Characterization of a hygro-regulated Wall Base Ventilation System for Treatment of Rising Damp in Historical Buildings

Transcription

1 Characterization of a hygro-regulated Wall Base Ventilation System for Treatment of Rising Damp in Historical Buildings Vasco Peixoto de Freitas, Full Professor Building Physics Laboratory Faculty of Engineering Porto University, Portugal vpfreita@fe.up.pt, Ana Sofia Guimarães Building Physics Laboratory Faculty of Engineering Porto University, Portugal anasofia@fe.up.pt, KEYWORDS: Rising Damp; Treatment; Wall Base Ventilation System; Historical Buildings; Condensation. SUMMARY: The treatment of rising damp in the walls of historic buildings is very complex, due to the thickness and heterogeneity of the walls. The techniques traditionally used for dealing with this problem (such as watertight barriers, injection of hydrofuge products, etc.) have sometimes proved ineffective, and that is why it is necessary to find a new approach. In recent years, the Building Physics Laboratory at the Faculty of Engineering, University of Porto has been conducting experimental research on the effectiveness of the wall base ventilation system, using natural or mechanical higro-regulate systems to reduce the level of the damp area. This experimental research and the simulations that were performed, clearly show that wall base ventilation is a system with potential. This paper presents the characterization of the hygro-regulated systems operation based on experimental studies developed in laboratory, which allowed the influence of the velocity of the air, condensation risk and the possibility of salt crystallization. This will also present the results of measurements conducted in churches in the North of Portugal in which these systems were implemented, with the purpose of validating its operation and establishing criteria for programming the ventilator. 1. Introduction For the last few years, the Building Physics Laboratory (LFC) of the Faculty of Engineering, University of Porto (FEUP) has been conducting experimental research to validate a technique for treating rising damp in the walls of older buildings [1, 2 and 3]. The technique consists of ventilating the base of the walls using a natural ventilation process or by installing a hygro-regulated mechanical ventilation device. The experiments were designed to show how these walls are affected by rising damp in view of different boundary conditions. The configurations used are shown in Fig. 1. To assess moisture transfer inside the walls, probes were inserted at different heights and depths to measure relative humidity and temperature. These probes were then connected to a data acquisition and recording system [1]. In Configuration 1, we measured the behaviour of a wall with both sides underground by placing sand on both sides of the wall up to a height of 45 cm above its base. In Configuration 2, in order to assess the effect of a ventilation system at the base of the wall, a ventilation box was placed on both sides of the wall. Two openings were left, to which flexible tubes were attached to ventilate the box. A mechanical extractor was placed at one opening, while the other was left free to allow air to enter freely. This extraction system was left running during the experiment to ensure that the temperature and relative humidity inside the box were identical to the conditions in the laboratory. Figure 2 illustrates the change in relative humidity in the cross-sections located 61.5 cm (level 9) above the wall base.

2 Configuration 1 Without ventilation. Configuration 2 With mechanical ventilation. 9A 9B 9A 9B 8A 8B 8A 8B 6A 6B 6A 6B Vent. Sist. Vent. Sist. 2A 2B Water 2A 2B Water FIG. 1: Variations in relative humidity at level 9 in configurations 1 and 2 [2, 3]. Configuration 1 Configuration A 8A 9B 8B 61.5cm from base Relative Humidity (%) hours 500 hours 600 hours 700 hours 800 hours Relative Humidity (%) hours 500 hours 600 hours 700 hours 800 hours 7A 6A 5A 4A 3A 2A 1A 7B 6B 5B 4B 3B 2B 1B 16.5cm from base Water Thickness (cm) Thickness (cm) FIG. 2: Variation in relative humidity at level 9 [3]. We can conclude from the experimental research that a ventilation system placed at the base of the wall reduces the level of the rising damp (see Fig. 2). The object of this paper is to present the results of experimental characterisation of a hygro-regulated ventilation system: Laboratory measurements; In field measurements.

