1. Introduction
Energy refurbishment of existing buildings is one of the priorities of the European policies to reduce fuel consumption, starting from the recognition of the ‘exemplary role of public bodies’ buildings (art.5 2012/27/UE) [
1] to activate effective strategies in the private building stock. Existing buildings in the European Union are, indeed, responsible for 40% of final energy consumption [
2] and for 36% of carbon dioxide (CO
2) emissions [
3]. Approximately 35% of the buildings are more than 50 years old [
3]. Considering the low rate of new buildings construction, 3% in Europe [
4], and 2% in the USA [
5], energy efficiency in existing, historical, and historic buildings is one of the greatest opportunities towards a sustainable future.
Besides the social and cultural value of all historic buildings, the specific value of heritage assets in Italy strongly justifies the origin of the current research: according to the Italian Ministry for Cultural Heritage and Activities, there are more than 20,000 historic centers of different ages. In light of such numbers, it is evident that many Italian cities are largely made up of historic buildings, which almost often require a greater commitment in the design of conservative and improving interventions than those devoted to the process of new construction. Nonetheless, this is also verified in many other European heritage city centers. Two examples could be pointed out: Edinburgh (Scotland) and Antwerp (Belgium). In 1995, UNESCO added the Old and New Towns of Edinburgh as a World Heritage Site [
6]. In this site, 75% of the 4500 individual buildings are listed for their special architectural or historic interest. This city’s latest management plan concerning the heritage site (2017–2022) has 6 main objectives and 39 actions, of which stands out” strengthening care and maintenance of buildings and streets’ and the ‘sustainable re-use of underused and unused buildings” [
6]. In Antwerp, instead, there are several listed buildings, such as the Vleeshuis Museum located in the historic center. This significant building has been the object of study [
7] in various domains (e.g., evaluation of brick masonry or the assessment of hygrothermal parameters and conservation of important housed collection).
The interest in historic and historical buildings has been gaining cultural and social strategic roles. One important way of preserving built heritage for the future is to keep it in use and to accommodate new uses, avoiding its transformation into a ‘museum’ and preserving its cultural memory. In order to make this operation successful, it is mandatory for their adaptation to today’s comfort requests for indoor human activities. Moreover, promoting the control of hygrothermal parameters and indoor air quality in such buildings also means assuring better conservation of the decorative features that make them distinguishable and enhance their architectural quality.
The building envelope plays an important role in terms of energy transmission. Particularly, the opaque surface in historic buildings constitutes the largest surface of the envelope, and heat losses through this element are, therefore, of most importance [
8,
9,
10]. In fact, some authors defend that in historic buildings heat loss through windows is only 10%, while walls and roof account for 60% (35% and 25%, respectively) [
11,
12]. This means that the intervention aiming to enhance the energy performance of the building should involve the envelope’s components to reach a high level of efficiency. As well-known, sometimes, it is impossible due to the presence of architectural features to be preserved, and the project has to focus on different strategies. In other cases, the envelope’s insulation is possible operating only on the inner façade of the building. Unfortunately, also in these situations, other difficulties may occur, hindering the good result of the operation.
One of the most significant issues in the field of efficiency topics is the buildings energy consumption gap [
13,
14,
15] between design and post-occupancy phase [
16,
17]. In many cases, it has been verified that this gap is due to occupants’ behavior [
18,
19], but it can also be justified by erroneous decisions or values accepted at the design phase (i.e., poor practice or uncertainty in building energy simulation—BES [
20]). Many authors have been demonstrating the limitations of traditional BES tools and procedures for the estimation of energy performance of historical buildings [
21,
22,
23,
24,
25]. This topic reaches a significant dimension in historic buildings refurbishment, once the real wall composition of such buildings is frequently unknown [
26] and, for practical matters, in many occasions, several projects and estimations are based on general assumptions [
27].
The calibration of the hygrothermal models with measured data is very important to avoid irreparable damage to historic buildings. The combination of several hygrothermal variables [e.g., heat flux (φ), surface temperature (Ts), air temperature (Ta), and relative humidity (RH)] should lead to more reliable models.
