THE DEHUMIDIFICATION PROJECT
Causes of humidity
There are four main causes of humidity in the cloister and these interact closely with each other.
a) Alterations to the original buildings
Up until the thirteenth century, the cloister corridors were on one floor only and the perimeter buildings to the east and south were not so high as they are nowadays. So the proportions of the cloister’s open area were in keeping with the size of these buildings. When the height of these buildings was later increased, the open area in the cloister became a sort of "well", with the result that there is poor air circulation within the cloister and the amount of direct sunlight is much reduced to the extent that some walls, especially those on the southern part, are in almost permanent shadow.
b) Changes in floor levels
Surveys have shown that floor levels and the level of the land on the southern side of the complex have been significantly raised, but not in equal proportion, with respect to the levels of the original construction.
The following differences in level are especially noticeable: the surrounding land (now the monastery garden) is higher than the floor level inside the building; the floor level in the southern building is a meter above the level of the cloister corridors; the open area of the cloister is above the level of the corridors (see section A-A). The two main results are:
1) the corridors are at the lowest level of the whole area and are thus most exposed to humidity from the subsoil;
2) the lower parts of most of the cloister walls are in direct contact, on one or other of their lateral surfaces, with soil filling.
Probes have shown that these soil fillings are mostly made up of soil mixed with various types of detritus (see archeological excavation cards), which means they have a high absorption capacity and a high rate of water seepage which directly affects the wall surfaces in contact with them causing substantial rising damp in the overlying walls. Therefore it is not surprising that the most pronounced deterioration has taken place on the cloister’s southern perimeter wall where the soil filling behind the wall is higher.
Probes have also shown that the floors are separated from the soil beneath them only by a thin pozzolana based layer. This means that rising damp spreads unhindered, which explains most of the damage that can be seen on the floor surfaces.
c) Marked presence of salts
Analysis has shown that there is widespread presence of salts and nitrates on many of the cloister surfaces. This can be explained by some of the typical activities that have taken place in the monastery over the years: the cloister area was also a burial ground while the surrounding premises were used as storerooms for food, much of which was preserved in salt. These areas were also used as stalls for animals whose dung is rich in nitrates, and this use continued right up until the twentieth century in the southern part of the cloister. In addition, the surveys have also shown that it was common practice, not only here but also in other parts of the monastery, to use garden soil for filling purposes. These gardens were also the site of the monastery’s main rubbish tips that were periodically burnt and covered with soil. So the soil used for filling purposes has a high content of partially burnt food remains mixed with bone fragments.
All these factors mean that there are large quantities of mineral salts dissolved in the soil. As water passes through the soil, it carries the salts into the walls where they are deposited internally or on the surface. Given the fact that these salts are strongly hygroscopic, they absorb more humidity from the air (which is saturated due to poor ventilation), thus giving rise to damp surfaces. The situation is made worse, though not to the same extent, by condensation.
d) High level of humidity in the subsoil
The siting of the complex on a hill which is partly artificial means that there is no danger of infiltration from a water-bearing stratum in the vicinity. Notwithstanding this, the subsoil beneath the cloister and the surrounding buildings contains significant quantities of water which rise through capillary action to the higher levels. So the presence of this water is due to other reasons.
One of the main factors is that the cloister is situated at the center of a complex water disposal system, with large quantities of water draining into it from various sources, for instance:
1) rain water falling directly on the exposed parts;
2) rain water on the roofing that converges on the cloister through a system of gutters and drain pipes;
3) waste water from the fountain in the middle of the cloister garden and from the twelve washbasins located on the upper floor of the surrounding buildings.
The exposed area of the cloister is 140 sq. m. and the
roof area that drains into it is 685 sq. m. Therefore, the total area
that drains into the cloister is 825 sq. m. Since the average rainfall
in Rome is 800 mm., the total flow of rain water into the cloister can
be estimated at 660 c. m. per year, to which must be added about 40 c.
m. of waste water from the wash basins and the fountain, making a grand
total of about 700 c. m. / year. Furthermore, the maximum rainfall in
Rome over brief periods is about 40 mm. in twenty minutes, so flash flooding
of water flowing into the cloister can reach the very high figure of 30
In order to understand the present situation, it is useful
to look back over the various alterations that have been made to the system.
Today, the floor is broken up and the base consists of
a flattened layer of tufa (outcrop) which slopes to the west.
After the works carried out by Muñoz, this drainage
system was closed off, but it seems that the channels, even though filled
with soil, continued to drain off part of the water into the subsoil.
Perhaps a similar system was arranged for the roofs of
the perimeter buildings, as seems to be the case from a photograph dated
Muñoz also kept the garden layout of the open
area but did not provide it with a drainage system, so rain water goes
directly into the subsoil and from there seeps into the corridors which
are at a slightly lower level.
To examine ways of rationalizing the system, a survey of the sewage system was carried out. The monastery is connected to the public sewage system at six points: four on Via dei Ss. Quattro Coronati and two on Via dei Querceti. Of these latter two, the one carrying the waste water from the southern area of the complex dates from about 1970. The waste water from the bathrooms in the area of the cloister flows into a tank situated in the garden to the south of the library. The water flows through underground channels to a second tank at a much lower level situated at the foot of the south-west tower.