The influence of ground slab permeability on wall moisture in a historic building

Overall assessment:

Most queries raised from the previous manuscript have been addressed, well justified and discussed providing appropriate references. These have been carefully considered and the content of the article has been accordingly modified and improved. The clarity of the manuscript has also been improved.

However, please consider that there are still some important queries to address that could further improve the discussion and outcomes of this case study.

1-Regarding to the following response made by the authors: ‘ Response – The conclusion of the paper is that the capillary forces are not causing moisture flow because the capillaries/pores are not connected. Hence any properties to describe flow through porous materials that is based on a continuum will not be applicable. So, properties such as the open porosity, pore size distribution, tortuosity, capillarity coefficient will not be helpful for understanding the flow. They were also not measured, and it would be difficult to know how to measure them to represent the whole wall and the composite nature of the various layers. For these reasons, we chose in-situ monitoring to gain information about the wall as a system, rather than measuring the individual elements/materials forming the wall, which we could not do.’

Building materials such as the described in this case study (limestone, sandstone, lime, cement, should have certain degree of open porosity, capillary pores and connected porosity. It would be very rare having not connectivity at all since these are sedimentary type materials which pores and connectivity may vary but is not usually low, as it could be for volcanic or other igneous rocks with vitreous matrix with closed porosity and low or not connected pores.

Therefore, instead of saying that the capillaries/pores are not connected...which is a strong statement considering that porosity has not been measured, it is suggested to correct it. This has been better explained by the authors in the conclusions’ section when talking about why the in-situ measurements of wall moisture contradicted predictions based on a theoretical model of capillary rise for an idealised wall:

i.e. ‘ This is because (better to say ‘might be’) the heterogeneous fabric of the rubble-fill wall ( better to say ‘seems to’ contain) contained a discontinuous pore network and therefore restricted capillary flow and capillary rise within the wall.

It is also recommended to suggest this possibility as one of the causes that contradicts the prediction by the model rather than state that this is the cause for this.

2- Regarding the comment on porosity measurements in the same ‘Response’ (paragraph above) :

There could be some possibilities to infer the porosity and related properties such as water absorption by capillarity or ‘moisture flow’. Although not specifically to measure porosity or capillary pores on site, indirect correlations of the wall (as a system, not for the individual layers) could be made from onsite measurements of non-destructive and portable techniques, such as ultrasonic velocity waves ( Vp ), which speed is related to porosity and density of materials. Vp waves change when materials are wet or dry, so, measurements taken in different weather conditions/ seasons and several measurements per point could be taken to map the obtained values measured on selected areas of the wall. In this way heterogeneities could be plotted for having a better understanding of the physical properties and hygric behaviours of the building materials.

Other non-destructive onsite measurements to support this could be air permeability, sponge, or Karsten pipe tests.

3- Regarding former Figure 2:

Despite the picture of former Figure 2 was blurry and there is no replacement photo, it would be better to show it instead of being removed.

A better description of building materials and vapour-proof barrier have been provided by the authors showing a new picture of the wall with the stratigraphy of the building materials, the core and the different vertical layers (render and coating).

Despite being blurry, former figure 2 was providing a lot of information since this was showing the internal face of the wall, dowel and moisture probe access holes, the state of conservation of the wall (which showed signs of rising damp…) and the polyethylene sheet that was installed on the floor.

4- Regarding the following Response made by the authors : ‘Response-The local climate was measured with a weather station. The results are typical of the seasonal temperate climate in Wales and the UK. It would be possible to compare these values to long-term averages (LTAs) but the results would not contribute to the results or conclusions of the case study. Fill has been changed to made ground, to align with the termed used by the British Geological Survey. A citation has been added. In terms of the geology, we could include general information about open porosity and capillarity rates, but from my experience these will not be enough to reliably model the flow through these materials. Also, the bedrock geology was not encountered during excavation, so the moisture supply and flow will be from the near-surface topsoil layers (and made ground). Not only do we not have that information, it would not give us any further insight into the results than was provided by the measurements (e.g. see the approach used in the Smethurst citation).

