Experimental assessment of thermal break strips performance in load-bearing and non-load-bearing LSF walls
Introduction
A reliable and accurate thermal characterization of building envelope components is very important at design stage to predict future thermal behaviour and energy efficiency, as well as in existing buildings for energy audits [1]. Several thermal performance assessment methods are available nowadays, such as the use of catalogues [2,3], analytical calculations [4,5], numerical simulations [6,7] and measurements (in situ or under controlled laboratory conditions) [8,9].
The use of catalogues is limited to the existing component configurations in the provided database, which oftentimes could not be enough. The use of standard analytical methods such as the ones prescribed by ISO 6946 [5] are limited to building components with homogeneous and/or inhomogeneous layers, being the level of heterogeneity restricted. In fact, the ISO 6946 Combined Method [5] is not applicable to building components where the thermal insulation is bridged by metal, such as happens frequently in lightweight steel frame (LSF) elements.
In fact, given the huge thermal conductivity contrast between the steel frame and the batt thermal insulation materials, there is a strong thermal bridge effect, being harder to accurately evaluate the overall thermal resistance (or transmittance) of these type of building elements [10]. There are several analytical methods developed specifically for LSF building components, such as the Gorgolewski methods [11], the ASHRAE Zone Method [10] and ASHRAE Modified Zone Method [12]. Santos et al. [4] recently performed a calculation procedures review and accuracy comparison of these analytical methods to estimate the thermal transmittance of LSF walls.
With the increasing computer speed and calculation capacity, the heat transfer numerical models had become more detailed and accurate. These numerical simulations could be simpler two-dimensional (2D) models [6,13] or more complex/detailed three-dimensional (3D) models [14,15], allowing a quick comparison between several building component configurations. However, to be fully reliable these simulations need to be validated against measured data or at least verified by comparison with benchmark results.
In fact, none of the previously mentioned thermal performance assessment methods fully replace the measurements under controlled laboratory conditions or in situ, having each method some advantages and/or limitations [8]. As recently reviewed by Soares et al. [8], there are several measurement methods for thermal characterization of building elements, including: (1) Heat flow meter (HFM) method [9] [16] [17] [18]; (2) Guarded hot plate (GHP) method [19]; (3) Hot box (HB) method [20], which could be calibrated (CHB) or guarded (GHB), and; (4) Infrared thermography (IRT) method [21].
The HFM method is one of the most used experimental technique for in situ assessment of thermal performance of building components, being attracting the researcher’s attention in order to minimize his usual long duration [22,23], evaluate uncertainty [24], compare with other methods [25] and increase precision [22]. It was concluded that the local operative conditions (e.g. high temperature gradient variation and heat flow inversion) can significantly influence the obtained in situ thermal transmittance [24]. Thus, whenever possible the HFM measurements should be performed under controlled temperature conditions (e.g. in laboratory). Moreover, it was found that the use of an additional heat flux sensor can significantly reduce test duration and increase precision [22].
As mentioned before, besides all these measurement inherent issues, LSF building components exhibits a particular additional challenge related with the strong thermal conductivity inhomogeneity of cavity insulation and steel frame materials [26]. This issue also addressed the attention of researchers by comparing several methods to evaluate thermal performance of LSF elements [27] and even developing new assessment methods for in situ measurements [28], as well as the evaluation of lateral heat transmission or flanking thermal losses [14].
Another LSF elements thermal performance research trend is the development, evaluation and comparison between thermal bridges mitigation strategies. This assessment could be performed making use of two- [6] or three-dimensional parametric studies [15]. Besides the use of slotted steel studs [29] (which have the major drawback of reducing the mechanical resistance of load-bearing LSF walls), the use of thermal break (TB) strips is one of the most used strategy to mitigate the steel studs thermal bridge effect [1], being this the main focus of research project Tyre4BuildIns – “Recycled tyre rubber resin-bonded for building insulation systems towards energy efficiency” [30]. Nowadays, there are available in the market several TB strip materials, which were specifically developed for this purpose (e.g., aerogel TB strips from SpaceTherm®) or could be easily adapted for this use (e.g., recycled rubber MS-R1 from AmorimCorkComposites® and cork/rubber composite MS-R0 from AmorimCorkComposites®).
It was not found in the literature any systematic experimental campaign for the evaluation of different TB strip materials, neither for the assessment of the best performance positioning for these TB strips (inner or outer steel flanges), nor to evaluate if the TB strips are more efficient in load- or non-load-bearing LSF walls, being all these features the major novelties of this study.
In this work, the overall surface-to-surface thermal resistance (-value) of twenty different configurations of load- and non-load-bearing partition walls are measured in controlled laboratory conditions, to evaluate the thermal break (TB) strips performance for the mitigation of the thermal bridges originated by the steel studs. Three tests are performed for each wall, with the sensors at different high locations (top, middle and bottom) within the LSF wall test-sample surfaces, totalizing sixty lab tests. Three TB strip materials are tested, namely: (1) recycled rubber MS-R1; (2) cork and rubber composite MS-R0, and; (3) aerogel CBS. Three different TB strip positions within the steel stud flange are tested: (1) inner; (2) outer, and; (3) two TB strips.
The paper is structured as follows. After this brief introduction, the related materials and methods are presented, including LSF characterization, experimental lab tests and numerical simulations. Then, the obtained results regarding the overall surface-to-surface -values, the infrared surface images and horizontal surface temperature lines are presented and discussed for the assessed structural and non-structural LSF partition walls. Finally, the main conclusions from this research work are summarized.
Section snippets
LSF walls characterization
In this section the load- and non-load-bearing LSF walls, as well as the thermal break (TB) strips are characterized regarding materials, geometries/dimensions and thermal properties.
Non-load-bearing LSF walls
Table 4 display the conductive thermal resistances predicted by THERM software 2D FEM models and the measured values for non-load-bearing (NLB) walls, as well as the absolute and percentage differences between measured and predicted -values. The results are organized into four groups: (1) the reference non-load-bearing -value (NLBref), i.e., for the LSF wall without any thermal break (TB) strip; (2) the LSF walls with an inner TB strip (NLBTBin) made of different materials (R1, R0 and AG);
Discussion
To allow an easier overall performance comparison, Fig. 14 displays a graphical overview of the results previously presented for load- and non-load-bearing LSF walls, regarding the thermal resistance improvement provided by the TB strips.
Comparing the thermal resistance improvement for load- and non-load-bearing LSF walls, it can be concluded that the TB strips are slightly more efficient in LB walls, mainly when applied in the outer or in both stud flanges. This feature could be related with
Conclusions
In this work the thermal performance of thermal break (TB) strips in lightweight steel frame (LSF) partition walls was assessed making use of experimental lab measurements. Load- and non-load-bearing LSF walls were evaluated. Inner, outer and double TB strips were tested and three TB strip materials were evaluated: R1 – recycled rubber; R0 – rubber and cork composite, and; AG – aerogel.
The main conclusions of this research work could be summarized as follows:
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The inner and outer TB strips have
Funding
This research was funded by FEDER funds through the Competitivity Factors Operational Programme – COMPETE and by national funds through FCT – Foundation for Science and Technology, within the scope of the project POCI-01-0145-FEDER-032061.
Author statement
Both authors participated equally to this work.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors also want to thank the support provided by the following companies: Pertecno, Gyptec Ibéria, Volcalis, Sotinco, Kronospan, Hulkseflux, Hilti and Metabo.
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