Abstract
Based on modeling and measurements, the authors investigated the performance of expanded polystyrene and lightweight aerated concrete slabs. The objective of the work was to assess the performance of the brick wall (having various thickness and layering) commonly built in the Silesia Region (Poland) in the 1920s and the changes of water content in layers of the flat wall through the measurement of temperature and relative humidity in the selected layers of the masonry wall.
Introduction
The literature offers descriptions of the design methodology for interior thermal insulation (Gonçalves, 2003; Straube and Schumacher, 2007). Some of the publications are of practical character (Schöberl et al., 2012; Straube et al., 2007) and present design details as well as insulation thickness. The works by Hens (1998) and Wilkinson et al. (2009) present a complete design for the existing public utility buildings with historical features. The analysis involves both hygrothermal modeling of the whole envelope and measurements of selected physical parameters (temperature and humidity) at selected locations. Sometimes, the research was used to verify models of transport processes (Fechner et al., 1999; Stopp et al., 2016). Sometimes, several measurements were conducted (Kloseiko et al., 2015; Rousseau and Maurenbrecher, 1990) when analysis involved buildings of significant historical value. Yet, for single-family houses, or multifamily residential buildings, where the interior insulation is undertaken by the owners generally available, inexpensive insulation materials are used.
The authors attempted to analyze the water content (WC) of walls with thermal insulation from the inside made from two insulation materials of different diffusion resistance factors. The objective of this work was to find answers to the following questions:
Does the type of insulation material in the applied system of brick walls insulated from the inside have any impact on the moisture level of layers in the envelope and to what degree?
How is the applied insulation material going to influence the process of water transfer in the brick wall and how is it going to react to the changing hygrothermal conditions of the surrounding environment?
The scope of research is composed of in situ tests and modeling tests—simulation with the use of the program WUFI® 2D (Künzel, 1994, 1995).
Measurement of temperature and humidity inside the wall
The building, with gravitational ventilation, was located in the region of Silesia. It was built in the 1930s using traditional full brick technology. The thickness of the external weight-bearing walls is 38 cm and that of the external curtain walls is 25 cm. All the walls are plastered on one side with cement–lime plaster to a thickness of 1.5 cm. The floors are supported by wooden beams. The test area included living room joined with a kitchenette that was normally used.
Thermal upgrade was executed on a south-facing, load-bearing wall using two variants: (W_1)—expanded polystyrene (EPS) of thickness of 10 cm and (W_2)—a slab from lightweight concrete of density of 115 kg/m3 and thickness of 10 cm. The material data provided by the manufacturer were as follows: EPS: λD = 0.040 W/m K and diffusion resistance factor µ = 45; lightweight concrete: λD = 0.042 W/m K and diffusion resistance factor µ = 4. The insulation covered an area of 8 m2 divided into two equal parts (Figure 1). In the selected layers of the envelope, a measurement system was installed for data acquisition of relative humidity (RH; ST-171) and the distribution of temperature (Pt-100) in particular planes of the envelope in the room. Additionally, the measurement apparatus installed on the building enabled the measurement of quantities characterizing the external environment: RH and the temperature of the outside air. The measurement of all the quantities was continuous, with the time step of 1 h. To measure the temperature and RH, we applied a multichannel logger type MA56902M09TG3 and sensors Pt-100 and ST-171 for the measurement of temperature and RH. Prior to the actual measurements, the storage humidity (which is the ratio of the mass of water or water vapor contained in the material to the mass of dry material) was measured using the apparatus Testo 435-2 (www.testo.com.pl)—for brick walls—2.5%.
Schematic diagram of the test: (a) cross section and (b) thermal insulation slabs adhered to the wall.
In situ tests
Results from 4000 measurements of temperature and RH are presented in Figures 2 to 6. One can observe a difference in water transport (Figures 4 and 6). For both envelopes, the average temperatures across the contact surfaces assume the values ranging from around 12°C for the winter months and 25°C for the summer months. Low temperatures occurring across that plane in the winter season (<10°C) may cause condensation of water vapor (Figures 2 and 5). The average temperature for the month of February is much higher than that which is given to the nearest weather station for this town (2.3°C Katowice).
Measured temperature of the contact surface between the wall and insulation materials.
Relative humidity of the exterior and interior air.
Relative humidity on the contact surface between the wall and insulation material.
Average values of temperature in the selected months of the measurement period (notation 1—place of installed sensor).
Average values of relative humidity in the selected months of the measurement period (notation 1—place of installed sensor).