3. State of the Art
The literature shows two different kinds of in situ tests: (i) test for determining the thermal performance of building elements, in terms of thermal resistance (R-value or R), thermal conductance (C-value or C), or thermal transmittance (U-value or U) [
33]; and (ii) hygrothermal monitoring for determining the hygrothermal behaviour of the various wall layers [
7,
34].
First, commonly used standard tests to experimentally determine the thermal performances of walls [
35] were divided in two groups: (i) In situ tests measurements based on the use of the heat flow meter (HFM) method [
36,
37,
38] or the quantitative infrared thermography testing (ITT) [
39,
40]; and (ii) laboratory tests performed on hot box chambers [
41,
42]. Soares et al. [
33] and Bienvenido-Huertas et al. [
43] have performed two of the most significant literature reviews on this subject. HFM method is a non-destructive testing (NDT) for determining the thermal transmission properties (R, C, or U values) of an existing building directly in situ. The apparatus was composed of a data-logger equipped with two thermal sensors and one heat flux plate for gathering the internal and external Ta or Ts and the φ through the element. The international standard ISO 9869 [
37] defined the calibration and the installation procedures, the data processing techniques, the methodology for correcting systematic errors, and the reporting format. In parallel, the literature presents several methods to solve meteorological and practical issues to reduce the errors and the uncertainties due to the measurement location [
44,
45], the influence of the boundary conditions [
44], [
45], or the presence of non-homogeneity, high thermal inertia [
44], or moisture content [
45] in the structure. In addition, the quantitative ITT permits to measure directly in situ the R-value of a masonry, avoiding the problems related to non-correct locations, non-homogeneity in the walls, or the influence of the boundary conditions [
46]. Otherwise, ITT was also used in a qualitative way to measure the thermal pattern of walls [
47]. Laboratory tests permit to measure the thermal properties of building components in steady-state or dynamic controlled conditions. The guarded hot plate (GHP) measures the steady-state thermal conductivity (λ-value or λ) of homogeneous flat walls [
46,
47]. The international standard ISO 8302 [
48] and the ASTM C177 [
49] defined the minimum requirements for designing the apparatus and the testing procedure. The main problem was related to the errors connected to gaps and edge losses. Several studies proposed analytical calculation models for reducing this error [
50,
51,
52]. The hot box apparatus measures the steady-state and the dynamic thermal performance (R, C, and U values, Ts, internal T, and RH) of inhomogeneous samples. Basically, it is composed of two climatic chambers maintained at different temperatures that simulate the internal and external conditions. The building element under measurement was inserted between the two chambers, and the thermal performance was obtained, measuring the power required to keep the hot chamber at a constant temperature. The ISO standard 8990 [
41], the American ASTM C1363 [
53], the European EN 1934 [
54], and the Russian GOST 26602.1 [
55] defined the minimum requirements for designing the hot box apparatus and the measurement procedure. Two alternative methods are available: the guarded hot box (GHB) and the calibrated hot box (CHB). GHB is composed of a climatic chamber for simulating the exterior temperature, a metering chamber heated to simulate the indoor conditions, and a guard chamber for minimizing the lateral heat flows at the edges of the metering chamber [
41,
53]. CHB is composed only by a climatic and a metering chamber, surrounded by a “temperature-controlled space” to reduce the errors generated by the apparatus [
41,
53]. Concerning the hot box method, many researchers have developed their own compact facility, but only a very few correspond to in situ affectations. A significant majority of the examples found in the literature correspond to variations of the hot box method, e.g., facilities for laboratory tests more targeted at wall/materials sample testing [
44,
56]. In [
57], the authors showed the new design of a compact hot box apparatus used for determining properties of wall samples, developed according to ISO 8990 [
41]. Though upfront and useful in laboratory, this tool was not developed for in situ measurement and the test rig dimensions are ruled by the sample size requirement. One variation of these models are full scale boxes simulating entire ambiences/buildings [
58,
59], among which are distinguished outdoor test boxes solutions for building envelope experimental characterisation [
60]. Once again, these intend to study new materials/walls, and not existing building construction solutions, for example: window shutters [
61], heat insulation solar glass (a type of multifunction PV module) [
62], glazed façades with water film [
63], multilayer, inhomogeneous, and massive walls [
64,
65]. More common instead is the use of a combined strategy for data comparison, as for example the in situ testing coupled with computer modelling and steady-state testing in a GHB [
66] or, the comparison of steady-state and in situ testing of high R walls incorporating vacuum insulation panels [
66].