3 2. Experimental characterization of the Wall Base Ventilation System 2.1 Model adopted and description of the experiment We study three cells corresponding to specific boundary conditions (Configurations 2A, 2B and 2C) (Fig. 3) that were associated to a mechanical ventilation system with controlled velocity. Probes were placed in such a way as to obtain readings of the temperature and relative humidity inside the ventilation system all the time (Fig. 3). Probes were installed in each cell: Probe S1 Temperature (T) and Relative Humidity (RH) of the air at the entrance of the system; Probe S2 T and RH of the air in the middle of the system; Probe S3 T and RH of the air in the middle of the system and in the surface of the wall and Probe S4 T and RH of the air at the exit of the system. DL S1 Hr (%) T (ºC) 0,30 0,30 0,30 S2 S3 0,15 0,20 0,15 0,20 0,15 0,20 Configuration 2A Configuration 2B Configuration 2C S4 w System without waterproofing Superior waterproofing of the ventilation system Waterproofing of the ventilation system FIG. 3: Physical Model Probes positions. Table 1 shows the speeds throughout the time. The velocity was regulated and measured with a high precision anemometer. TABLE. 1: Variation in velocity over time. Test duration 5 months Days Saturation of the wall Velocity (m/s) During the drying process, the interior of the ventilation system was monitored in accordance to the principle defined in Fig Results The analysis of the graphs of vapour pressure variation at the entrance and the exit, for the three configurations, clearly shows the occurrence of periods with negative differences of flow for the configurations 2A (Fig. 4) and 2B, which means the occurrence of condensations. It was possible to visualize that, in these periods of time, the system generates condensation moisture, had being seen the occurrence of drops of water. On the other hand, it was verified that the inversion of the gradient of pressures occurs near the exit. The length of the system presents a basic role in the functioning of the system. Using the temperature and relative humidity values at the entrance and exit of the systems, we calculated the quantity of water vapour transported. Table 2 shows the model used to calculate the amount of water transported by the ventilation system.

4 FIG. 4: Condensation occurrence in configuration 2A (red entrance, yellow middle and blue exit). TABLE. 2: Method of quantifying transported mass flows [4] * θ θ P = e * RH P Vapour pressure (Pa); θ Temperature (ºC); (I) (II) (III) RH Relative Humidity (%). w = *P/ θ w Absolute air humidity (kg/m 3 ); P Vapour pressure (Pa); θ Air temperature (K). W = (w exit-w entrance)*q* t W Quantity of water removed (kg); w exit Absolute air humidity at exit of system (kg/m 3 ); w entrance Absolute air humidity at entrance of system (kg/m 3 ); Q Ventilation flow (m 3 /s); t Time interval (s). Note: As the measurements were taken every half hour, the temperature and relative humidity are considered to be constant for this time interval. Figure 5 shows the quantity of accumulated water vapour transported during the testing period, for various air circulation speeds. The functioning of the system is greatly influenced by outside conditions. In Configuration 2A and 2B we can see that, in some periods of time, the quantity of accumulated water vapour transported decreases which mean that we have condensations inside the system. FIG. 5: Quantity of accumulated water vapour (kg) taken from the ventilation system.

5 INSTALADO APROX. À COTA 1,20 (CERCA DE 2,5m ABAIXO DO TERRENO ACTUAL) COM A INCLINAÇÃO MÁXIMA DE 0,5% Ø315 CAL EIRA CALEIRA Ø200 CALEIRA CALEIRA CALEIRA INSTALADO À MESMA PROFUNDIDADE DO TROÇO EXISTENTE Ø200 CALEIRA Ø300-1,50-1,60-0,85-0,50-0,60-0, Main conclusions obtained The following conclusions were drawn from the results obtained: The Configuration 2B had transported more water vapour; In configurations 2A and 2B we had problems of condensations inside the systems; The best behaviour is achieved by the configuration 2C; The outside conditions of the air at the entrance influence the system functioning; The velocity or flow extracted by the ventilation system, though necessary, contributes little to the efficiency of this system. A hygro-regulated system is essential to control possible condensation inside the system. A hygro-regulated system is the one in which the vapour pressure is controlled by sensors placed at the entrance and exit, which instruct the system to switch off whenever the exit flow is less than the entrance flow. 3. In Field Characterization of the Wall Base Ventilation System 3.1 Description To validate the system in field, a wall base ventilation system was installed in a church in the North of Portugal. On the outer face of the church walls, a natural ventilation channel was created, consisting of prefabricated concrete parts. This channel could be opened to the outside air by means of a ventilation device (Fig. 6). Inside the building, there were two distinct subsystems, both with hygro-regulated mechanical ventilation. Air was admitted into one subsystem through an opening in the façade. The other subsystem involved two admission grids located inside the building, with extraction to the exterior of the church. Extractions were controlled by an hygro-regulated variable speed engine, which would stop operating whenever the difference between the entrance and exit relative humidity was lower than 5%. Ø200 MOTOR DE VELOCIDADE VARIÁVEL HIGROREGULÁVEL -0,65 EXIST. Ø315 System of higro-regulable variable speed EXI ST. Ø300 Ø300 System of higro-regulable variable speed FIG. 6: Plan of the wall base ventilation system (inside and outside) system of higro-regulated variable speed. MOTOR DE VELOCIDADE VARIÁVEL HIGROREGULÁVEL -1,65 Each ventilation subsystem had two probes and two temperature and relative humidity transmitters, a control module and a data acquisition system for recording and operating the air extraction device. 3.2 Results In Fig. 7 it is presented the differences of vapor pressure and the periods of functioning of the system. We can see that we have some periods of time where the system is operating but where we have condensation problems.