The calculated average monthly values of RH across the contact planes of the envelope layers are considerably higher between the insulation layer and the layer of plastered ceramic wall (Figures 4 and 6). Temporarily, the humidity of the material (plaster) was higher than the RH of the external environment, and in the remaining winter months, it was at the level of 80% RH. It means that the material absorbed a considerable amount of moisture and was transferring it to the surrounding layers. For the wall insulated with lightweight concrete, the boundary value of RH for February was exceeded, that is, 83% (Figures 4 and 6). Such a level of RH in such conditions requires that the saturation degree (relation of WC in air to the amount of WC in air filled with steam) should be analyzed.
In the summer months, in the variant W_2, a drop in RH takes place, which means that the process of moisture return takes place until, in the summer months, it reaches the value of ∼40% RH. The dependence of thermal conductivity on moisture for the material of that type has a linear character, and for air, RH higher than 70% the thermal conductivity of material rises within 60%–70% (Trochonowicz et al., 2013).
For both cases, the measurement of thermal transmittance was carried out in each month, using the apparatus Testo 435-2 (multipurpose measuring instrument for air conditioning, ventilation, and heat quality assessment divisions). For the wall insulated with EPS (W_1), the measurement was within the range of 0.157–0.46 W/m2 K, and for the wall insulated with aerated concrete (W_2), it was 0.42–0.69 W/m2 K (the device gives instantaneous measurement, shows the minimum and maximum values of the measurements, and the results are the values obtained from measurements of instantaneous).
For the contact surface of the wall with the EPS, we can observe a tendency to inhibit the impact of the internal environment. The objective of such a solution is to reduce the penetration of moisture into the wall structure, and it would be reasonable to accept that the material behaves in line with the assumed solution. The only thing that might be viewed as disturbing is the high level of humidity along the contact EPS/wall: 65%–80% RH (for monthly average values) but still within safety limits and at the constant level over the year. We can add that in the investigated time period, the RH in the room was at a level of ∼35%—winter season, which can be regarded as acceptable from the viewpoint of humidity balance of the envelope (not necessarily in terms of the comfort experienced by the user of the room).
Modeling
Hygrothermal modeling was carried out using the WUFI 2D. WUFI takes into account the following: thermal conduction, enthalpy flows through moisture movement with phase change, short-wave solar radiation, and nighttime long-wave radiation cooling. The vapor transport mechanisms included in WUFI are vapor diffusion and convective vapor transport by airflows, which has been ignored (Tables 1 and 2).
Input data to WUFI.
EPS: expanded polystyrene; RH: relative humidity.
Physical properties of the interior insulations.
The liquid transport mechanisms taken into account are capillary conduction and surface diffusion. The initial temperature in the envelope was accepted at the level of 20°C.
The boundary conditions for each time step are expressed in terms of meteorological data (temperature, RH, driving rain, and radiation) since in building physics, these are the relevant parameters specifying the conditions at surfaces exposed to natural weather. For the needs of calculations, we accepted the parameters of external climatic conditions nearest to the research stand—weather station in Krakow. The climatic conditions on the inside wall were in accordance with Standard EN 15026:2007 (2007). The following conditions were accepted: the temperature fluctuates from 20°C (winter) to 25°C (summer), and RH changes from 30% (winter) to 60% (summer).
All the simulations were initiated with the start of the heating period, that is, 1 February. Hygrothermal conditions in the state of dynamic equilibrium were considered. Therefore, all the simulations were carried out to the moment when the fluctuations of WC were effected solely by seasonal changes. The results of the analyses were presented for the selected 1 year (full cycle, according to the guidelines of Standard PN-EN ISO 13788:2013-05 (2013). The results of hygrothermal calculations (total WC and RH) were presented for the selected places in the envelope (Figure 7).
Diagram of the envelope with the marked places for analysis.
Figures 8 and 9 present the calculated hygrothermal conditions in the selected planes of the insulated envelopes (W_1 and W_2) in line with the 3D simulation. In all the investigated cases, initially the WC and the RH increased. The highest WC is observed along the contact area between the insulation and brick mortar, maximum 8.70 kg/m3 for W_2 and 7.80 kg/m3 for W_1.
Water content in the selected planes of the wall insulated with EPS (results obtained in the program WUFI).
Water content in the selected planes of the wall insulated with lightweight concrete (results obtained in the program WUFI).
The results of the carried out analyses demonstrate that for each of the insulation variants, on the back side of the insulation, the boundary value of the RH of 95% is not surpassed even during the winter time—maximum 73% (W_1) and 70% (W_2). In summer, the drying-out level is sufficiently good: 65%–54% RH (Figure 10) for W_1 and 60%–52% RH for W_2 (Figure 11). The levels of RH do not surpass the boundary values, and hence, there is no need to investigate the saturation level and freezing risk. For both envelopes insulated from the inside, we can observe a drying-out tendency and moisture drop (Figure 12).
