Precast Concrete Building Construction and Envelope Thermal Behavior: A Case Study on Portuguese Public Social Housing
Abstract
:1. Introduction
- Firstly, one related to relevant social housing retrofit projects, where the recognition of the importance of a successful intervention in these contexts is growing, as proved by the 2019 Mies van der Rohe Award, awarded to a constructed retrofit project of 530 dwellings in the Grand Parc Bordeaux [13]. Here, a key strategy was to rethink the existing envelope regarding specific measures to improve habitability, such as introducing flexibility in spaces for balconies or winter gardens, alongside other relevant features such as window improvements. Another example can be observed in the Municipality of Covilhã (where the present study took place) for a social housing building neighborhood, which is currently being retrofitted under the European Union (EU)-supported “Portugal 2020” program, directed at energy efficiency improvement regarding actions in the building envelope such as the introduction of external insulation and window replacement (Figure 1);
- Secondly, one related to specific literature developed for several SEC such as Portugal, Spain, or Italy, regarding, among others, retrofit interventions in vertical envelope components. The work from Oteiza et al. [14] provides a general approach to retrofit these types of buildings, highlighting envelope interventions such as exterior enclosing walls retrofit as appropriate examples of combined actions to improve both energy performance and other relevant needs such as building and neighborhood images. The work of Suarez and Fragoso [15] studied the repercussion of envelope retrofit measures such as insulation, solar protection, and window improvements. For the latter, specific recommendations are made to achieve air infiltration reduction through the frames, glass solar control improvements using solar films, and window U-value reduction through the higher spacing between glass panes. A combination of all those measures alongside other relevant passive strategies such as ventilation can reduce heating energy demand by 3–6 kWh/m² and cooling energy demand by 5–6 kWh/m². Alonso et al. [16] propose a methodological approach for monitoring energy refurbishment, applying it in a case study in which several actions were detected as necessary to minimize opaque and open envelope sections debilities, for both winter—increased insulation, window air infiltration control compatible with low risk of condensation, elimination of thermal bridges and advantages from solar gains through glazing elements; and summer—the role of solar protection to reduce the effect of climate change frequent extreme events. The aforementioned studies approached envelope retrofit through a combination of specific measures. However, other approaches have also been studied regarding the effect of a specific measure. Curado and Freitas [17] present an analysis on thermal comfort for a reference dwelling in representative Iberian climate scenarios without heating/cooling energy consumption, considering only the effect of external insulation in facades, and the results show that additional insulation alone may be unnecessary or insufficient for winter/summer mild and severest scenarios, respectively. Literature such as that by Boeri et al. [18] should also be mentioned considering its importance in the field of social housing, mainly for its focus on economic conditioning of envelope retrofit interventions.
2. Materials and Methods
2.1. Case Study
2.2. Methodology
- Stage 1 was a preparation stage, consisting of assessments resorting to Municipality Archives, other entities, and in-situ visits to obtain quantitative and qualitative data that make it possible to perform the next stages;
- Stage 2 was a diagnosis stage, where quantitative and qualitative assessments were performed—verifying U-value requirements defined by SCE and an indoor thermography survey, respectively—to properly identify existing physical and thermal envelope characteristics;
- Stage 3 was a testing stage, resorting to the SCE framework to evaluate the impact of feasible constructive envelope retrofit measures regarding potential overall energy savings for this type of building in both coastal and inland Portuguese climates.
