Evaluation of the Impact of Bricks of Various Characteristics on Internally Insulated Masonry Walls in Cold Climate

Abstract

Energy consumption in historic building stock is high compared to current energy efficiency standards. The heritage value of the façade and the limited space on the external surface in densely populated urban streets limit the application of external insulation. Internal insulation can be applied instead. However, it is considered to be a riskier technology due to moisture-related damage. In addition to mold growth and wood rot, frost damage should be considered in cold climates. This study aims to assess the impact of a vapor-open capillary-active calcium silicate internal insulation system with and without adhesive glue on the hygrothermal behavior of masonry from various historic bricks in cold climates by performing numerical simulations in the software Delphin. Test results of hygrothermal properties of 40 historic brick samples were used in numerical experiments to assess the impact of a brick type, the quality of the application of calcium silicate (with or without adhesive), and the impact of cold climate on the hygrothermal behavior. Results show that temperature behavior is similar to all wall types whereas a large difference is observed in moisture behavior. The application of adhesive glue tends to reduce moisture spikes caused by rain events when compared to the same samples without adhesive. Findings only partly correspond to other studies on factors affecting moisture behavior.

1. Introduction

The high energy consumption in historic buildings is determined by the poor thermal properties of a building envelope. As a significant share of the thermal energy losses are through the opaque building envelope (exterior walls), the insulation of exterior walls can significantly reduce energy consumption of the building and thereby reduce greenhouse gas emissions [1]. Existing buildings can face restrictions for applying external wall insulation. First, due to the cultural heritage value of a building, and, second, technical limitations due to the geometry of the building or location within the densely built urban environment. In these cases, internal insulation should be applied. This type of energy efficiency measure has a high impact on the hygrothermal behavior of the wall, leading to a higher risk of frost damage, mold growth, and decay of embedded wooden beams [2]. If thermal insulation is applied without preliminary investigation of the existing moisture issues in the building, it can lead to increased moisture-caused problems. Moreover, if these problems are not eliminated prior to the refurbishing, they will intensify in a short period of time and can cause irreversible damage to the building’s thermal envelope and construction [3].

The assessment of heat and moisture transfer in porous building materials is the central issue when internal insulation is applied in buildings. Material properties are crucial when internal insulation is considered. They include general, thermal, and hygric properties. General properties that are essential for hygrothermal behavior are bulk density and open porosity. Thermal properties include thermal conductivity and specific heat capacity. Moisture storage in a material is characterized by saturated moisture content, capillary moisture content, sorption isotherms, and retention curves. Transport properties include the capillary absorption coefficient, vapor permeability, liquid permeability, and liquid diffusivity [4]. Various test methods are available to test the thermal and hygric properties of porous building materials in the full humidity range [5].

Existing masonry and historic bricks and their hygrothermal properties have been studied [6,7,8,9,10,11,12]. If historic masonry is covered with an external render, the hygrothermal performance of internally insulated walls depends mainly on the liquid permeability of the exterior finishing render [13]. De Mets et al. (2017) found that brick type and driving rain load significantly impact the moisture levels in masonry with internal insulation. They concluded that the absorption coefficient of a brick cannot be used as a single factor to assess the hygrothermal impact of interior insulation, and the liquid water conductivity has a higher impact [2]. However, the liquid water conductivity is difficult to estimate because there is a lack of a single method to directly measure the liquid permeability at low and intermediate moisture content [14]. Isothermal measurements alone are not sufficient to correctly simulate the capillary condensation process and the drying-calibrated model underestimates the moisture content [15]. A vapor-open exterior water-repellent façade impregnation or rendering is an important measure against wind-driven rain if internal insulation is applied [2]. Another study found that the pore size of a material strongly influences its hygric properties. Small pores mainly increase the hygroscopicity (e.g., the sorption isotherms), while large pores primarily enhance the capillarity (e.g., the capillary absorption coefficient). If pore size distribution is known, one can estimate the overall hygric performance of a material [14].

For capillary-active internal insulation systems, driving rain load and the drying potential of the wall are the main factors affecting their hygrothermal performance [2]. They are safe if capillary-active internal insulation is applied in spaces with low or normal occupancy with no additional moisture source. Capillary-active internal insulation is considered to be safe if the relative humidity under the insulation does not exceed 99% [16].

A capillary-active insulation system of calcium silicate insulation is adhered to the masonry wall by a glue mortar so that the interstitial condensation can be buffered [17,18,19,20,21]. Good contact between the masonry wall and the insulation should be provided [20]. The functionality of the system is disturbed if contact between the masonry wall and the capillary-active material is discontinued. The adhesive glue limits the redistribution of the moisture from the interstitial condensation that occurs at the warm side of the masonry wall by the capillary-active material. A low capillary absorption coefficient of the adhesive glue diminishes the risk of moisture from wind-driven rain. The glue reduces the redistribution of potential condensation toward the masonry. The absorption coefficient of the glue depends on the curing conditions. The glue mortar can capture a large amount of moisture [22]. In another study, [23] conclusions are that capillary-active interior insulation cannot be applied in a cold continental climate and vapor-tight insulation has to be applied. They found that condensation in the masonry can appear if the capillary-active interior insulation with a relatively small water vapor resistance factor, high indoor moisture load, and no adhesive glue is applied.

