The Behaviour of Vapour Transfer on Building Material Surfaces: The Vapour Transfer Resistance

Abstract

While the consideration of the heat transfer resistances on surfaces is natural in practice, the similar effect of the moisture transport is not considered. New investigations showed that in some cases the transition resistance of vapour should not be neglected. Experimental studies have been carried out to determine the rate of vapour transfer at various surfaces. For the transition of vapour at different surfaces specific differences could be detected. A definition by cases concerning the involved surface must be made. This distinction can be met with the aid of the so called vapour transfer resistance, similar to the usual description of heat transfer. Appropriate numerical values are presented and discussed.

Conservative modelling of the moisture and heat transfer in building components under atmospheric excitation

“…At present such CFD-based relations are not available though. The surface transfer coefficients furthermore depend on the pore structure, and on the roughness of the external building material [31,32]. Worch [31] furthermore stated that the moisture transfer coefficients depend on the moisture content at the surface.…”

“…While the latter relation has occasionally been measured in wind tunnels [31,32], the relation between the reference wind speed and the local air velocity is still lacking. Like for wind-driven rain, this could be quantified by numerical CFD simulations.…”

Wind-driven rain as a boundary condition for HAM simulations: Analysis of simplified modelling approaches

“…Moisture and heat transfer at external surfaces are dominated by the forced convection due to wind, and thus depend strongly on the local air velocity. While the latter relation has occasionally been measured in wind tunnels [31,32], the relation between the reference wind speed and the local air velocity is still lacking. Like for wind-driven rain, this could be quantified by numerical CFD simulations.…”

“…The surface transfer coefficients furthermore depend on the pore structure, and on the roughness of the external building material [31,32]. Worch [31] furthermore stated that the moisture transfer coefficients depend on the moisture content at the surface. Presently none of these influences has been integrated in models yet.…”

Bestimmung des Trocknungsverhaltens von Baustoffen unter definierten Randbedingungen

“…4. Accessories of the drying apparatus: a data logger (7), a drawer closed (8) and with its parts -drawer, template for the specimen and hight adjustable inserts (9), example specimen (10), a balance to weight the specimen (11) and the controlling gear for the ventilators (12) …”