on experience from these projects, the method and the tools have been evaluated and revised. This paper presents the validated method and some associated tools that can be used in the building process.

A transient technique for determining diffusion coefficients in hygroscopic materials

Abstract A transient technique is described to find the diffusion coefficients of moisture transfer in building materials as a function of moisture content. Measurements that take many months by steady-state methods can be done in days or even hours. The material is subjected to a staircase function of relative humidity steps, the transient weight changes noted, and a numerical algorithm used which extracts the Kirchhoff potential, one value for each relative humidity step and corresponding moisture content. Diffusion coefficients are then calculated from the Kirchhoff potential. Experimental results show fair agreement between Kirchhoff potentials found by steady-state methods and those found by this transient technique, but good agreement between the corresponding diffusion coefficients. An important limitation to the technique at this state of its development is the need to know the material sorption curve.

Moisture Safety in Cold Attics with Thick Thermal Insulation

AbstractDamage caused by moisture and mold in attics has increased in Sweden during recent decades. The main reasons are thicker thermal insulation between the living space and the attic and lack of a warm chimney. Very thick thermal insulation will result in attics that are more sensitive to convection from the living space. With only a thin layer of thermal insulation, the attic will be warmer as a result of convective heat transfer from the living space. With a warmer attic during the winter, a certain amount of vapor can enter into the attic from the occupied area without causing any damage. With very thick thermal insulation, however, vapor cannot be allowed to enter the attic. Without a chimney, the temperature in the attic will decrease (especially in winter), which will result in higher relative humidity and lower ventilation owing to a lack of buoyancy. A warm chimney will also increase the natural ventilation in the occupied area and hence decrease the amount of vapor entering into the attic by ...