Relative humidity in the selected planes of wall insulated with EPS (results obtained in the program WUFI).
Relative humidity in the selected planes of wall insulated with lightweight concrete (results obtained in the program WUFI).
Total water content for wall with EPS and wall with lightweight concrete.
In Figures 13 and 14, the authors have compared the results obtained on the basis of measurement and modeling. Temperature and RH were analyzed. In the case of changes in the RH that was obtained through measurement, it can be observed that there are discrepancies in the quantitative description in relation to model values for both kinds of the insulated wall (W_1 and W_2). These differences are due to accepting the parameters of external climate from the program base (Cracow) and not for the actual localization. In addition, the program uses climate average values, which is also responsible for the differences. However, the results of modeling confirm the general tendency of the walls reaction during the measurement process.
Comparison of the modeling results with the research results—measured relative humidity of the contact surface between the wall and insulation materials (s_4).
Comparison of the modeling results with the research results—measured temperature of the contact surface between the wall and insulation materials (s_4).
The temperature changes on the meeting point of the wall and insulation material are similar for both types of the insulations that were used—and this is confirmed by the measurement results as well as numerical analysis (Figure 14). The results obtained on the basis of the model are less advantageous than the measurement results because other boundary conditions—different from those in the given measurement period of time—were accepted.
Summary and critical comments
The results discussed in this study may seem controversial. There are two main reasons for this: one is that the research period of time was short, and the other is that discrepancies occur between the measurement results and modeling.
It must be highlighted that the authors have been the first ones to analyze brick walls in old buildings that are owned by people. Such buildings represent a high percentage of houses built in Silesia (Poland) and in other cities where there is old housing infrastructure. Obtaining the permission to conduct the examination and the possibility to monitor the partition and rooms proved to be a huge challenge.
The authors’ aim was to state—on the basis of modeling—that the tendency/direction of hygrothermal changes takes place in a partition insulated from the internal side. The measurement results confirm the partition reaction in the research period of time with the simulations that were carried out. The authors are aware of the fact that there are discrepancies between the computer simulation and the actual course of phenomena, but they claim that this is due to a short research period and the lack of up-to-date climate database concerning the measurement localization or another place situated in close proximity to the building examined.
The research has the usefulness of both materials used as thermal insulation on the internal side. At the end of the research period, the brick wall in the habitable room did not show any increase in the WC level in conditions of regular exploitation when compared to the initial situation.
RH measured in specific layers during the research period did not pose the risk of fungoid growths (RH values above 80% which occurred in February were temporary and were not observed for the rest of the months). Having observed changes of RH on the contact points of different materials, it can be concluded that light concrete used here is characteristic of a great capability to change quickly the levels of moisture, but in the longer term, it stabilizes water flow through the whole partition keeping it at an adequate level.
The simulation carried out in the WUFI 2D program shows that despite temporary dampness in winter, the partition is likely to channel away the accumulated water later in summer no matter what insulation material was used. The results of the numerical simulation of water flow, carried out in the same conditions as in the research period, confirm a general tendency: there is no increase in the water amount in the brick wall insulated from internal side.
The research that has been conducted gives unequivocal answers to questions concerning the low amount of water in the brick wall after using two thermal insulations with different values of moisture permeability. However, it must be clearly stated that this research refers only to a flat surface of the wall distant from construction joints. Areas such as joist supports, junctions of internal and external walls as well as building corners were not subject to the examination.
Inhabitants had not been instructed prior to the research on how to behave as far as interior moisture production was concerned assuming that they would behave as usual. Weather conditions during the research period were a favorable factor. Winter–spring season was much warmer than in the past years and was different from a typical meteorological year. The average temperature of outer air was higher by about 1.4 K than the temperature of a typical meteorological year. This leads to some deviations of the results. Despite this, both the research and the simulation clearly indicated the same phenomenon—decrease in the water amount in a brick wall.
We can accept that the measurements of that type should be carried out over longer time periods of research studies, for example, 5 years, instead of the period specified in the guidelines of Standard PN-EN ISO 13788:2013-05 (2013; 1 year). Yet, such long research studies would be too tiresome for the inhabitants and would not contribute much more knowledge. For the design purposes of internal thermal insulation, the simulation results are acceptable and representative to carry out when we take into account the local climate and real exploitation conditions of the rooms.
Footnotes
Declaration of conflicting interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) received no financial support for the research, authorship, and/or publication of this article.