Interface Influence on Moisture Transport in Building Components

A critique on characterising interface resistances with maximum moisture flows

“…In February 2018, this journal published the article "Interface influence on moisture transport in buildings" [1], wherein an enquiry into the impact of material interfaces on moisture absorption in building materials is documented. This article continues previous research of this group on this topic [2][3][4][5][6], which was originally initiated with the seminal paper of de Freitas et al in 1996. In all of these studies, the interface impacts are characterised with maximum moisture flows over the interface, contrary to the much more widely employed characterisation with interface resistances though [7][8][9][10][11][12][13][14]. The investigation in [1] targets three types of material interfaces: 'air space' (two brick samples separated by an air space), 'perfect contact' (two brick samples in direct contact) and 'hydraulic contact' (bonded brick-mortar samples).…”

“…This implies that [1] severely overestimates the effect of the material interfaces, since its post-interface flows highly underrate the actual moisture absorption after the interface. It should also be noted that these new post-interface flows have been calculated quite long after the moisture front passes the interface, while [6] asserts that flows are calculated right after that passing. When limiting the regression for the 5 cm brick B and cement mortar case to the initial three bold data points, see Figure 2, then the post-interface flow would become 30.9•10 -5 kg/m²s, which is much higher still than new value in Table 1.…”

“…Other publications on the theme of material interfaces [7][8][9][10][11][12][13][14] have consistently opted to describe their impact with an interface resistance, the flow through which hence is controlled by the difference in capillary pressure over that interface. The critiqued publication [1], as well as its related articles [2][3][4][5][6], chooses for another approach, by characterising the interface impact via the more direct post-interface flow. This is a peculiar decision, since this strongly restricts the generalisa- Initially, one can best understand this by thinking about the thermal analogon.…”

“…and 'air layer' configurations only, where often large interface resistances can be expected. The approach was though abstractly expanded to 'hydraulic contact' configurations in [4,5] and really applied for 'hydraulic contact' configurations in [1,6], without verifying however whether the premise of location-independent and time-constant post-interface flows still applies for their typically smaller interface resistances. The flow values in Table 1 actually disclose that the location-independent assumption has been abandoned, as distinct values are reported for interfaces at different heights.…”

“…Introductorily, it should be noted that presenting moisture flows as hydric resistances is strongly counterintuitive: low moisture flows correspond to high hydric resistances and vice versa. This label, only recently formally introduced [1,6], should therefore be reconsidered. This being said, this critique aims at two much more critical defects in [1] though.…”