Hygrothermal Analysis of CLT-Based Retrofit Strategy of Existing Wall Assemblies According to EN 13788 Standard

In 2018, the amendment to the Energy Performance of Buildings Directive (EPBD) stated that each Member State shall establish a long-term strategy for the renovation of their national building stock so as to facilitate the cost-effective transformation of existing buildings into nearly zero-energy buildings. In particular, the States are committed to define a roadmap leading to the reduction of greenhouse gases (GHG) in the EU by 80-95% by 2050 compared to 1990 levels [1]. Residential and non-residential buildings are currently responsible for 40% of the final energy demand in the EU, and for approximately 36% of all emissions of GHG. In order to reach these ambitious targets, deep renovation of the existing EU building stock is sought. In particular, according to the EU building stock observatory, around 78% of the residential buildings in the EU have been constructed before 1990 [2]. Moreover, about 35% of the EU buildings are over 50 years old, and almost 75% are highly energy inefficient [3]. Despite the urgent need to improve the energy efficiency of the EU building stock, the renovation rate is still highly unsatisfactory [3]. Furthermore, energy efficiency is not the only problem faced by the European building stock because about 50% of the European territory is earthquake-prone [4]. In these countries, energy renovation actions must therefore strategically combine with seismic retrofitting.

In this framework, the ongoing EU-funded innovation project called e-SAFE (energy and Seismic Affordable rEnovation solutions) addresses various solutions for the combined energy and seismic deep renovation of residential reinforced-concrete (RC) framed buildings in the EU. One of these solutions makes use of cross-laminated timber (CLT) panels connected to the existing RC frame to increase the seismic and energy performance of the existing envelope. This research presents preliminary results concerning the thermal and hygrometric analysis of CLT panels when applied to existing wall structures located in different climate zones in Italy, by following the calculation procedures set by EN 13788 Standard [5].

The assessment of the thermal and hygrometric behavior of the e-CLT solution for the refurbishment of existing wall structures is made through the methods prescribed by the EN 13788 Standard [5] and considering different climate conditions and indoor vapor concentrations. The hygrothermal risk assessment of the various wall assemblies is carried out through the freeware software PAN v.7.1.0.4, a tool developed by the Italian National Association for Thermal Insulation (ANIT) that complies with all the relevant European and National Standards [10].

3.1 Typical Italian wall structures analyzed as reference

The wall structures considered in this study are some of those listed in the final report of the EU Tabula project [11], since they are representative of a large share of existing walls for the non-historic Italian residential building stock. In detail, the investigated walls are:

• Wall structure ID1: solid bricks (25 cm) finished by internal (1 cm) and external (2 cm) plaster layers;
• Wall structure ID2: the same as Wall ID1 except for the addition of a wooden-based insulation layer of 4 cm between the external plaster and solid bricks layers;
• Wall structure ID3: hollow clay bricks (8 cm on the inner side and 12 cm on the outer side) separated by an air gap (7 cm) and finished internally and externally by a plaster layer of 1 cm and 2 cm thickness respectively;
• Wall structure ID4: reinforced concrete wall (15 cm) finished by internal (1 cm) and external (2 cm) plaster layers;
• Wall structure ID5: the same as Wall ID3 except for the addition of a wooden-based insulation layer (4 cm) in the air gap on the external hollow brick side and the reduction of the air gap down to 3 cm;
• Wall structure ID6: the same as Wall ID4 except for the addition of a wooden-based insulation layer (6 cm) between the external plaster and reinforced concrete layers;
• Wall structure ID7: the same as Wall ID 3 except for the addition of a wooden-based insulation layer (8 cm) in the air gap on the external hollow brick side and the reduction of the air gap down to 3 cm.

The thermophysical properties of the materials used in the various wall assemblies, along with those pertaining to CLT panels, are listed in Table 1 and are gathered from the EN ISO 10456:2007 Standard [12].

Table 1. Thermophysical properties of the wall assemblies’ materials

*this is the equivalent thermal conductivity corresponding to a thermal resistance of 0.18 m²∙K∙W^-1

3.2 Hygrothermal assessment

3.2.1 EN 13788 method

Surface condensation and mold growth on the internal surface of building components are directly linked with their surface temperature. This in turn depends on the U-value of the components, the presence of thermal bridges and the psychrometric conditions of indoor air.

The standard approach employed to assess the risk of surface condensation and mold growth is defined in the European Standard EN ISO 13788 [5]. Here, the so called “temperature factor” (otherwise known as f-factor), is defined as a bulk index that describes the thermal quality of an envelope component in terms of surface condensation and mold formation avoidance, and can be calculated as follows:

${{f}_{RSI}}=\frac{{{T}_{si}}-{{T}_{o}}}{{{T}_{op}}-{{T}_{o}}}$      (1)

Being T_SI the risk surface temperature, T_O the outdoor air temperature and T_OP the indoor operative temperature (℃).

Operatively speaking, the f-factor calculated for the analyzed component is then compared with a minimum allowable temperature factor f_RSI,min, derived by imposing a threshold condition to the surface relative humidity. The critical relative humidity value considered in Italy is 80%.

However, condensation can also take place in the inner layers of a building component, and as such, it would not be visible from the outside. It is the case of interstitial condensation, which can be triggered by various complex physical phenomena like vapour convection and vapour diffusion when the water is in its gaseous state, or by capillary transport and surface diffusion when the water is in its liquid state.

Notwithstanding such a complexity, the approach prescribed by the EN ISO 13788 Standard only considers the vapour diffusion mechanism generated by the difference in partial vapour pressure between the indoors and the outdoors.

