Royal Institute for Cultural Heritage (KIK-IRPA), Laboratories Department, Brussels, Belgium; 2 Daidalos-Peutz Engineers, Leuven, Belgium

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Hygrothermal analysis of the façades of the former veterinary school in Anderlecht (Belgium) for the risk assessment of internal thermal insulation Roel Hendrickx 1 ; Hilde De Clercq 1 ; Friedl

1 Hygrothermal analysis of the façades of the former veterinary school in Anderlecht (Belgium) for the risk assessment of internal thermal insulation Roel Hendrickx 1 ; Hilde De Clercq 1 ; Friedl Decock 2 ; Filip Descamps 2 1 Royal Institute for Cultural Heritage (KIK-IRPA), Laboratories Department, Brussels, Belgium; 2 Daidalos-Peutz Engineers, Leuven, Belgium 1. Introduction 1.1. The veterinary school in Anderlecht The veterinary school of Anderlecht consists of a complex of some 20 large and small buildings on a 4 hectares domain, constructed between 1903 and 1909 in the Cureghem area. Since 1990 the whole site is protected as a monument and the faculty which occupied it moved its activities to a modern campus outside the city. From 2000 on the largest part of the site was developed and renovated for private housing and offices, except for the administrative building, which was acquired by the town of Anderlecht. For several years, the restoration strategy and the adaptive reuse of the administrative building have been the subject of debate. Several projects were submitted and refused. The current project by the architecture offices ARTER and HASA proposes to create 2400 m² of low energy offices for a group of small companies. Daidalos-Peutz engineers were involved for the study on the energy efficiency, including the feasibility and the concept design of the interventions. The aim is to comply with the low energy standard defined by the regional government in order to get subsidies and make the office spaces more attractive for potential tenants. However since the start of the project, the decision-process has been a balance exercise between the conflicting interests of energy effi ciency (and economic benefit) on one side and heritage values on the other. The Royal Institute for Cultural Heritage was asked for advice about the restoration of mortars and stones, and was also charged to investigate the hygro-thermal behaviour of the walls and to assess the risks related to internal thermal insulation. The whole complex is built in a style which is a mix of Flemish Renaissance and French Classicism. Technically however it was a modern building for that Fig.1 - Rear façade and left façade (left image) and the angle of the front and right façades (right 1092 image)

period, with cast iron columns, large windows, and floors in early concrete between steel beams, similar to industrial and art nouveau buildings of that time.

2 period, with cast iron columns, large windows, and floors in early concrete between steel beams, similar to industrial and art nouveau buildings of that time. The outside masonry of most buildings is in red brick on a bluestone pediment, with decorative elements in Luxembourg sandstone (Fig.1). The administrative building however, which is located along the street, has 3 façades in French Euville limestone with Belgian bluestone on the ground floor while the rear façade, facing the other buildings, is in brickwork (Fig.1) Scenarios to reduce primary energy use The reference scenario is the actual building with new standard installations for heating and lighting, but no cooling. Cooling seems desirable for the 2nd floor, where a lecture room and a hall are illuminated by large windows. There is no thermal insulation and windows have single glazing. Air tightness is rather poor. The project foresees utilities and meeting rooms in the basement, offices on the ground and 1st fl oor, and common spaces on the 2nd floor as well as under the roof. In the feasibility study 2 scenarios have been investigated, both proposing thermal insulation of the roof, walls and floor, and double windows placed behind the original windows. The difference between both scenarios lies in the ventilation system, which is based on natural infiltration through existing openings and mechanical extraction for scenario A and on a more controlled mechanical infi ltration and extraction with heat recovery for scenario B. In this early stage of the project, the proposed thermal insulation system for walls was 12 cm of calcium silicate board, glued on the inside surface of the walls. Based on the fi rst estimations, the reduction of primary energy use for heating could be 53% or 58% for respectively scenario A and B. For both scenarios, the largest gain is obtained by insulating the roof and retrofitting the windows. The heat transmission coeffi cient for the walls ( U value ) decreases from 1.0 W/m²K to 0.27 W/m²K ; the contribution of the façades in the total transmission losses increasing slightly from 20% in the existing situation to 22% in the insulated situation. This is due to the particularly important effect of insulating the roof. Table 1. Calculated energy demand for the existing situation and two scenarios for energetic retrofi t This result is however based on static calculations of heat losses using the standard software. A dynamic simulation based on a constant internal temperature of 20 C gives different results: 2900 kwh/m²year without insulation versus 870 kwh/m²year with insulation. This is the value obtained as convective/ conduction heat exchange between the inside space and the inside surface of the wall. The absolute numbers are an overestimation because of the rigid assumption of the inside climate and the difficulty to account for radiation effects. 1093

