1. Introduction
The most relevant energy consumer in European Union (EU) is the buildings’ sector [
1]. In fact, during 2019, forty percent of the EU27 final energy consumption was spent in buildings [
2]. Within this share, almost 50% are used for space heating and cooling [
1]. These facts are related to reduced energy efficiency of buildings and consequent wasted energy. Indeed, in the EU three out of four buildings are classified as inefficient [
2]. To make Europe’s building sector compatible with the Paris Agreement, two goals need to be achieved: (1) reduce energy demand through energy efficiency measures, and; (2) increase the use of renewable energy sources [
3].
As is well known [
4], the reduction of undesirable heat losses is one of the possible strategies to improve energy efficiency. This heat loss reduction could be achieved by mitigating each heat transfer mechanism across the building envelope: radiation, convection, and conduction. The most forthright and simplest approach to increase the thermal resistance of building envelope components is the usage of thermal insulation, reducing significantly the heat transfer by conduction. Nevertheless, the effectiveness of the thermal insulation depends also on their position within the building element, as previously demonstrated by Roque and Santos [
5]. Additionally, this insulation material also endorses sound insulation, principally when porous batt insulation materials are used inside the air cavities [
6].
Currently, extremely efficient insulation materials (sometimes designated by SIMs—super insulating materials) are emerging in the market, having very small thermal conductivities [
7]. Aerogels [
8] and vacuum insulating panels (VIPs) [
9] are nowadays two of the most common examples of SIMs. Notice that increasing discontinuous thermal insulation along building envelopes may rise the relevance of thermal bridges, being this effect even more significant in steel structures, given the huge thermal conductivity of steel [
10]. In fact, as concluded by Erhorn-Klutting and Erhorn [
11] up to near one-third of the heating energy needs could be originated by thermal bridges in traditional buildings (reinforced concrete and masonry).
During the last years, the lightweight steel frame (LSF) construction system is being more used, mainly for low-rise residential houses [
12], due to their intrinsic benefits. Some of these advantages are: fast construction, high mechanical strength, and low weight, high potential for recycling and reuse, reduced on-site disruption, great suitability for retrofitting, high architectural flexibility, economical transportation and handling, easy prefabrication, precise tolerances, superior quality, insect damage resistance, and humidity stability shape [
4].
Nowadays, several techniques could be used to mitigate thermal bridges in LSF buildings’ elements, such as slotted thermal steel studs [
13,
14,
15], thermal break (TB) strips [
13,
16,
17], and continuous thermal insulation layers (e.g., ETICS—external thermal insulation composite system) [
18,
19,
20]. Moreover, the steel frame is so important that even minor changes in the stud flanges shape and size could have a relevant effect on the thermal performance of LSF walls [
21].
Moreover, when there is an air cavity inside the wall, one effective way to improve the thermal performance is by reducing the heat transfer by radiation. This could be achieved by using reflective low-emissivity paint or foil inside the air gaps of the building components [
22,
23]. This thermal performance improvement solution has supplementary benefits, such as easy installation and low cost.
As recently mentioned by Bruno et al. [
23], there is a very small number of research works related to thermal resistance improvement due to low-emissivity materials placed inside air cavities. This fact is even more perceptible in LSF double-pane building elements.
Recently, Santos and Ribeiro [
24] studied the thermal performance of double-pane lightweight steel-framed walls with and without a reflective foil. This assessment was mainly experimental under laboratory-controlled conditions, but the measurements were compared with 2D finite element numerical simulations. Several air cavity thicknesses (0 mm up to 50 mm, with an increment of 10 mm) were evaluated, but only one aluminium reflective (AR) foil was considered (on the outer surface of the air cavity). It was concluded that “the use of a reflective foil is a very effective way to increase the thermal resistance of double pane LSF walls, without increasing the wall thickness and weight”. However, when using an AR foil, it is not worthy to have an air cavity higher than 30 mm. One research gap that was not investigated in this work was if it is worthy to use two low-emissivity aluminium foils (one in each air-gap surface). Moreover, another interesting question is if the performance of the AR foil is similar when used in the outer or inner surface of the air cavity.