The specific flux of water vapour through a material layer of a building component can then be appraised through the Fick’s diffusion equation as follows:

$g={{\delta }_{0}}\cdot \frac{{{p}_{i}}-{{p}_{o}}}{{{s}_{D}}}$     (2)

Here, δ₀ is the water vapour permeability of air (187.5∙10^-12 kg∙m^-1∙s^-1∙Pa^-1), p_i and p_O are the indoor and outdoor water vapour partial pressure (Pa) respectively, and s_D is the equivalent air layer thickness (m) for water vapour diffusion. The latter one is defined as the product of a layer’s material thickness (m) by its vapour resistance factor μ (-), a non-dimensional quantity that expresses the resistance opposed to water vapour diffusion by a material compared to still air (the higher the value, the higher the resistance to the flow).

The water vapour contained in the air and flowing through a square meter of a building material can eventually condensate, i.e. changing its state from vapour to liquid, if its partial vapour pressure reaches the corresponding saturation pressure p_S (Pa), a quantity that depends only on the material’s temperature.

For a multi-layer construction assembly, the EN 13788 Standard recommends to follow the so-called Glaser’s method, which accounts for these mechanisms.

3.2.2 Boundary conditions for hygrothermal assessment

It is necessary to first set reference climate conditions indoors and outdoors, which can be made relying on the EN ISO 15927-1:2004 Standard [13]. Various locations in Italy – ranging from warm to cold – are thus investigated according to their Heating Degree Days (HDD). In fact, HDD provide a simple yet effective measure of the severity of a specific climate. HDD are defined as the summation of all the positive differences between a conventional indoor set point temperature (T_i) and the average daily outdoor air temperature (T_o) over a defined period. In Italy, the conventional indoor set point temperature is fixed to 20℃, while the various areas have been classified into six climate zones ranging from A (warmest) to F (coldest) according to their HDD (see Table 2). This classification is dictated by the Presidential Decree n. 412 of the 26^th August 1993 [14] and determines the conventional heating period of buildings, as reported in Table 2.

Table 2. Climate zones considered in the analysis

For each city representative of the Italian climate zones, the average monthly climate conditions gathered from the Italian Standard series UNI 10349:2016 [15] are used to derive the corresponding indoor conditions as dictated by the EN ISO 13788 Standard [5] for walls exposed to outdoor conditions.

The assessment of indoor conditions is carried out considering a worst-case scenario, i.e. supposing a heating system able to provide sensible heating only and not to manage latent heat. This means that indoor relative humidity is expected to rise, especially in case of high occupancy rate and high vapour production from the occupants, which depends on their activity.

More in detail, indoor air temperature is set to 20℃ during the heating periods listed in Table 2, while it coincides with the outdoor air temperature value in the remaining of the year (a lower threshold of 18℃ applies in case of particularly cold outdoor conditions). As for indoor relative humidity values, these are derived by adding to the vapour partial pressure in the outdoor a contribution associated to indoor vapour sources, as a function of the intended use of the indoor space (e.g., offices, spaces with or without a mechanical ventilation system, kitchens, etc.) and the average monthly outdoor air temperature.

Since internal vapour production can significantly influence the hygrothermal performances of the walls, in this research both the suggested “vapour class production 3” – houses without mechanical ventilation and with unspecified occupancy pattern – and the more demanding “class production 4” – gyms, kitchens and canteens – are taken into account. This allows performing a sensitivity analysis of mould growth, surface condensation and interstitial condensation risks to indoor humidity values.

Finally, the amount of condensate that is detected in the various assemblies’ layers is compared against the threshold values set by the EN ISO 13788 Standard and reported in Table 3 that, in any case, should be below the suggested value of 500 g∙m^-2.

Table 3. Threshold values for interstitial condensation for various construction materials

This research studied the thermal and hygrometric behavior of the e-CLT system developed in the framework of the H2020 Innovation Project e-SAFE. Such a system is made of CLT panels connected to existing wall structures through energy dissipative devices on the inner side while being covered with a wooden-based insulation material and a cladding layer on the outer side. The e-CLT performance, assessed through the stationary method dictated by the European EN 13788 Standard, revealed that this energy retrofit intervention significantly improve both the thermal and hygrometric behavior of typical wall structures located in different climate zones in Italy.

As examples, the U-values achieved after applying CLT panels to solid brick walls and cavity walls drop down from 1.81 W∙m^-2∙K^-1 and 1.11 W∙m^-2∙K^-1 to 0.31 W∙m^-2∙K^-1 and 0.28 W∙m^-2∙K^-1, respectively, thus making these constructions complying with the maximum admitted U-values for walls after major renovations. In addition, the summer thermal performance is significantly improved thanks to the supplementary thermal mass brought by CLT panels, which typically shows density values in the range of 400 to 600 kg∙m^-3, and the improved thermal resistance.

From the hygrometric point of view, the e-CLT solution eliminates any surface condensation and mold growth risks present in various typical wall structures (most notably solid bricks and concrete walls), and reduces the amount of interstitial condensation within their structure. In fact, the additional water vapor resistance (μ ≈ 60) makes it more difficult for the water vapor to migrate from the indoors and reach the coldest points of the wall. In any case, the maximum amount of condensate predicted is only 20 g∙m^-2 in the worst scenario (renovated concrete walls in coldest climate zone F, indoor vapor production class 4). This figure is well below the maximum threshold of 500 g∙m^-2 set by the normative and the corresponding amount achieved by the same wall without the application of e-CLT (1400 g∙m^-2).

Further studies are planned to assess other condensation mechanisms such as vapor convection, capillary transport and surface diffusion through advanced transient hygrothermal simulations.