- The first concern is related to performing the study in an unoccupied dwelling. For the specific case of social housing buildings, occupancy also plays a vital role in the building’s overall thermal performance, mainly because of passive strategies used to control indoor environments. Studies such as that by Serrano-Jiménez et al. [34] resort to indoor measurements to investigate indoor environmental quality in social housing dwellings, and the used methodology includes high relevance on occupancy regarding issues such as ventilation patterns and window habits. Curado and Freitas [17] performed building energy simulations for a reference social housing dwelling, and an experimental campaign was performed alongside extensive interviews with the residents, to clearly define occupancy profiles to perform a successful calibration of the simulation model. Nevertheless, this was not the scope of the present paper. As the focus was to analyze the existing thermal envelope characteristics, common non-destructive procedures, such as indoor measurements and building energy/thermal modelling techniques were not applied as they are more suitable for studies on the thermal performance of a building where the influence of occupancy is also considered;
- The second refers to the study’s comprehensiveness. Although this study aims to present the analysis of NK1 thermal characteristics focusing on the building envelope, restrictions in accessing dwellings and other parts of the building limited this intention, especially regarding in situ visits and the thermography survey. On the other hand, the scope of this study is centered on the thermal characteristics of specific envelope components, and this is the reason why other relevant fields related to indoor thermal performance, such as ventilation, indoor partitions, or internal gains were not studied. Therefore, the envelope analysis was made only of the vertical envelope’s components, such as walls and glazed areas, and of the envelope’s connections between vertical components, indoor floors/ceilings, and the building structure.
2.2.1. Stage 1
2.2.2. Stage 2
Assessment Resorting to SCE (Energy Performance Certification System)
Thermography Survey
- Regarding the measurement method, it was selected to be qualitative, so color patterns obtained in recorded thermal images were evaluated to detect relevant superficial temperature differences to identify where and how possible anomalies may occur;
- Regarding the definition of the analysis scheme, the limitation of an unoccupied dwelling without electricity made it impossible to provide heating recurring to electrical devices. Therefore, passive thermography—by which the target is observed with temperature gradients resulting from current temperature states [32]—was selected instead of active thermography, in which the target is observed exposed to an external stimulus to obtain relevant temperature gradients [32]. Thus, the following targets of qualitative analysis were used to detect the existence of individual or repeated anomalies [31]: target symmetry, when the thermography is performed on different areas of the same surface; and target comparison, when the thermography is performed on surfaces located in different elevations with common constructive properties;
- Regarding the location where the survey was to be performed, two main procedures needed to be selected. The first consisted of selecting the approach to perform the survey. The traditional method, walk-through thermography, the most popular passive method used, which consists of a technician scanning relevant building envelope components from both internal and external sides using a thermal camera to detect thermal anomalies, which are then recorded as thermal images for analysis and inclusion in a report [31], was selected instead of street pass-by thermography, a quicker and cheaper procedure which consists of driving past buildings to capture single thermal images of elevation external surfaces, obtaining a considerable amount of information [31]. Studies performed comparing both methodologies have shown that walk-through thermography is more suitable for qualitative analysis [31], besides other aspects like the study feasibility. The second procedure consisted of selecting the possibilities within the previously selected approach, considering that the walk-through methodology means that surveys can be performed from the inside, outside or both. Some authors [39] defend the position that internal inspections should only be performed to validate the findings of external surveys. Other authors state that anomalies are more clearly shown in internal thermography [31,40], and some internally identified anomalies are not always detected in external surveys [31]. For these reasons, internal thermography itself was selected as adequate to the study.
2.2.3. Stage 3
- Case A: the application of external insulation, with repercussions in both heating/cooling seasons for opaque envelope elements;
- Case B: the substitution of existing windows by new ones, considering the effect in both heating/cooling seasons for glazed envelope elements.
- Thus, two more cases were analyzed considering their usefulness in cooling seasons as economical alternatives to window replacement in residential buildings [12]:
- Case C: the application of solar films in existing glazed elements;
- Case D: the application of rolling shades on the internal side of the existing windows.
3. Results
3.1. Stage 1
- The connections between both concrete layers are made during the industrial production process recurring to galvanized metallic clamps that go through the insulation layer;
- External walls accommodate both electric and domestic water supply infrastructures at specific points, and in some interventions where larger volume components must be installed, the insulation layer may be removed to accommodate them.
- Window U-value corresponds to the mentioned Uw value in Table 2. Nevertheless, considering that existing PVC roller shutters improve Uw value when activated, the Uwdn value is presented, which consists of a daily average value considering the activation of shutters during night periods;
- The glazed area corresponds to almost 13% of the dwelling exposed area;
- Among others (such as building exposure to wind or the existence of kitchen ducts), the type of window installed highly affects the expected air infiltration rates in buildings. As no information was found about this value in related documents, an SCE recommended LNEC tool [45] was used to calculate it as predicted. The value obtained was 0.98 ac/h for Covilhã, which is considerably high according to its potential impact on dwelling indoor thermal behavior;
- No information about the connections between glazed areas and external walls was found.
