## 1 Introduction  
In July 2020, this journal published the article \"A User-Friendly Tool to Characterize the Moisture Transfer in Porous Building Materials: FLoW1D\" [1]. In this article, the authors present an Excel-based tool for the numerical simulation of moisture transfer in porous building materials, employing finite differences for the spatial discretisation and forward Euler for the temporal discretisation. The hygric properties of the material involved are parameterized via the unimodal van Genuchten and Mualem equations [2], which require five parameters, including porosity. Porosity is independently determined, and the four remaining parameters are obtained via inverse identification exclusively based on a capillary absorption test. In their 'featured application', the authors declare that \"FLoW1D is a numerical tool that is designed as support for the analysis of the conventional experimental tests for the hygric characterization of porous building materials\", hence alleging its general applicability for building materials. Their 'conclusions' say that \"FLoW1D allows both the simulation of two WAC tests and the estimation of hygric parameters with robustness and ease\", therefore averring the robust nature of their approach in general and their tool in particular. These feats--the forward-Euler-based moisture transfer simulation of building materials wherein the hygric properties were universally described with unimodal van Genuchten-Mualem equations, the parameters of which could be inversely identified exclusively based on a capillary absorption test--are impressive successes for research on moisture transfer simulation, hygric property description, and hygric property characterisation for porous building materials. If valid, these three feats would indeed radically progress the current state-of-the-art on moisture transfer simulation, hygric property description, and hygric property characterisation for porous building materials. Regarding moisture transfer simulation, backward Euler with adaptive time stepping [3] is considered the least complex approach to overcome the usual inefficiencies of forward Euler for the small time steps and/or element sizes [4] usually required in the simulation of the highly nonlinear moisture transfer equations. In relation to hygric property description, the unimodal van Genuchten equations are often assumed to be insufficient to accurately capture hygroscopic and capillary moisture storage [5], while the Mualem equations are usually seen as noncompliant with the complex variation of the moisture permeability [6]. Concerning hygric property characterisation, an extensive and time-consuming experimental campaign, comprising a myriad of measurements, is often deemed necessary [6; 7; 8]. The three feats reported in [1] would therefore form major steps forward in these three areas if valid. This critique does, however, formulate severe doubts on the reliability of [1]'s findings. Below, it is revealed that the applicability of the FLoW1D tool and the suitability of the unimodal van Genuchten-Mualem equations are limited. More crucially, however, this critique establishes that such an inverse identification of the hygric properties based on capillary absorption only cannot be unique and therefore cannot be considered robust or reliable. Below, a concise digest of the disputed article is first given. Subsequently, the characterisation of the Cuenca limestone's storage and transport properties is critically examined by comparing it with other data on this material. The next section then discusses the non-uniqueness of parameterisations inversely identified from capillary absorption only and demonstrates that this non-uniqueness leads to non-unique hygothermal performance evaluations. Next, the shortcomings of the unimodal van Genuchten-Mualem equations for the moisture storage and transport properties of porous building materials are substantiated by applying them to several sandstones.