Please, consider to comment or discuss the influence of the presence of organic topsoil layers and 'made ground'.. mixed with clays and the thickness of these layers in the ground underneath the building compared to other soil types. These should condition the results of this case study considering that clays have low permeability, low infiltration rates, displaying poor drainage and impeding water flow compared to sands or other soil types in which the outcomes would have been different.

This should be mentioned or discuss somehow in the results and discussion section since it is also related to soil moisture deficit (SMD) and soil moisture content and the water transfer to building materials.

5- Regarding the following Response made by the authors : ‘ Response – We could include information on the long-term average (LTA) values for this site but that is not the focus of the paper. Nor is the paper about climate change or future implications. The weather data are there to test the Hall & Hoff model and the influence of sealing the floor. Other implications would not be well-served by this case study. However, to allow comparison with long term (+10 years) weather data, citations for two sites in southern England have been added for readers interested in this comparison.’

Please, just to clarify that the query raised about making a better discussion of results and the influence of climate and the weather during the monitoring period was not about discussing climate change or future implications. This was about discussing the results from the model and the influence of sealing the floor under the environmental conditions registered by the weather station during the monitoring period, e.g. the influence or correlations between the environmental data recorded (i.e. solar radiation (kWm-2), air temperature (⁰C), humidity (%rainfall (mm/tip), wind speed (ms-1), wind direction (⁰) and the results of wetting or water content of the wall and the implications on the water absorption and evaporating drying processes of the ground and walls in this case study.

6-Regarding the following question and the response by the authors:

What can be the causes of discrepancy between model and measurements? Is there something that the model is missing? ‘Response – In this instance the model is testing whether moisture movement is due to capillary flow of water from the ground. The results suggest that this capillary flow is not occurring. This is due to the heterogeneous composition of the wall, with pores that are not connected. If the material porosity was fully defined, more complex models of liquid and vapour water flow would also simulate capillary flow (if the pores were assumed to be continuous). But the in-situ wall moisture data and inspection of the wall composition shows that this is unlikely to be the case.’

A better discussion and conclusion are still needed when talking about contradictions from model and measurements outcomes. It should be good to mention that more monitoring studies should be needed to have other outcomes and more representative conclusions based on monitoring: (i) different building materials (eg. bricks, stone ashlars, etc versus this case study of a complex composite based on mortar and stone rubble-filled core bedded in cement render, etc) and (ii) the ground soils underneath the building (eg. sands, gravels, etc. versus clay soils), as well as (iii) other climate conditions.

Moisture content of building and soil materials depends on their physical properties (eg. different permeabilities, pore network (including heterogeneities, fissures and cracks) and capillary coefficients or water absorption rates through capillary pores. This case study shows a nice example of a three-year monitoring measurements under specific environmental, geologic and building materials conditions and physical properties encountered onsite. It shouldn’t be used to generalize the behaviour of moisture levels in external walls of all historic buildings in response to seasonal potential evaporation rates and the influence by the installation of impermeable ground bearing slabs in all cases since the outcomes may change depending on the aforementioned factors.

Additionally, it should be discussed that the model was designed to be applied to predict rising damp and moisture movement within a porous masonry wall without finishes. Therefore, the values for input parameters for the model such as sorptivity that were used by the authors should be reconsidered since according to them:

‘The sorptivity ( S ) and moisture content of the wetted part of the wall ( θ w ) were not measured, but were assumed to be 1.0 mm.min -1/2 and 0.2 respectively, as used by Hall & Hoff (2007).’

Should these values being assumed to have lower levels than those used by Hall & Hoff (2007) and more according to the system with the described characteristics which also include finishes such as render and coating? Please, consider this together with the comments previously made on the porosity, capillarity, permeability, of both building materials and the ground composed by clay soils mixtures, and discuss if these could also have had an influence in the discrepancy between model and measurements.

7-Figure 8 shows that the intervention (installation of polyethylene sheet on the floor) was done in mid-September 2019, in the middle of a period of high Soil Moisture Deficit (mm) (according to figure 8) and a very low soil moisture content (according to figure 7).

Could this have influenced the results obtained from the measurements of moisture content in the timber dowels (rubble wall) which does not show changes before and after installation?

