Microclimate analysis

by Carlo Cacace

  

The environment-masonry system must be considered as a physical system, whose evolution can be controlled by understanding and measuring its parameters. Based on this assumption, a microclimate survey plays the special role of analysing the interaction between air and the materials, which helps control environmental aggression, on one hand, and understand how to minimize material deterioration, on the other hand.

This theoretical outline is useful to describe the majority of the phenomena that can be observed in a degradation process. When the functional interactions between the physical characteristics identified to describe the evolution of the system are known, these variables can be measured. The experimental data acquired with this procedure, in addition to characterizing the system, represent the basic knowledge to develop any environmental rehabilitation and, as a consequence, to control conservation conditions. Degradation happens when the state of conservation of the constitutive materials of a masonry changes. Generally speaking, degradation, in other words the state of conservation, depends on the energy balance between the two elements of the system. When a thermo-hygrometric unbalance is triggered between them, the system changes its state of conservation. In other words, a process is activated that will either improve or worsen the state of conservation of the material. Therefore, understanding the environmental conditions of a building, with regard to the chemical-physical relations of its constitutive materials, as well as to the state of aggregation and conservation of such materials, is an essential condition to control the process of deterioration. On the other hand, the so-called 'natural ageing', that is, the natural processes caused by the spontaneous interaction with the environment, must not be neglected. However, the way degradation develops and how fast it progresses not only depend on the amount of energy exchanged through the surfaces of the building, but also on the “initial conditions” and on the “boundary conditions” of the system, and namely on the way the latter can vary or be changed.

Spreading a protective coat on a dry finish or on a fresh one (different types of surfaces) during restoration can modify the boundary conditions and evolve into a different conservation pattern.

Also, clearly enough, masonry is not isolated with respect to the surrounding environment, but makes with it a physical system evolving with time. The evolution pattern depends on the type of energy involved (heat, mechanical, chemical, electro-chemical, etc.) and on the factors mentioned above. The 'environment-masonry' system can then be defined by the physical characteristics and by the mathematical relations that quantitatively describe the evolution in time and in space. Such evolution can be detected with the help of control units capable of recording the change over time of such physical parameters as air temperature, masonry temperature, relative humidity, air velocity, solar radiation, etc. Understanding the characteristics and the functional relations governing the transition from one state to another is essential to control and modify, following some selected indications, the pattern of degradation. However, one should always bear in mind that the physical characteristics have a different influence, depending on the nature and shape of the material.

Taking the model described above as a reference, the objective of a microclimate survey is to understand and identify the patterns governing the thermohygrometric balance of the environment-masonry system and to establish the conditions to slow down the process of deterioration of the material. The presence of water vapour in air and the hydrophilic characteristics of masonry are the main cause of deterioration generated by thermohygrometric exchange.

The most important parameters governing this type of interaction essentially depend on the physical state of water (vapour, liquid, solid). With mortar, they concern the nature of the binder and the mineral composition of the material, which, owed to high polarity, attracts water, also because of the porous microstructure of plaster generated by carbonation. For a thorough understanding of the ongoing and potential deterioration mechanisms, it is essential to learn about the porosity, hygroscopicity and wettability of a surface, because these parameters, when the thermohygronometric pattern remains equal, are capable of diversifying the behaviour of a system in the processes of absorption, adsorption, desorption, condensation, evaporation and sublimation.    If we rule out the case of water infiltration or rising damp, owed to leaks from faulty piping or bad sealing of windows or roofing, the behaviour of any hydrophilic and porous material in a large enough environment that will not be influenced by any water absorption or desorption by the material itself is governed by the thermohygrometric pattern of air, and therefore of local climate.

As a general rule, the variation of water content in masonry is linked to these patterns and tends to diminish as temperature rises, because the latter is associated to the decreasing trend of relative humidity, and vice versa. The material tends to release water into air when temperature rises and relative humidity decreases, and to absorb water in the opposite conditions (figures 1,2,3). In these cases, it is theoretically beneficial to annul vapour exchange through the surface and therefore reach stable microclimatic conditions.

To minimize thermohygrometric variations induced by the exchange and recirculation of internal and external air, it is convenient to enhance the insulation of masonry and roofing alike, the seal of doors and windows, and the insulating conditions of the whole environment. All these interventions will inhibit heat and water vapour exchanges and strike a stable balance between air and the constitutive materials. In some special conditions, e.g. high relative humidity, it is difficult to identify and understand whether the causes for deterioration actually depend on the wetting of a surface owed to rising damp, or to condensation after the dew point is reached, (figure 4), or to adsorption, or to condensation caused by the presence of hygroscopic salts and/or chemicals used during cleaning, consolidation and protection works, or to deposits of particulate or gaseous pollutants.

However, degradation caused by wetting may also be owed to the presence of a thin water film of few microns that is likely to form on a surface under conditions other than saturation, which is hardly detected with direct observation.

The absorption of sulphur trioxide (SO3), for instance, varies depending on the material, also as a function of superficial microporosity, and on the values of relative humidity. The presence of chlorides generates a different pattern of absorption, which needs to be assessed each time. In these cases, the critical threshold can fall far below the conditions of air saturation. Another important parameter to be considered is the heat strain caused by daily solar radiation. The energy released by a heat source propagates in different ways, depending on the type of source and on the physical state of the materials where heat is propagated. Any time there is a difference in temperature (heat gradient) between two physical systems or even two elements of a system, thermal energy is transmitted by conduction, convection and/or radiation, depending on the medium intervening between the heat source and the receiving system. Heat energy can, however, be measured instrumentally by measuring temperature and observing the heat-induced phenomena (figure 5).

 

[by Carlo Cacace, Director section microclimate models and data management IsCR]