This study aims to assess the impact of a vapor-open capillary-active calcium silicate internal insulation system with and without adhesive glue on the hygrothermal behavior of masonry from various historic bricks in cold climates by performing numerical simulations.

2. Methodology

In this study, the simulation software Delphin 6.1 is applied. Delphin is designed to simulate the heat, moisture, and air transportation and storage processes in porous construction materials. The brick files to be used in Delphin were created during laboratory tests of hygric and thermal parameters of 40 historic bricks from Latvian historic buildings from the 17th to the 20th century (Appendix A). Tests were carried out based on the testing standards defined by Dresden Technical University and ISO standards. The following parameters were determined: density, open porosity, thermal conductivity, specific thermal capacity, sorption isotherms, retention isotherm, water absorption coefficient, capillary saturation moisture content, water vapor diffusion resistance factor, vapor diffusivity, and vapor conductivity. Tests and test results have been published [24]. A clustering analysis of tested brick samples was carried out after material tests were performed. The clustering results of hygrothermal properties were cross-examined with clustering results of the Delphin simulation data. Six and three clusters were found to be optimal, accordingly for the hygrothermal properties and the Delphin results data groups. After cross-examination, a total of nine combined clusters were recognized, with two dominant clusters containing 67.5% of all samples (30% and 37.5%), four of the clusters containing only one sample in them, and other clusters containing two, three, and four samples in them [25].

Two slightly different Delphin models were created: one for the situation where calcium silicate insulation was glued to the masonry and another where calcium silicate was not glued to the masonry. Figure 1 shows the simulated structure with masonry (0.25 m) and calcium silicate (0.05 m), with an adhesive layer of 5 mm. Simulations were carried out for each of the 40 types of masonries: with and without adhesive glue under the insulation material. For the model without adhesive, the adhesive layer was not present in the model and the gap in between the insulation and the masonry was removed.

The properties of the insulation material and the adhesive glue are shown in

Table 1. Properties of the insulation material and the adhesive glue.

Parameter	Calcium Silicate	Adhesive (Glue)	Unit
Bulk density	225.04	820.033	kg/m3
Specific heat capacity	1129	1306.5	J/kgK
Open porosity	0.9136	0.690553	m3/m3
Effective saturation	0.90408	0.34	m3/m3
Capillary saturation	0.7211	0.316	m3/m3
Hygroscopic sorption value at 80% relative humidity	0.00622307	0.139355	m3/m3
Thermal conductivity	0.061	0.216	W/mK
Water uptake coefficient	0.7831	0.00801211	kg/m2s0.5
Water vapor diffusion resistance factor	2.3973	18.9365	-
Liquid water conductivity	3.08745 × 10−10	6.80328 × 10−12	s

.

As there are no strict criteria or threshold values that are used to determine whether climate can be considered to be a cold climate, in this study, a cold climate is referred to as climatic conditions where four seasons can be distinguished and outdoor temperature below 0 degrees Celsius are common during one of those seasons as well as a yearly average temperature close to 0 degrees Celsius. In the simulations, the outdoor climate uses data from the year 2022 from the Riga University weather station, gathered by the Latvian Environment, Geology and Meteorology Centre. Hourly rain loads are shown in Figure 2 [26].

Indoor climate conditions are set as a function of outdoor temperature (see Figure 3). The simulation period is set to 3 years to allow for stabilization of the initial values.

3. Results

3.1. Hygrothermal Behavior of Masonry with Capillary-Active Insulation without the Adhesive Glue

Figure 4 presents temperature profiles under insulation material for each brick type. The overall trend follows outdoor temperature behavior. However, the distribution of temperature varies due to the thermal conductivity of the bricks (varies from 0.4 W/mK to 2.7 W/mK). The higher the thermal conductivity, the lower the temperature level under the insulation. Only five walls reached the freezing point during the winter months.

Figure 5 illustrates that the amplitude of the temperature under the insulation is different for various bricks. The higher the temperature under the brick, the lower the amplitude is.

The relative humidity under the insulation also follows the trend of outdoor temperature (see Figure 6). The relative humidity does not exceed 96% in winter for any of the wall types. The relative humidity level reaches different values. Some walls are more susceptible to rain events than others. However, no correlation between relative humidity level and any other parameters (water absorption coefficient, liquid water conductivity, porosity, density) is seen.

Figure 7 shows the impact of rain on the relative humidity under the insulation. Before the rain, the distribution of relative humidity was 10%. During the rain, the relative humidity behavior changes: some of the bricks are more sensitive to rain, and the relative humidity level increases soon after the rain while, in other walls, it either happens with a delay or the wall does not show an impact from the rain.

An example of relative humidity under the insulation for two wall types shows (see Figure 8) that, in wall 20_8, relative humidity increases at a slower pace during autumn and decreases at a higher rate during spring compared to wall 18_4. Wall 18_4 stabilizes at 95%, but wall 20_8 does not stabilize and it reaches 96% for a short period of time.