3 Anyway, it seems reasonable to assume that the heat losses through the walls can be reduced by a factor 3 by applying the insulation Insulation methods for walls and their implications Due to the impossibility of external insulation or cavity insulation, the only option for the school building is to apply insulation on the inner surface of the walls. From a hygro-thermal point of view, this is not the preferred option, because of several drawbacks: 1. the connection of structural elements such as walls and floors leads to thermal bridges; 2. there is a risk for interstitial condensation when water vapour diffuses through the wall; 3. the masonry is colder because of the reduced heat losses from the interior, and hence it dries out slower and remains wetter throughout the year, which might increase the risk of frost damage; 4. the heat capacity of the walls is thermally disconnected from the inner space, which might lead to a risk of overheating during summer. The first risk leads to a decreased energy efficiency and the last to a decrease in comfort, but this paper focuses on the other two, which form a direct risk for the heritage value of the building. The type of insulation material and the way of applying it, have an important influence on this risk. In general, the available systems can be divided into two types [Roels, Vereecken, 2012]: 1. systems with a traditional insulation material like mineral wool, extruded polystyrene (XPS) or polyurethane (PUR) which are vapour-tight or which have a vapour barrier on the inside, and are finished with a gypsum board; 2. porous mineral or vegetal materials which allow to buffer and transport water ( capillary active materials), either glued on the wall on a thin continuous glue mortar or fi xed with screws or staples. Calcium silicate, insulating plasters and wood fi bre board are examples of this type of systems. The principle of both systems is visualised in Fig.2. The vapour barrier can be a smart vapour retarder, which is more permeable to vapour when wet. In both cases, we assume a good contact with the wall and the absence of important air leaks, which otherwise may cause the system to be inefficient. Fig.2 - Principle sketches of vapour-retarding insulation systems (left) and capillary active insulation systems (right). The blue shading indicates the places where moisture can be expected The dotted block arrows indicate a drying flux

2. Measurement of material parameters and methods for modelling In order to be able to simulate the hygro-thermal performance of the walls in the present condition and compare it to their performance

The most critical parameters for a successful application are the moisture transport parameters of the original brick, the Euville limestone, jointing and bedding mortars, as well as the new

4 2. Measurement of material parameters and methods for modelling In order to be able to simulate the hygro-thermal performance of the walls in the present condition and compare it to their performance when insulated, a numerical HAM ( Heat, Air and Moisture ) simulation tool can be used. The most critical parameters for a successful application are the moisture transport parameters of the original brick, the Euville limestone, jointing and bedding mortars, as well as the new materials: glue mortar, calcium silicate, insulating render and fi nishing render. Table 2 gives an overview of the important parameters and the tests which were performed to obtain them [Hendrickx, De Clercq, 2013]. The parameters of the new materials were based on material data sheets and on the standard parameters in the library of the Delphin code, which was used to perform the simulations [Nicolai, 2007]. Table 2. Overview of moisture transport parameters and the method to measure them The effect of two insulation systems has been simulated (with 12 cm calcium silicate or with 3 cm insulating render) on three 1D models of a wall (the brick wall, the stone wall or a continuous mortar joint) (Fig. 3). Investigation of the mortars revealed that the bedding mortar was the same for the brick and the stone, but that the jointing mortars, which had also a distinct appearance, were different in nature. The thickness of the wall is set at 70 cm, which is representative for most sections. Inside conditions were set at 20 C and a relative humidity of 50%. The outside conditions were based on the climate data of the German city Essen, which has a climate comparable to Brussels, using the most exposed orientations of the building. Simulations were done for a period of 2 or 3 years, the last one of which is visualised in the results. This was done to allow for the system to reach equilibrium, because thick Fig.3 - Overview of the 1D simulation models of walls. The outside surface is at the bottom. The total thickness of the existing wall is approximately 70 cm 1095

