Conclusions

An influence of environmental climate on the temperature and humidity distribution in the different building stones was detected. The measurements taken show the moisture balance and temperature of the stones in comparison to each other. They show clear gradients of relative humidity (RH) and temperature in the ashlars of different stone materials at various depths at identical exposure areas. The correlation of these gradients to the outside conditions is depth-specific. Variations can be seen according to the exposition of the building section. Relative moisture distribution is detected by measurements of relative humidity and temperature. A conversion to absolute and material-relative moisture contents would be the next step of the evaluation, which needs further comparative “thermo gravimetric” analyses and laboratory testing (see Fischer 1993).

The measured temperature values show only small variations between the outside and the material temperature inside the stone. The highest differences – of about 2°C – are observed for the northern façade (MF02) and the southern façade (MF07) in March/April 2011. The sensor in the Krensheim Muschelkalk oriented towards the East (MF04; sensor close to the adjacent Drachenfels trachyte – A-KS03) shows values with a constant degree of variance of about 3–4°C; this might be a question of a malfunctioning sensor. The almost uniform courses of the curves show slight retardations at temperature increases or decreases. In the cold winter months (October to March) the temperatures inside the stone are lower than the outside temperature. In the warmer summer months this is reversed – the temperature inside the stones is higher than the outside temperature. This is observable for the western façade (MF01), the northern façade (MF02x), and also on the eastern façade (MF04), though not as significantly. At the southern façade (MF07), the temperature inside the stones is higher throughout than the outside temperature. As expected, a clear trend of higher temperatures is observed for the southern façade (MF07; Fig. 3.15 e) in comparison to the northern (MF02x; Fig. 1.13 a), western (MF01; Fig. 3.13 e), and eastern sides (MF04; Fig, 3.14 g).

This is paralleled by lower RH values, which correlate with temperature.

In terms of the relative humidity, clear gradients are detectable. The deeper inside the stone the sensor is, the flatter – more amplitude-reduced – the course of the curve. This indicates a decreasing direct impact of the outside humidity on the inside moisture. This may reflect to a certain degree the hysteresis of the sorption isotherms (see Fischer 1993); much more than this, however, it indicates a separate “stone climate” (see Schuh 1988).

At the western façade (MF01), where the sensor is placed deeper inside the stone, a tendency of higher relative humidity can be observed. The lower the outdoor RH, the

difference is higher. This indicates that an almost constant moisture content of 85–95% RH is established at a depth of 32 mm. This decreases in very warm summer months down to 70%

RH.

In the eastern-oriented measuring field (MF04), the sensors are all placed at the same depth inside the stones but at different distances to the joints or the adjacent stones. The RH curves of the sensors inside the ashlars are highly amplitude-reduced. In the winter months, RH in the Drachenfels trachyte is around 95–100%; in the Krensheim Muschelkalk, RH values range between 90–100%. In respect of several days with minus temperatures, frost action inside the stones can be assumed. In the warmer months (April to September/October), RH ranges are lower, at an average of around 80% with the lowest values around 70%. The stone climate differs very much from the outside climate. RH shows higher values than the outside humidity.

It is noticeable that, in this monitoring field, the sensors in the Drachenfels trachyte show different values in correlation to the distance to the adjacent stone. It still has to be ascertained whether this is due to a certain moisture impact by the joints, or whether the relatively higher RH values of the Krensheimer Muschelkalk are responsible. Regarding the four sensors inside the Krensheim Muschelkalk, the two placed closer to the Drachenfels trachyte and Obernkirchen sandstone show lower RH values than the two sensors in the Krensheim Muschelkalk. A generally higher water balance inside the Krensheimer Muschelkalk can be concluded. From this, a feedback mechanism can be deduced of adjacent stones, leading to a higher moisture load into the stone with the originally lower water balance, i.e., a higher load from the Krensheimer Muschelkalk to the Drachenfels trachyte and from the Krensheim Muschelkalk to the Obernkirchen sandstone.

On the northern façade (MF02x), only a small amount of data could be collected due to several problems with the installation of the sensors. The curves of the sensors inside the stone are amplitude-reduced and run in an average range of the outside humidity. The sensor at 37 mm depth reveals a tendency of higher RH in comparison to the sensor at 23 mm depth. Similar observations can be made for MF07 at the southern façade with relatively higher temperatures and correspondingly lower RH values.

As expected, a correlation of the stone climate to the cardinal direction can be seen. An amplitude-delayed behavior of RH inside the stones is observed. Furthermore, the measurements show a general tendency of continuously higher relative humidity at deeper sensors compared to those placed closer to the surface of the respective stone. Thirdly, in one and the same stone different RH values were measured according to distance to the joints or adjacent stones, indicating interference from the water balance and moisture contents of neighboring natural building stone ashlars.

Time- and amplitude-delayed behavior of humidity measurements in sandstone over the course of a day was first detected by Schuh (1988). The moisture content inside the sandstone (11 mm depth) shows less variance (low amplitude) than the outside and surface sensors. The humidity inside the stone is generally higher except in heavy rainfall, when outside humidity increases rapidly to high values (90–100% RH). Thus, Schuh 1988 refers to an individual “stone climate”.

The collected data show tendencies of moisture balance inside different building stones.

Different gradients of moisture content are ascertained in the different building stones – higher-resolution measurements would be a great help here. The reliability of the recorded data still has to be confirmed by further measurements; the choice of method also has to be evaluated. For example, the behavior of the sensors after a condensation event should be further investigated. Numerous publications exist concerning technical moisture-measuring in porous material for comparable single measurements (see above). However, technical requirements of devices, i.e., sensors and data collectors, need to be formulated to improve devices and installation, and thus reproducible measurements. The chosen method allows low-invasive long-term measurements.