Regarding the use of TB strips, there is also a lack of scientific research works available. Perhaps the most relevant is the experimental campaign completed by Santos and Mateus [
17] for the assessment of thermal break strip performance in load-bearing and non-load-bearing LSF walls. They concluded that an outer or inner TB strip has very similar thermal performances, being the best performance achieved for two TB strips (one on each steel stud flange) and for the aerogel TB strip material. Notice that these walls were single-pane LSF walls, not found in the literature any research work related with the use of TB strips in double-pane LSF walls, or in combined use with AR foils.
In this work, the authors seek the answers for some of these research gaps and questions, by assessing the thermal performance improvement of double-pane LSF walls using thermal break strips and reflective foils. The strategy was to start with a reference double-pane LSF wall (30 mm air gap) and compare the thermal resistance increase only due to aerogel TB strips and only due to AR foils, by performing measurements under laboratory-controlled conditions. Moreover, on the next set of measurements, the combined effect of both aerogel TB strips, and AR foils was evaluated. Notice that the aerogel was selected as the material for the TB strips since it is one of the highest performant materials available in the market, having a very reduced thermal conductivity (in this case, 0.015 W/m K). Moreover, aluminium was selected as the material for the reflective foil since it is the most currently used for this purpose, having a very reduced emissivity value (below 0.05).
This article is structured as follows. After this small introduction and contextualization, it is presented a section with the materials and methods, where the LSF walls and used materials are characterized. Moreover, the experimental lab tests are described, including the experimental setup, as well as the set-points and test procedures. Additionally, to ensure the reliability of the measurements and to check the test procedures, two verifications were performed: (1) comparison between the measured thermal conductivity of a homogeneous XPS panel with the value provided by the manufacturer, and; (2) comparison between the measured thermal resistance of four double-pane LSF walls, namely: (1) reference; (2) with a single TB strip; (3) with a single AR foil, and; (4) with both two TB strips and two AR foils, with the predictions provided by numerical simulation models. The obtained results are presented and discussed next, being grouped into three sets: (1) only TB strips; (2) only AR foils; (3) combined TB strips and AR foils. Finally, the main conclusions of this research work are summarized. Notice, that the various scenarios evaluated in this research work were based on previous papers from authors, namely reference [
24] for the aluminium reflective foil scenarios and reference [
17] for the thermal break strips.
3. Results and Discussion
This section is divided into three parts. First, the measurement results related with the thermal resistance improvement of double-pane LSF walls, due to aerogel thermal break strips, are presented and discussed. After, a similar discussion is presented regarding the thermal performance enhancement due to aluminium reflective foils. Finally, the measured -values for combined aerogel thermal break strips and aluminium reflective foils, in double-pane LSF walls, are displayed and discussed.
3.1. Thermal Performance Improvement Due to Aerogel Thermal Break Strips
Table 4 displays the measured thermal resistance values of the double-pane LSF walls, as well as the thermal performance improvement due to aerogel thermal break (TB) strips, providing the values graphically displayed in
Figure 6.
When there are no thermal break strips (reference LSF wall) the measured thermal resistance is 2.456 m2·°C/W. Adding an inner or an outer TB strip provides similar -values (around 2.75 m2·°C/W), corresponding to a thermal resistance increase of about +0.29 m2·K/W (+12%). This -value increase is similar to the one achieved by adding 10 mm of continuous mineral wool (MW). The higher measured -value (2.928 m2·°C/W) is achieved when using two TB strips (inner and outer), allowing a thermal resistance improvement of +0.472 m2·K/W (+19%). Notice that the combined effect of these two TB strips (-value increase of +19%), is smaller than the summation of two single TB strips (+24%), i.e., inner (+12%) and outer (+12%).