It would be good to mention that the outcomes of this research are specific to this case study with this type of underground and building materials. Therefore, it shouldn’t be used to generalise the behaviour of moisture levels in other climates, geological settlements and different types of building materials.

8- Regarding the following questions and corresponding response: ‘where is the water from the wall coming from’? and (according to Figure 9) Why is there more moisture content at the base of the wall compared to higher height? Response - We do not have a clear answer to this question as we did not design the experiment to explore wall moisture distribution, but instead to explore changes in wall moisture in response to the intervention. In our experience, the moisture content of walls in historic buildings is usually greater at the base than further up (unless there is a roofing, guttering or plumbing defect). At Court House there may have been some water uptake at the base of the wall from the adjacent soil during wet weather, but because of the composite construction and lack of capillary pathways its ability to rise was limited. Alternatively, the moisture distribution in the wall may reflect moisture equilibrium with the room by a process of diffusion rather than capillary rise (which was shown to be not occurring).

Please, provide a better evidence, justification, correlation and discussion of results on the statement about the lack of capillary pores and limited capillary rise due to the composite construction, and the presence of water in the walls considering monitored data from the weather station (eg, rain fall), picture of the indoor wall in former Figure 2 (eg. showing signs of decay by rising damp), sorptivity properties of building materials (eg. Gummerson, Hall & Hoff, 1980), soil moisture content and deficit (Figs 7 and 8) at the time of the intervention, etc.

The paper presents the results of in-situ monitoring in a case-study building, analysing the influence of reducing the floor vapour diffusion on the moisture balance of a surrounding wall. The analysis of the case study shows that the wall moisture content did not change as a result of sealing the floor area with a polyethylene membrane, and the monitoring results were found to be in contrast with the numerical results from a sharp-front model on capillary rise. Below are some more detailed points that need addressing:

1. The paper doesn't provide information on the hygrothermal properties of the existing building materials considered in the analysis. A geological map is mentioned (please add a reference), but there is no information on the hygrothermal properties of the materials used in this house for the wall and floor.
2. The concrete slab is represented by the installation of a polyethylene sheet; however, there is no comparison between the two:
   • In the method section, please add information on the hygrothermal properties of both, at least on the vapour resistance.
   • It is likely that the polyethylene sheet has a lower vapour resistance than the slab; also, construction moisture is not considered in the analysis. In the discussion section, please add a discussion point on the difference between the two.
   1. The title refers to "ground slab permeability "; it is more appropriate to rename it to "ground floor vapour permeability"
3. In the methods section, some points are unclear:
   • Why were monthly intervals considered? For some of the methods considered, a more frequent sampling is possible, and this could have been beneficial for the analysis. Also, other measurements could have been considered for this analysis.
   • Are all the measurement points presented? A Tinytag logger was mentioned in the results section, but it is not clear if it's the indoor logger or a new logger beneath the polyethylene sheet .
4. Most of the text found in the discussion consists of the results of a sharp-front model on capillary rise and the comparison with measurements:
   • Please move this part to the result section
   • Introduce this comparative analysis in the methodology. E.g. "The wall moisture changes were then calculated using the potential evaporative drying measurements, and the results of this calculation were compared with the wall moisture measurements".
5. In figure 10, it would be best to refer to WME (wood moisture equivalent); the text already explains the limitations of this reading. Also, if this chart presents unreliable measurements (as mentioned in the text), what is the value of having it in this paper?
6. The discussion section is very limited; expand the discussion section,  elaborating on the relevance of these results and limitations, including the following:
   • Is the polyethylene sheet appropriate to represent a concrete slab? Some vapour accumulated initially, but there is not information on the moisture levels under the polyethylene sheet in the long-term, nor any discussion on where the initial vapour might have transferred to (absorbed by the floor material? Through the polyethylene sheet?)
   • The available ground moisture is measured considering the soil moisture deficit; please add information on how this compares with other locations. Is this a location with particularly low water table or is this representative of an average ground moisture?
   • Is the timeframe of this analysis long enough? The conclusion refer to long-term changes, but moisture can build up in years, and three years is possibly not long enough.
   • Is this measurement method the most appropriate? How could this methodology be improved for more conclusive results? How have other researchers tackled similar problems?
   • What can be the causes of discrepancy between model and measurements? Is there something that the model is missing?