- In the points of the building envelope where the connections between external wall panels are located, the correspondent thermal insulation is not continuous, although it is partially or totally achieved in the connections with structural elements such as beams and columns;
- Vertical and horizontal joints between the panels and/or structural elements were made using a sealed insulation strip, normally mastic-based.
- Internal partitions were found to be constructed using precast concrete panels, while internal wall and ceiling surfaces were found to be finished with thick paint. Alongside the external wall panel solution, the dwellings may benefit from the effect of the high thermal mass;
- The ground floor is constructed with a concrete slab on a gravel base, with textile flooring or ceramic tiles as floor finishing;
- Internal floors are constructed with beam and block slabs, with textile flooring or ceramic tiles as floor finishing;
- The roof is constructed with beam and block slabs with fiber cement roofing. Thermal insulation is sometimes applied depending on the location of each project, although no information was found regarding its application in this specific building.
3.2. Stage 2
3.2.1. Assessment Resorting to SCE
3.2.2. Thermography Survey
- In northern orientated façades, no relevant anomalies were identified;
- In eastern orientated façades, two commonly anomalies related to high superficial temperatures were detected: (1) a considerable and continuous thermal bridge area below the beam, which matches the zone where the connection between external wall panels is made; (2) regular disposition of high-temperature points that were detected only in some of the surveyed façades, which may correspond to the points where the metallic clamps were applied to connect both panel’s concrete layers;
- In the southern orientated façade, two main anomalies related to high superficial temperatures were identified: (1) a considerable and continuous thermal bridge area below the beam, with the same characteristics of observed anomalies in eastern façades; (2) an isolated area detected above the floor, and that was not identified in any other surveyed envelope component, and that might be related to damage or no insulation due to the accommodation of specific infrastructure, according to the available data.
3.3. Stage 3
- Case A: the ETICS (External Thermal Insulation Composite System) system was tested as external insulation, using EPS 100 (20 kg/m³) with a 0.036 W/m °C thermal conductivity and 60 mm thickness. This system is a common wall insulation type in Portugal; the mentioned characteristics are the same as used in the example of Figure 1;
- Case B: the new windows tested consist of a PVC frame with clear double glazing of 6mm (outside) and 5mm (inside) and a 6 mm space between the panes. This solution was selected as its current market cost is moderate, compared to other available solutions. The possibility of highly efficient solar films was not applied to evaluate it separately from frame and double-glazing performances, as made in Case C. Therefore, the solar factor used was 0.75. Further related values were taken from the SCE database and indicative literature [44] once both already considered proper market information;
- Case C: the tested solution of solar films applied to existing glazing aimed to decrease the existing solar factor from 0.10 to a final 0.78 value as the building´s existing windows still use clear and uncoated glass. This value was taken from pertinent market information;
- Case D: the tested solution for internal rolling shades considered the possibility of considerable solar radiation restriction while allowing proper daylighting. Therefore, a solution was retrieved from the SCE database presenting solar transmittance between 0.15 and 0.25 and an absorbing factor of 0.50.
4. Discussion
- Regarding external walls, the sandwich solution clearly benefits NK1 compared to other constructive solutions applied until then [27], as it contributes to achieving U-values compatible with SCE requirements for retrofitting in any of the aforementioned locations. Nevertheless, the achieved values are considerably far from SCE required maximum U-values for new buildings, and this is the reason why it is advised that improvements are made. On the other hand, the use of metallic clamps to connect concrete layers may pose a weakness if the observed anomalies detected in some external wall panels during thermography surveys are considered. Improvements to solve this issue are strongly advised, considering the potential heat gain/loss through these components;
- In what concerns glazed areas, it is noticeable that existing U-values are problematic when compared to SCE required maximum U-values for both new buildings (Uw) and retrofit (Uwdn). For new buildings, the existing values are considerably far from SCE requirements applied to all winter scenarios. For retrofit, SCE requirements are achieved only in coastal locations (I1 climate zone) for both sliding and casement window types, while for regions with harsher winters, only casement window types achieve the required values. Thus, reducing high air infiltration is the most likely window solution, considering few other existing possibilities of air infiltration;
- Regarding connections between vertical components, indoor floors/ceilings and/or building structure, the solution applied in connections between external wall panels presents a considerable weakness regarding discontinuous thermal insulation, resulting in pronounced anomalies related to high conductivity gains due to thermal bridges. Nevertheless, the solutions applied in connections and/or joints between external wall panels, internal ceilings/floors, and the building structure (columns and beams) seem to obtain positive results, although a more complete thermography survey of the entire building envelope would be essential to confirm this assumption.