3.2. Thermal Performance Improvement due to Aluminium Reflective Foils
The measured thermal resistance values and thermal performance enhancement due to aluminium reflective foils are displayed in
Table 5, while
Figure 7 exhibits a plot of these values.
Notice that the reference -value (wall without aluminium reflective foil) remains the same, as in the previous subsection (2.456 m2·K/W). Adding an inner or an outer aluminium reflective foil provides similar -values (around 2.92 m2·K/W), corresponding to a thermal resistance increase of about +0.46 m2·K/W (+19%). This -value increase is similar to the one achieved by adding 16 mm of continuous mineral wool (MW). The higher measured -value (2.972 m2·K/W) is achieved when using two reflective foils (inner and outer), allowing a thermal resistance improvement of +0.516 m2·K/W (+21%).
Comparing these values with the previous ones for the TB strips (
Table 4), it can be concluded that the thermal performance improvement due to aluminium reflective (AR) foils is more effective, allowing to achieve higher thermal resistances of the double-pane LSF wall. In fact, the
-value improvement due to a single AR foil (inner or outer) is similar to the
-value improvement provided by two aerogel TB strips (+19%).
Another interesting conclusion is that the use of two reflective foils is not so effective, since the thermal performance improvement, in comparison with only one aluminium reflective foil, is very reduced (about +2%).
3.3. Thermal Performance Improvement Due to Thermal Break Strips and Reflective Foils
Table 6 exhibits the measured
-values for combined aerogel thermal break (TB) strips and aluminium reflective (AR) foils, as well as the corresponding thermal resistance increase.
Figure 8 displays a graphical representation of these values, being grouped into three sets of
R-values, depending on the number and location of TB strips (inner, outer, and both inner and outer). Notice that the reference
-value remains the same (2.456 m
2·°C/W) for all three sets of measurements.
The combined thermal resistance improvement, due to a single TB strip (inner or outer) and a single AR foil (inner or outer), is around 0.85 m
2·K/W (+35%), being this value slightly higher than the summation of individual
-values increase (+31%), obtained from
Table 4 (+12%) and
Table 5 (+19%).
Looking to the LSF walls with inner or outer TB strip, having two AR foils (inner and outer), the
-value increase is around +1.01 m
2·K/W (+41%). Again, this thermal resistance improvement (+41%) is higher than the summation of individual
-values increase (+33%), obtained from
Table 4 (+12%) and
Table 5 (+21%), evidencing a bigger synergy outcome between the TB strips and the AR foils.
As expected, the higher measured thermal resistances are provided by the LSF walls with two TB strips. In this circumstance, when having an inner or an outer AR foil, the -value increase is about 1.23 m2·K/W (+50%). Once more, the summation of individual thermal performance improvement due to double TB strips (+19%) and due to one (inner or outer) AR foil (+19%) is smaller (only +38%) than their combined effect (about +50%).
The use of two AR foils, instead of a single one, only improved the thermal resistance by an additional 5%, i.e., +55% (+1.352 m
2·°C/W) in relation to the reference LSF wall. The synergy effect remains, being even higher since this value (+55%) is considerably bigger than the summation (+40%) of individual improvement contribution from two TB strips (+19%,
Table 4) and from two AR foils (+21%,
Table 5). The
-value increase measured for double TB strips combined with double AR foils (about +1.35 m
2·°C/W) is similar to the one achieved by adding 47 mm of continuous mineral wool (MW).
4. Conclusions
In this paper, the thermal performance improvement Due to the use of aerogel thermal break (TB) strips and aluminium reflective (AR) foils in double-pane lightweight steel-framed walls were experimentally assessed. The lab measurements were performed using two mini climatic chambers (hot and cold) and the double-sided heat flow meter technic. Three sets of measurements were performed, considering the thermal performance improvement due to: (1) only aerogel TB strips; (2) only AR foils, and; (3) both TB strips and AR foils. Taking into account the three TB strips and AR foils locations, namely: (i) inner; (ii) outer; (iii) double (i.e., both inner and outer), as well as the reference LSF wall (without TB strips and AR foils), sixteen LSF walls’ configurations were measured.