Level of importance

The publication is relevant for both the academic community and practitioners within the conservation industry working in historic buildings. It deals with the effect and consequences of the application of impermeable ground-bearing slabs on the floor of the interior of old buildings without a damp-proof course. This is an important topic because there is limited evidential data based on measurements to validate the hypothesis and belief among practitioners about the action and effect of capillary rise that may cause decay to the adjacent walls of the building. This case study shows that ground moisture would not necessarily be ‘driven’ up adjacent walls. However, models of the process may display different outcome results.

Providing accurate data based either on models, case studies, or laboratory work is critical to ensure the appropriate preservation of the structure and to avoid further decay.

This research work is based on a long-term case study which provides some interesting insights despite this it's a very specific case study with some missing information that may condition the representability of results. However, it represents a novel methodological approach and provides new findings in comparison to other publications in the field.

Level of validity

Overall, the hypothesis is clearly formulated but the argumentation is not very stringent. How damp and moisture may promote the decay of construction building materials and how it affects the health of occupants is not well justified. The data and results are interesting but the description of the building materials (e.g. l imestone, sandstone ashlars? gypsum, lime or cement plaster?, painting layer , etc) and their petrophysical properties, are specific to this case study but these were not well described nor measured and hence maybe not statistically significant.

The interpretation of results is also unsound, and a better discussion of results is missing to draw a solid conclusion. By contrast, the methodology is described with an exhaustive level of detail that wouldn’t have been necessary unless the focus of the research is based on this. Nevertheless, some important useful methods applied in this research are not clearly described (e.g. explanation on how the impermeable layer was installed and fitted (sealed) onto the ground; how the collected samples for gravimetry analyses were preserved to avoid dehydration until analyses were performed in the lab; the identification for the location of the drills to introduce the timber dowels and deep wall probes, etc.). More onsite and/or laboratory measurements (from collected samples) would be very supportive to validate the research findings.

More specifically, there are some issues that would benefit from an improvement or clarification:

1. From the title of this article ‘ The influence of ground slab permeability on wall moisture in a historic building’ the reader expects to have more accurate and representative results to establish that influence’. However, the paper deals mostly with the methodology used to establish this in a very particular case in which the type of building materials and underlying ground where the building is settled is very specific to this study. Different outcomes could have been obtained for other types of materials where the physical properties should be known either by measurements or at least by references.

2. As mentioned above, the final hydric, capillary, and evaporation behavior, water transport, and mechanisms of decay of building materials that could happen or not or being more or less severe, will be conditioned, among other factors by the local climate and weather and type of petrophysical properties of building materials and soil/ground/rocks where they're settled. More information of materials’ properties obtained from measurements or at least references would be important to better discussion and conclusions from results; e.g. Type and main physical properties of the buildings materials and soil/ground of this historic building? Are these frequently used in the UK, England, other countries? Are the type of soil and location-ground settlement representative of many historic buildings in the UK, other countries? In this case study, it seems that the ground underneath the building is clay soil (mixed with ' fill '?? this term should be clarified) which is mostly impermeable; the influence of this for example should have also been discussed. There should be references providing generic petrophysical characteristics of the sandstone rock geological formation underneath that would be relevant and important to mention and discuss here (eg. open porosity, capillarity rates, etc). Also regarding the building materials; are the stone ashlars of the building also sandstone?

Additionally, a discussion about the climate and the weather during the monitoring period would have been relevant e.g the discussion on evaporating drying is missing a link/references to climate/historic weather conditions of this particular UK region and future implications (also for other climates/regions in other countries); discussion the orientation of the building, seasonally rainfall? predominant rain/ wind direction? etc.

Some info about the Physico-chemical properties and context of the building structure, location, and weather would provide a more consistent and realistic interpretation of results.

3. The discussion about why meter readings were erratic and did not show a temporal trend in contrast to gravimetric analyses that is mentioned at the end of the Results section should be better discussed and moved to the corresponding Discussion of Results section.