- Regarding the influence of façade orientation, Alonso et al. [46] studied the specific effect of external insulation in buildings located in Madrid, a city with severe winter and summer seasons, concluding that 15.4% energy demand reduction is achievable by adding external insulation to south-facing facades; nevertheless, this solution is not as effective when applied in facades exposed to solar radiation in warm climate conditions. For this reason, this is an issue that must be regarded for both coastal and inland locations;
- In terms of possible effects on thermal comfort, the work of Curado and Freitas [17] highlights the effect of this measure in several Iberian climate contexts. Results show that additional external insulation alone may be unnecessary for winter/summer milder scenarios to achieve thermal comfort. For severe winter/summer scenarios, this measure only constitutes an improvement, being that heating/cooling is required. Thus, the effect of occupancy should be considered when sizing this measure for both coastal and inland locations climates;
- Concerning possible effects when combined with the existent thermal mass, the work of Gonçalves and Graça [47] strongly advises the combination of these measures for locations with high annual and daily temperature variability. Nevertheless, some issues must be regarded. As stated by Stazi et al. [48], recommending adaptable constructive measures to find a proper solution for both winter and summer seasons can constitute a major challenge in Mediterranean climates. The study of Tribuiani et al. [49] highlights the importance of the type of insulation material used to achieve optimal construction solutions on high thermal mass walls in Mediterranean climates with warm summers. Therefore, different external insulation solutions may need to be studied for each one of the mentioned locations.
- The first regards the influence of existing rolling shutters. Being part of the existing dwelling envelope, their inclusion in the study somehow conditioned the impact that glazing improvements and internal roller shades might have. On the other hand, improved results might be obtained using other similar solutions, such as exterior insulated rolling shutters, a very suitable option considering that the existing wall system already presents the space needed for its application. The benefits of this system are described in the work of Ariosto et al. [12], being identified as a remarkably effective solution for thermal efficiency and low risk of condensation when the system features an insulating foam core and is placed on the outside face of the window. The use of sealed tracks can also improve air leakage issues. Among other positive effects, such as simultaneously controlling daylight and allowing ventilation, all these features make this measure very suitable to be applied in any of the mentioned locations;
- The second regards the influence of insulation and window substitution in IAQ. This issue is particularly relevant in Mediterranean contexts for residents who usually reside in reduced spaces for long periods, such as the elderly. Therefore, further studies are advised to be carried out, such as those from Canha et al. [53] and Serrano-Jiménez et al. [34], the latter related to Spanish social housing, which identified excessive CO2 levels in indoor measurements with elderly occupants, especially in winter seasons. Therefore, studies regarding building airtightness before and after window retrofit also gained importance within Mediterranean climates to prevent such problems. Alfano et al. [54] recommend that proper window sealing must be selected considering possible IAQ and/or condensation issues, while Ghoreishi et al. [55] suggested that proper air renewal may be achieved using non-related window solutions, such as solar air collectors;
- The third regards the NK1 roof solution. If not already applied, a constructive intervention related to thermal insulation increase is strongly advised to improve this solution, especially to resolve probable summer overheating of dwellings located on the last floors.