As expected, the key findings address the research questions of this research work, being the main conclusions summarized as follows:
Both aerogel TB strips and AR foils allowed to improve the thermal performance of a reference double-pane LSF wall, which has a conductive thermal resistance of 2.456 m2·K/W.
Placing the aerogel TB strip on the inner or outer steel stud flange provides similar conductive -values, being the thermal resistance increment of about +0.29 m2·K/W (+12%).
The use of inner or outer AR foils inside the air cavity also provides similar -values, but the effectiveness of this improvement measure is higher, exhibiting a thermal resistance increment of about +0.47 m2·K/W (+19%).
In fact, using an AR foil inside the air cavity of double-pane LSF walls is much more effective than using aerogel TB strips along the steel flange, since only one AR foil (inner or outer) provides similar thermal resistance increase than two aerogel TB strips, i.e., around +0.47 m2·K/W (+19%).
However, the use of two AR foils, instead of a single one, is not effective, since the relative thermal resistance increase is only about +0.04 m2·K/W (+2%).
The combined effect of both TB strips and AR foils allowed to achieve a maximum -value increase of +1.35 m2·K/W (+55%).
When combining these two thermal performance strategies, it was found a synergy effect between them, since the measured combined -value increase, when using both TB strips and AR foils, is bigger than the summation of individual -values increments.
This synergy effect ranged from only +0.10 m2·K/W (+4%) for the thermal resistance increase due to a single TB strip and a single AR foil, up to +0.37 m2·K/W (+15%) when using two TB strips and two AR foils.
Notice that the higher effectiveness of the AR foils in comparison with the aerogel TB strips could be justified by the continuous air cavity (3 cm) and reflective foil, while the TB strips are restricted to the steel studs’ flanges. Moreover, the steel studs in the double-pane LSF wall are separated in two different frames (inner and outer wall panes), not crossing the wall and, this way, the related steel thermal bridge effect is very reduced.
The main practical applications and implications of the research findings are related the design of double-pane LSF walls, or refurbishment, whenever their thermal performance is not satisfactory. As mentioned above, the use of AR foil is more efficient than TB strips, even when they are made with a super insulation material (aerogel). Moreover, adding this AR foil to the inner or outer side of the air cavity originates a similar thermal performance increase. Furthermore, the use of two AR foils, instead of a singleone, is not effective.
Cost assessment is not within the scope of this research, being an interesting research idea for future work. However, the acquisition cost of aerogel TB strips is much higher than the cost of an AR foil. For example, the cost of an aerogel Spacetherm® CBS (Cold Bridge Strip) is around 3.80 €/m (50 mm wide and 10 mm thick), while the cost of a Space-Reflex® AR foil (4 mm thick) is around 2.35 €/m2. Assuming an LSF wall with a stud spacing equal to 400 mm and a high of 2.70 m, the unit consumption of a single TB strip is 3.30 m per wall square meter. Thus, the unit cost of this TB strip is around 12.54 €/m2 for this LSF wall. Therefore, the unit cost of the AR foil is around five times lower than the aerogel TB strip. Consequently, it can be concluded that besides the higher thermal efficiency of the AR foil, this performance improvement strategy is also much cheaper.
The main limitations of this research are: only one air cavity was assessed (30 mm thick); only one TB strip material was assessed (aerogel); only one reflective foil was assessed (aluminium); the measurements were performed in a double-pane LSF wall test-sample (without any wall ties, connectors or other bridging elements inside the air cavity) under controlled lab conditions in a near steady-state regimen (not having into account any transient effects due to daily temperature variations).
Energy consumption is not within the scope of this research, being a good research suggestion for future work. Nevertheless, the achieved thermal resistance increments (maximum absolute increased value of +1.35 m2·°C/W, corresponding to +55%) will allow for sure to reduce the heat losses across the building opaque envelope during the winter season and, consequently, to reduce energy consumption for space heating. This energy consumption reduction will be larger for colder climates.