The importance of thermal bridge correction in energy refurbishment of existing buildings

Retrofitting of existing buildings plays a critical role to achieve sustainable development. In fact, the building sector is one of the largest energy consumers and GHG emitters [1]. Building energy consumption sums up to 40% of primary energy consumption in the world. Improving the energy efficiency in buildings is vital to address the climate change and achieve energy independence [2].

Several actions have been taken by the European Commission to reduce buildings energy consumption through many revisions of Energy Performance Building Directives (EPBD) [3-5].

In Italy, some mandatory laws have come into force to transpose European Directives [6]. In particular, the Decree 26.06.2015 [7], concerning minimum requirements of new and existing buildings object of energy refurbishment, fixes limit values for the following parameters referred to the building envelope:

mean thermal transmittance U_m of single opaque elements to be respected in both cases of simple energy refurbishment, that involves a surface ≤ 25% of the total gross leaking building surface, and major 2^nd level energy renovation, that involves a surface > 25% of the total gross leaking building surface;

mean global heat transmission coefficient H'_T of an entire façade including opaque and glazed elements to be respected in case of major 2^nd level energy renovation, as above defined.

Moreover, in case of simple energy refurbishment, a 30% derogation on limit values of mean thermal transmittance is allowed in two cases: insulation of the inner part of building elements and filling cavity walls of existing buildings with insulating materials. This derogation is not allowed in case of important 2^nd level energy renovation.

The cavity insulation is very significant, since almost all Italian '45-'80 years residential buildings were built in reinforced concrete frame structure with beams and pillars completed by cavity wall technique.

In an urban contest, an eventual coat insulation increases wall thickness and frequently is hardly achievable due to limited available distances among buildings and to property border reasons. Moreover, coat insulation of external walls frequently requires extra works, i.e. thresholds and lintels substitution, eventual new displacement of water and gas pipes, reposition of shutters and reconstruction of architectural and decorative elements. On the other hand, internal insulation leads to a reduction of net indoor spaces and involves the risk of interstitial condensation [8, 9].

A useful technique is therefore to fill the air layer with granular insulating materials. The main problem related to this technology is the failure of thermal bridges correction with consequent higher energy needs and risk of mould growth. A study concerning buildings in Greece [10] has demonstrated that buildings heating need in case of insulated wall cavities without thermal bridges correction is by 50% higher than the one calculated for the more recent buildings with coat insulations and thermal bridges correction. Consequently, a low thickness insulation of thermal bridges with a high performance insulating material is necessary to reduce building envelope energy need. A study concerning thermal bridges correction in a dwelling by means of cork-based mortars is developed in [11]. In [12] an aerogel insulating coating is used to limit windows offset thermal bridge effects: applying 1 cm to 2 cm of insulation reduces the windows offset energy load by about 24–50%.

In previous papers [13,14], the authors demonstrated that the mean thermal transmittance limit values imposed by Decree 26.06.2015 are impossible to be comply without a correction of thermal bridges, even in the presence of derogation. A simple case study concerning an opaque vertical wall with thermal bridges due to beams and pillars has been investigated in [13]. A more complex case concerning a typical repetitive module of a building façade with window, sub window and roller shutter box is analysed in [14].

The purpose of this paper is to study a further typical case consisting of a repetitive module of building façade with door window, balcony and roller shutter box. In addition to junctions due to beams and pillars, thermal bridges due to lintel, threshold, jambs and balcony are examined. The linear thermal transmittance y is calculated according to standard UNI EN ISO 10211 [15] by means of the 2-D finite element simulator THERM [16].

Starting from the actual state of an uninsulated building and considering the insulation of cavity wall, the correction of the thermal bridges necessary to respect limit values of both mean thermal transmittance U_m and mean global heat transmission coefficient H'_T is discussed. The insulation is achieved by an aerogel layer characterized by a very low declared thermal conductivity.

A very common case study has been investigated. Most of Italian buildings dating back to the '45-'80 years are characterized by frames with beams and pillars in reinforced concrete with cavity wall masonry completion. A building façade, consisting of a repetitive module with door window, balcony and roller shutter box, is considered (Figure 1). The module has a gross length of 5 m and a gross height of 3.3 m; its gross area is A_mod = 16.5 m². Pillars are 35 cm thick and 30 cm wide. There are no dividing walls behind the pillars. Floors have a 30 cm thickness. Two different lengths of balcony are considered: 3 m (Figure 1a) and 1.8 m (Figure 1b). Balcony is 1.1 m protruding and 25 cm thick.

figure_1.png

Figure 1. Repetitive module

The current wall presents a 15 cm tick cavity and its stratigraphy is reported in Table 3. Thermal conductivities of materials are provided by the standards UNI 10351 [20], UNI EN ISO 10456 [21] and UNI/TR 11552 [22]. Conventional surface resistances of UNI EN ISO 6946 [23] are used.

Table 3 provides thermal transmittance for both cases, non-insulated and insulated cavity wall. Insulation is achieved by filling the air layer with a traditional insulating material ($\lambda$= 0.037 W m^-1K^-1). Note that the insulated wall transmittance value is in the range allowed by the decree (Table 2), but it does not take into account the influence of thermal bridges.

Table 3. Non-insulated and insulated cavity wall stratigraphy

It is also necessary to hypothesize geometrical, constructive and thermal characteristics of door window and roller shutter box.