5. Conclusions
- For coastal locations, external insulation is a strongly advised retrofit solution, considering its potential in decreasing energy demand, especially during heating seasons. Window replacement may present satisfactory results for both heating and cooling seasons, the latter being internal roller shades and other solutions which might be applied to glazed areas from the inside. These achieve interesting results considering their economic feasibility;
- For inland locations, external insulation is also a key retrofit solution to decrease energy demand for both heating and cooling seasons, window replacement being also advised as a strong retrofit solution for its performance during heating seasons. Nevertheless, although some caution must be considered with these two measures regarding their repercussion in IAQ during colder periods. Internal roller shades can provide little improvement during cooling seasons, although they do not present relevant condensation issues, as in alternative solutions;
- For both locations, glazing improvement through solar films presents some benefits for both coastal and inland cooling seasons, although its use must be carried out with some caution, considering heat gains conditioning during heating seasons.
Author Contributions
Funding
Data Availability Statement
Acknowledgments
Conflicts of Interest
References
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Location | Position | Reference Altitude (m) | Building Site Altitude (m) | Winter Severity | Summer Severity |
---|---|---|---|---|---|
Covilhã | Inland | 507 | 620 | I3 | V2 |
Fundão | Inland | 507 | 480 | I2 | V3 |
C. Branco | Inland | 328 | 375 | I2 | V3 |
Lourinhã | Coastal | 99 | 30 | I1 | V2 |
T. Vedras | Coastal | 99 | 50 | I1 | V2 |
Moita | Coastal | 47 | 25 | I1 | V3 |
Component | Description | Thickness (mm) | U-Value (W/m² °C) |
---|---|---|---|
External walls | Precast sandwich panel (outermost to innermost): 50 mm reinforced concrete 30 mm Expanded Polystyrene (EPS) 70 mm reinforced concrete | 150 | 0.99 |
Glazed areas | Outside uninsulated PVC roller shutters Aluminum frame (no thermal break) Single clear 3 mm glazing | - | Uwdn: 3.90–4.10 Uw: 6.20–6.50 |
Component | New Buildings [35] | Retrofit [38] | Analyzed Building | ||||
---|---|---|---|---|---|---|---|
I1 | I2 | I3 | I1 | I2 | I3 | ||
External walls | 0.50 | 0.40 | 0.35 | 1.70 | 1.50 | 1.40 | 0.99 |
Glazed areas | 2.80 (Uw) | 2.40 (Uw) | 2.20 (Uw) | 4.50 (Uwdn) | 4.00 (Uwdn) | 4.00 (Uwdn) | 6.20–6.50 (Uw) 3.90–4.10 (Uwdn) |
City | Moita (Coastal) | Covilhã (Inland) | ||||||
---|---|---|---|---|---|---|---|---|
kWh/m²·Year | Nic | D | Nvc | D | Nic | D | Nvc | D |
Existing dwelling | 43.2 | – | 16.4 | – | 92.6 | – | 10.6 | – |
Case A | 31.7 | −11.5 | 15.2 | −1.2 | 70.0 | −22.6 | 10.1 | −0.5 |
Case B | 40.9 | −2.3 | 13.3 | −3.1 | 86.5 | −6.1 | 8.4 | −2.2 |
Case C | 46.9 | +3.7 | 14.1 | −2.3 | 98.8 | +6.2 | 8.8 | −1.8 |
Case D | 43.2 | 0.0 | 15.9 | −0.5 | 92.6 | 0.0 | 10.2 | −0.4 |
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Brandão, P.I.; Lanzinha, J.C.G. Precast Concrete Building Construction and Envelope Thermal Behavior: A Case Study on Portuguese Public Social Housing. CivilEng 2021, 2, 271-289. https://doi.org/10.3390/civileng2020015
Brandão PI, Lanzinha JCG. Precast Concrete Building Construction and Envelope Thermal Behavior: A Case Study on Portuguese Public Social Housing. CivilEng. 2021; 2(2):271-289. https://doi.org/10.3390/civileng2020015
Chicago/Turabian StyleBrandão, Pedro I., and João C. G. Lanzinha. 2021. "Precast Concrete Building Construction and Envelope Thermal Behavior: A Case Study on Portuguese Public Social Housing" CivilEng 2, no. 2: 271-289. https://doi.org/10.3390/civileng2020015