6 FIG. 7: Differences of vapor pressure and periods of functioning of the system. With the goal of analysing the occurrences of condensations inside the system we focus on the differences of pressure between the exit and the entrance of the system that sometimes were negative, what means that exist periods of time where we have condensation problems. To solve this problem or, at least, minimizing the possibility of occurrence of condensations we place the cited hygro-regulated system. Whenever the air that enters presents a relative humidity near the existing moistness in the interior of the system, it doesn t work having only natural drying. If we analyse Fig. 7 we can cross information between the periods where the system is working, mechanically and the periods where the condensations occur. We can see that the criterion of 5% of difference between the exit and the entrance humidity isn t effective. The system doesn t work in the periods of drying, sometimes functioning in periods where we have condensations, being able to contribute for its intensification. We propose a criterion based on vapour pressure where the system works when the difference between the vapour pressures at the exit an entrance is positive, which should be more efficient as we can see on Fig. 8. On the other hand, the occurrence of internal condensations can contribute to the crystallization/dissolution of salts present in the water warping the surface pores of the wall, putting in cause the effectiveness of this system. The criterion we used to stop the system should be function of vapour pressure instead of relative humidity. It is suggested to stop the system, whenever the vapour pressure at the entrance is superior to the pressure at the exit, minimizing the occurrence of condensations. It is considered that the recorded values of relative humidity, between 60% and 95%, do not cause serious risks of crystallization/dissolution of salts in the interior of the system.

7 FIG. 8: New criterion of system operating and periods of functioning of the system. 4. Conclusions Our assessment of the results led us to the following conclusions: The wall base ventilation system is effective in historical buildings with thick and heterogeneous walls; A mechanical hygro-regulated system is essential to control possible condensations inside the system; Although it will be interesting in future to assess the possibility of the occurrence of crystallization / salt dissolution, we believe that this problem will be prevented by the hygro-regulated system; The in field measurements, which are in course since October 2006, allow the results of the experiments to be validated in the laboratory. 5. Acknowledgement The authors would like to thank the FCT Fundação para a Ciência e Tecnologia, for supporting this research in the frame of POCI/ENR/61524/ References [1] Torres, M.I.M. (2004). Humidade Ascensional em Paredes de Construções Históricas. Ph.D. Thesis, Coimbra, FCTUC, 229 pgs (in Portuguese). [2] Freitas, V.P., Guimarães, A., Torres, M.I.(2007). The Experimental Characterization of a Wall Base Ventilation System for the Treatment of Rising Damp in Historical Buildings. 12th Symposium for Building Physics. Dresden, de Março de [3] Torres, M.I., Freitas, V.P. (2005). Treatment of rising damp in historical buildings: wall base ventilation. Building and Environment, 42 (2007), , January, [4] Kunzel, H.M. Connection between liquid water and vapour transport in porous media and its consequences for heat and moisture transfer models. Contribution to the Science. London. 12 pgs., December, 1992.