A door window constituted by a 6 cm thick PVC frame section (U_f = 2.12 W m^-2K^-1) and double glazed (4-12-4 mm) with air cavity (U_g = 1.88 W m^-2K^-1) and common spacer ($\psi_{\mathrm{g}}$= 0.07 W m^-1K^-1) is assumed. Thermal transmittances of the frame, glass and spacer are calculated according to UNI EN ISO 10077-2 [24]. Thermal transmittance of window, according to UNI EN ISO 10077-1 [25] is 2.08 W m^-2K^-1.

Table 4. Pillar stratigraphy

figure_2.png

Figure 2. Building elements and thermal bridges of the repetitive module

figure_3.png

Figure 3. Pillar-current wall thermal bridge (pc)

figure_4.png

Figure 4. Door window-current wall thermal bridge (wc) and roller shutter box-current wall thermal bridge (rc)

figure_5.png

Figure 5. Beam-current wall thermal bridge (bc) and balcony-current wall thermal bridge (bac)

A non-insulated soft wood ($\lambda$= 0.13 W m^-1K^-1) 5 mm thick roller shutter box is considered, with thermal characteristics and geometry according to UNI EN ISO 10077-2. The roller shutter box has the same length as the door window and its depth is about 30 cm, with a thermal transmittance of 5.06 W m^-2K^-1.

Figure 2 shows the nomenclature used in the present study for both buildings elements and thermal bridges that compete to the repetitive module. Figure 2 also reports geometrical sizes used in the calculations.

Figures 3-6 sketch thermal bridges constructive details. They correspond to THERM models [16]. In particular, light green lines represent boundary conditions. Numbers are referred to stratigraphies reported in Tables 3-5; sizes are expressed in cm.

L-shaped border beam, identified by number 11 in Figures 5 and 6, is 60 cm high and 15 cm thick and it is characterized by the same conductivity of pillars ($\lambda$= 1.91 W m^-1K^-1). The marble slab of the door window threshold, identified by number 12 in Figure 6, is 15 mm thick with $\lambda$= 3 W m^-1K^-1.

figure_6.png

Figure 6. Roller shutter box-balcony-door window thermal bridge (rbaw)

In Table 6 calculated mean thermal transmittance U_m and mean global heat transmission coefficient H'_T are reported for the repetitive module for both 1.8 m and 3 m balcony lengths and for both non-insulated and insulated wall cavities. Results are very far from respecting limits values shown in Tables 1 and 2, even in case of derogation on the mean thermal transmittance.

Table 6. U_m and H'_T of non-insulated and insulated wall cavities

A 2 cm thick aerogel layer, identified by number 13 in Figures 4 and 6, is applied inside the roller shutter box leading its thermal transmittance to 1.10 W m^-2K^-1.

Owing to the small thickness of external plaster used in buildings of '45-'80 years, no more than 2¸3 cm of useful thickness are available in order to insulate thermal bridges from outside and to maintain the original vertical shape of the façade.

Correction of vertical thermal bridges (pc), (wc) and (rc) is obtained in the same way as described in [14]. In particular, for the pillar-current wall bridge (pc), the insulation exceeds by 15 cm the pillar width from the right and left sides.

Correction of horizontal thermal bridges is depicted in Figure 7, where L is the beam width and d represents the aerogel thickness.

For the (bc) bridge $\ell_{1}$ dimension is referred to the aerogel that exceeds L dimension applied above and below the beam. For the (bac) bridge $\ell_{1}$ is referred to the aerogel surplus applied below the beam and $\ell_{2}$ to the aerogel surplus applied under and above the balcony. Finally, for the (rbaw) bridge $\ell_{2}$ is referred to the aerogel surplus applied under and above the balcony.

The effect of $\ell_{1}$ and $\ell_{2}$ parameters is considered by means of the percentage surplus ratios SR₁ and SR₂, defined as follows:

$\mathrm{SR}_{1}=\frac{\ell_{1}}{\mathrm{L}} \cdot 100 \quad \mathrm{SR}_{2}=\frac{\ell_{2}}{\mathrm{L}} \cdot 100$  (6)

In order to correct the thermal bridge (rbaw), an aerogel layer 1 cm thick is applied on the inner side of the beam, inside the roller shutter box, as shown in Figure 7.

figure_7.png

Figure 7. Correction of thermal bridges (bc), (bac) and (rbaw)

Envelope energy refurbishment of buildings by means of filling wall cavities with insulating material and thermal bridge correction has been carried out, taking into account limit values on thermal transmittance U_m and mean global heat transmission coefficient H'_T provided by Italian DM 26.06.2015. A repetitive module containing door window, roller shutter box and balcony has been considered. Thermal bridges correction is achieved by aerogel panels. The following conclusions can be drawn:

·     application of the aerogel panel under the balcony floor tiles produces a negligible effect on U_m;

·     application of 3 cm thick aerogel panel on all thermal bridges allows to comply derogated U_m limit values at 2021 only in A and B climatic zones;

·     mean transmittance U_m decreases as the length of the balcony decreases;

·     compliance with H'_T limit values is possible using high performances windows with transmittance values between 1.5 W m^-2K^-1 and 0.9 W m^-2K^-1, according to the climatic zone.