4. Regarding the following statement posed in the abstract and conclusions: ‘ moisture content within the wall did not increase following the installation of a vapour-proof barrier above the floor. This indicates that the moisture levels in the rubble wall were not driven by capillary rise .’ :

There are no consistent results and neither a thoughtful interpretation of these to assure this. Several questions should be important to address and discuss, such as: ‘ where is the moisture coming from’? Why the moisture content measured from the timber dowels is there if the source is not capillary rise?’ Why is there more moisture content at the base of the wall compared to higher height? This and other arguments would enrich the content of the manuscript and the quality of the research. Some clarifications would be useful to avoid some contradictory interpretation of results

Level of completeness

The authors reference appropriate international scientific publications and provide relevant information to follow their findings.

However, more references are missing to discuss these findings and obtained results, such as the influence of physical parameters related to the hydric behaviour of the different building materials and adjacent/subjacent soil/ground of this particular case study; the influence of climate/weather; the correlation/discussion between measuring and modeling data and the obtained results is unsubstantial and the volume of the latest scientific and technical references or publications to argument the hypothesis is scarce and not enough to draw the main stated conclusion.

More specifically, some other issues to clarify:

1. The second paragraph of the Introduction section that describes the hypothesis and related physical factors involved should be better deliberated. The factors that might influence the occurrence or severity of rising damp in the walls that may happen due to a lack of damp proof course and the installation of impermeable ground bearing slab installed in a historic building during renovation and how this may alter the moisture dynamic of the building should be better addressed. Despite the limited evidence-based on long-term monitoring, the authors mention that there are references to this phenomenon. The provision of those ‘ technical and product literature ’ references would be useful.

2. Among other factors, such as climate, weather, or geological features, rising damp will depend on the type and petrophysical properties of surrounding materials, i.e. of both the soil (ground) and building materials (walls) which will condition the capillary forces mentioned by the authors. The influence of their physical properties, such as open porosity, pore size distribution, tortuosity, capillarity coefficient, etc. should be mentioned and discussed. This is important since the final consequences (including rising damp) may be very different depending on the type of materials and hence on the type and kinetics of water transport and the eventual mechanisms of decay (e.g. freezing-thaw, salt crystallisation, etc).

3. Even though in the ‘Methods’ section (which should be better named as ‘Materials and Methods’), it was already explained in the Level of Validity query,  that the methodology is thoroughly described but some important information is still missing, such as the description of installation of impermeable layer, etc. (see these comments above).

Level of comprehensibility

The language is clear and easy to understand for an academic. The article is overall systematically and logically well structured. However, the description of some parts of each section is not clearly organised and the reader wonders about the principal and main content that should be indicated at the beginning of each section rather than suddenly being explained at the end.

More specifically , there are some minor issues to consider improvement or clarification:

1. In the last paragraph of the Introduction section, the term ‘ conductivity properties’ may be confusing ; consider using the term 'hydric properties' or physical properties of building materials that influence ‘water transport’. Additionally, specify the type of ‘ monitoring ’ (e.g. monitoring moisture content?).

2. In the ‘Methods’ section (second paragraph) briefly specify what type of ‘ instruments ’? e.g. data loggers? sensors? The ‘Instrumentation’ subsection should be specified that the moisture meters are not directly measuring 'moisture' when inserted through the building materials of the wall; these are moisture equivalent measurements WME (%) using electric resistance mode (pin mode).

3. The figure captions are properly described but the quality of some images and pictures (e.g. Fig. 1 and Fig.2) is not good enough or relevant to provide the necessary information (e.g. image of the external view of the building is missing; Fig.2. is blurry and building materials cannot easily being distinguished, etc.).

Overall Impression

Very interesting paper and relevant topic. However, the argumentation to describe the hypothesis and relevance of the study is unsubstantial. The description of this specific ground site, soil and type, and physical properties of construction and building materials is poor. A thoughtful interpretation of data and discussion of results is also missing. More references and other cases studies of these and other building materials, soil/ground and climates/weather, and better discussion of the findings would provide more solid conclusions.