## 7 Discussion  
The article \"A User-Friendly Tool to Characterize the Moisture Transfer in Porous Building Materials: FLoW1D\" [1] proposes three major innovations: the forward-Euler-based moisture transfer simulation of building materials wherein the hygric properties are universally described via unimodal van Genuchten-Mualem equations, of which the parameters can be inversely identified based on a capillary absorption test only. This paper presents severe doubts regarding the applicability of FLoW1D, the adequacy of the van Genuchten-Mualem equations, and (most fundamentally) the uniqueness of the inverse characterisation. The authors of [1] are invited to soothe these doubts in their reply. In concrete terms, the following 10 questions should be addressed:  
1. Section 4 shows that many parameter value sets allow for the reproduction of the capillary absorption, revealing that the inverse characterisation of multiple van Genuchten-Mualem parameters based on the capillary absorption coefficient only is non-unique and implying that the ensuing hygric property characterisation is also non-unique and therefore not reliable or robust. Could [1]'s authors counter by verifying that their method satisfies the requirements for structural and practical identifiability [14; 15]? Additionally, could [1]'s authors establish that this remains valid even if researchers choose a more reliable hygric property description involving more parameters? 2. Section 3 has illustrates that the achieved hygric properties for the OPS limestone disagree with other data on various fronts. Could [1]'s authors verify their smooth  
Figure 6: **(top left)** FLoW1D simulations of capillary absorption in the original OPS limestone and two variants, plus original measurements; **(top right, bottom left, bottom right)** FLoW1D-simulated moisture content profiles during capillary absorption, shown every 10 s for the initial minute and then every 20 s for the four remaining minutes. moisture retention curve with experimental data, possibly resulting from pressure plate tests [8]? Could [1]'s authors support their smooth liquid permeability curve with experimental data, possibly resulting from moisture diffusivity experiments based on visualised moisture content profiles [8,12]? Could [1]'s authors validate their smooth moisture content profiles during capillary absorption, possibly with visualised moisture content profiles again? Could [1]'s authors confirm that the moisture content during the capillary absorption indeed goes up to the saturated moisture content, possibly with visualised moisture content profiles again or otherwise by prolonging the capillary absorption experiment well into the second absorption phase? Could [1]'s authors establish that the liquid permeabilities in the hygroscopic range are well above the vapour permeabilities, possibly via dry and wet cup tests? Could [1]'s authors verify the tortuosity correction being equal to 1, possibly via dry cup tests again? 3. Section 6 demonstrates that FLoW1D suffers from inefficiency due to its instable forward-Euler-based solution. Could [1]'s authors demonstrate that FLoW1D can simulate capillary absorption in the OPS limestone variant 2, report which element size and time step would be needed for that, and report how much computation time is needed for that simulation? 4. Section 5 reveals that the unimodal van Genuchten-Mualem equations may be inadequate for the accurate hygric property description of building materials. Could [1]'s authors confirm that these equations do accurately describe the hygric properties of the OPS limestone, possibly with the outcomes of the experiments that are suggested in relation to point 2 above? In short, the authors of [1] were the first to propose that the hygric properties of building materials may be fully characterized exclusively based on the moisture mass increase measured during capillary absorption.