Figure 9 presents masonry moisture content in all wall types. The moisture content trend follows seasonal changes: increases during autumn and winter and decreases during spring and summer. The difference between the lowest and the highest moisture content in walls is fourfold. Spikes in the moisture content indicate rain events (see Figure 2).

For the majority of walls, the moisture under the insulation was not influenced by rain events if the adhesive glue was not applied under the insulation (see Figure 10). However, 16 walls exhibited behavior that was influenced by rain events in winter and summer.

3.2. Hygrothermal Behavior of Masonry with Capillary-Active Insulation with the Adhesive Glue

When adhesive glue is applied under the insulation, moisture content spikes in the insulation layer are eliminated because the glue acts as a vapor barrier and a moisture buffer. Figure 11 shows the moisture content in the insulation for the wall 20_16 with and without the adhesive glue. It represents a wall type with high peaks in moisture content after rain. Wall 20_16 has very steep and high peaks. When glue was applied, it eliminated moisture peaks.

When adhesive glue was applied on the walls that did not have peaks after rain, it did not affect the moisture content behavior. Figure 12 illustrates the behavior of the moisture content in the wall 20_18 with and without glue.

Figure 13 shows examples of the temperature, relative humidity, and moisture content distribution at the end of the simulation (in December); the integral moisture mass changed over time in the wall 20_16 when the adhesive glue was applied. The moisture content was slightly higher at the external part of the wall and was stable at a high level in the masonry in the deeper layers. It peaked in the glue and was at a very low level on the insulation side. Min and max levels during the simulation show similar trends. The relative humidity in the masonry was at a very high level at the end of the simulation and was reduced on the indoor side of the glue.

Figure 14 presents the temperature, relative humidity, and moisture content distributions, as well as the integral moisture mass, which changed over time in the wall 20_18 when the adhesive glue was applied. The moisture content was high at the external part of the wall and was significantly reduced in the first 10 cm of the masonry and stabilized at a relatively lower level in the deeper layers. The absorbed water was transported more slowly to the inside and was stored in the exterior part of the masonry. It peaked in the glue and was at a very low level on the insulation side. Max levels during the simulation show a similar trend to the final values of the simulation. The relative humidity in the masonry was at a very high level at the end of the simulation and was reduced on the indoor side of the glue.

The adhesive is a highly porous material (porosity 69%) and therefore can absorb a high content of water. Moreover, we can see a peak in water content for both the 20_16 and 20_18 samples (see Figure 13 and Figure 14).

4. Discussion

The numerical experiments on the hygrothermal performance of 40 types of massive masonry walls internally insulated with vapor-open capillary-active calcium silicate in cold climates showed that all wall envelopes showed very similar temperature trends, whereas a large difference was observed in moisture behavior. This corresponds to the findings of Zhou et al. (2018) [13] that the influence of brick type on relative humidity and temperature under insulation is complex. Temperature variations between various wall types depend on the thermal resistance of the bricks.

Moisture content levels vary fourfold between wall types due to the moisture content distribution in the masonry that varies significantly between stable moisture levels throughout the masonry while also displaying stable levels at the external part of the wall followed by steep decreases in the deeper layers of masonry. At the external part of the masonry (closer to the outdoor air), the maximum masonry moisture reaches relatively similar (around 100 kg/m³) values for all the brick types. Data analysis showed that no correlation between moisture content, relative humidity level, and any other parameters (water absorption coefficient, liquid water conductivity, porosity, density) was found in this study.

The findings do not support conclusions from other studies that the hygrothermal performance of a massive masonry wall internally insulated with vapor-open capillary-active materials depends on the pore size distribution which determines liquid water conductivity of bricks: the higher this parameter, the deeper the masonry penetrates rain [14]. It also does not correspond to the finding that wall envelopes with high capillary-active bricks show larger relative humidity and temperature indices [13]. The pore size of a material strongly influences its hygric properties. Small pores mainly increase the hygroscopicity (e.g., the sorption isotherms), while large pores primarily enhance the capillarity (e.g., the capillary absorption coefficient).

If the quality of the construction is low, and the adhesive glue does not provide full contact with the insulation material, the moisture from the rain can penetrate into the insulation material. If the adhesive glue is in full contact with the insulation material, the impact of wind-driven rain does not diffuse into insulation material. These findings correspond with conclusions from a different study [2]. The adhesive glue has a high moisture buffering capacity if properly applied. Similar conclusions were drawn by another study [22].

Simulation results showed that if internal insulation with capillary-active calcium silicate is applied in a cold climate with a normal indoor moisture load, the relative humidity does not exceed 96% and is considered safe [27]. This conclusion can be applied to both insulation with and without adhesive glue.

5. Conclusions

• Temperature trends display practically no influences by brick type;
• Brick type has a noticeable impact on the hygrothermal behavior of the insulated masonry wall;
• There are no correlations between individual brick parameters and moisture dynamics within brick samples;
• Capillary-active internal insulation use in cold climate is safe as long the indoor relative humidity does not exceed 65% in warm months and 35% in cold months;
• The application of adhesive glue tends to reduce moisture spikes caused by rain events when compared to the same samples without adhesive.