5 walls may have an important hygric inertia. Key questions to address are: (1) is there an important accumulation of moisture inside the wall, (2) is there any risk of wet stains on the inner surface due to penetration of water, and (3) is there an important increase of dangerous freeze-thaw cycles in zones close to the outside surface? 3. Results and discussion The results can be visualised in the form of colour plots of the moisture content over the depth and over time. As an example, the brick wall is shown in Fig. 4. The composition of the wall is as shown in the left 3 drawings in Fig. 3. The diagrams illustrate that the existing wall has a normal behaviour of wetting during periods of driving rain and fast drying during dry periods. The core of the wall becomes only slightly moist and the moisture front never reaches the inside. When 12 cm of insulation is applied, the cooling of the wall leads to much slower drying and higher moisture contents in the core of the wall. The glue mortar of the insulation is permanently moist, but there is no problem at the inside surface. The intermediate case of a thin insulating render causes only minor changes in comparison to the current state. These results can be used to assess the risk for frost damage. Wet stone, mortar and bricks can crack when they are subjected to frost-thaw cycles. Damage always occurs near the surface, because of the larger temperature and humidity amplitude, but also because of 3D stress state, which leads to important shear stresses. In Fig. 5 the temperature at 3 mm below the surface is plotted for the brick wall for the 3 scenarios. It is clear that the application of insulation leads to a decrease of the temperature by about 2 C for the 12 cm of calcium silicate and less for the insulating render (about 0.5 C). It is clear from this plot that the number of frost-thaw cycles may be influenced by this relatively small difference. However, because of the increased humidity of the wall, the risk of frost action associated to those cycles increases. In Table 3 the calculation results are shown for another type of model (the continuous joint) for the 3 scenarios. Here it becomes obvious that the number of wet cycles increases (relatively) much stronger than the overall number of cycles. As the material was considered as frost-sensitive, this result justifies the choice for an insulating render instead of a calcium silicate system. Whether or not a material is susceptible to frost damage is not always clear and it still is an important issue in conservation science [Ingham, 2005]. In the case of the veterinary school, an approximate estimation was done via an assessment of the porosity and the water absorption characteristics. However these results are merely indicative. About Euville limestone, some literature was available and experience of other research institutes was taken into consideration. The most important source of information on the sensitivity of the materials was found in the façades of the complex. In fact the 100 years of exposal to the climate allows to assess the frost damage in real life, although it is important (and far from evident) to distinguish the influence of sulphate attack on limestone and of other damaging phenomena from the influence of frost. The final project proposal, based on these investigations, consisted of a di- 1096

6 Fig.4 - Evolution of the moisture content of the brick wall (colour code expressed in kg/m³) during 1 year for 3 different build-ups of the wall. The inside surface is on top, the outside at the bottom 1097

7 Fig.5 - Temperature evolution 3 mm below the surface of the brick wall for 3 scenarios during 10 days in winter Table 3. Numerical calculation of the risk of frost damage, expressed in number of frost-thaw cycles at different levels of moisture saturation stributed application of calcium silicate in 2 thicknesses over the walls which were assessed as being not too frost-sensitive, and an insulating render over the rear façade, of which the joints were judged to be too vulnerable. The quantification of the increase of the risk and the thorough assessment of the materials susceptibility for damage, allowed for a well-founded solution to an otherwise very subjective problem. References Roels S.,Vereecken E., 2012, Inside insulation materials and techniques and their application in historical buildings (in dutch), in Innovatieve materialen en technieken in de monumentenzorg, Brussels, 23 November, De Clercq H., Hendrickx R., Herinckx S., Vanhellemont Y. and Vernimme N., Eds., KIK-IRPA, Onroerend Erfgoed, BBRI, VCB, Hendrickx R., De Clercq H., 2013, Parameter determination for heat and moisture simulations of repair mortars, in Proceedings of the 3rd Historic Mortar Conference, Glasgow, september, Hughes J., Ed. 2013, p. (submitted for review). Nicolai A., 2007, Modeling and numerical simulation of salt transport and phase transition in unsaturated porous building materials, PhD thesis, Syracuse University. Ingham J.P., 2005, Predicting the frost resistance of building stone, «Quarterly journal of engineering geology and hydrology», 38, 4,

AN EXPERIMENTAL STUDY OF

Micro-CT and nano-ct for the hierarchical analysis of materials May 24 th, 2011 AN EXPERIMENTAL STUDY OF THE HYGRIC PERFORMANCE OF DIFFERENT INTERIOR INSULATION SYSTEMS PhD Hygrothermal analysis of interior