On the other hand, only a few scientific studies combine both methodologies, and solely for measuring the thermal performances of building elements: The ‘chamber’/box and the HFM. In 2008, Peg and Wu [
67] approached this strategy by designating an entire room of an apartment situated in a new residential development district in Nanjing as a ‘test chamber’ where in situ measuring method for the R-value of buildings was tested (defining ‘measuring points’ arrangement in several walls), but no box was in fact generated. In 2015, in their turn, authors had verified the feasibility of a new developed simple hot box-HFM method (SHB-HFM) to address an in situ measurement of wall thermal transmittance [
68]. This SHB-HFM was preceded by another experiment developed by Chinese researchers in 2012, designated Temperature Control Box-HFM method (TCB-HFM) [
69] cited in [
70]. However, the authors of [
70] (p. 748) described this TCB-HFM as not suitable for the in situ measurement, also noticing that measurement thermal transmittance results obtained in [
69] were “55% higher than the design thermal transmittance and that the measurement error was attributed to high moisture”, denoting the problem of not controlling for humidity in the test.
Besides the final aim of monitoring hygrothermal parameters instead of exclusively the thermal transmittance, the most significant difference between the boxes presented in [
68] or [
70] and the new one now presented lies in the dimension—none of the SHB-HFM boxes surpasses 0.90 m × 0.90 m × 0.30 m. Further developments on this topic are presented in
Section 4.2.
In situ monitoring can be very significant in the case of historic buildings, since: (i) walls samples cannot be examined in the lab (for cultural heritage protection issues, no samples can be removed from original sites); (ii) many historic buildings are abandoned or not in use, and, therefore, are not heated; (iii) many of these building present particular features as high ceilings/volumes and therefore the traditional 1 m × 1 m lab measured surface might not be representative enough of the vertical heat stratification of a historic wall.
The hygrothermal monitoring of heritage buildings can be divided into: environmental monitoring and contact monitoring used, respectively, to assess the environmental condition of a room and the hygrothermal performance inside to a building element [
34]. Skills and procedures for the environmental monitoring of Ta and RH are defined by several standards that focus particularly on cultural heritage (CH) [
71,
72], in order to avoid damage and risks for CH object and surface and users’ discomfort [
73,
74,
75,
76,
77,
78]. Contact monitoring is used to quantify damage already occurred and to predict the presence of potential hygrothermal risks for CH [
79]. The methodologies used can be divided into: (i) surface monitoring of Ts and RHs; and (ii) monitoring of T and RH inside the walls [
34]. No standard procedures have been developed for the surface monitoring of CH building elements [
79]. As a matter of fact, the procedure normally used for new and existing buildings without any heritage value cannot be applied to historic surfaces as risks and losses of historic materials should be avoided.
Moisture content within walls has proved difficult to measure because several variables are unknown, including the influence of the probe on the test results [
80]. Moisture content inside the walls can be measured in two ways using: (i) direct methods based on the gravimetric analysis; and (ii) indirect methods based on the drilling of wooden dowels inserted into the building element. The gravimetric analysis consists in the measurement of water content in a building material sample, weighing its mass with analytical scales in a range of controlled wet and dry conditions [
81,
82]. Standard CEN EN 16,682 [
81] and UNI 11,085 [
82] define the operative procedure. This process involves the drill of samples at various heights and widths across the area being tested and thus, is not always suitable for CH building elements. Indirect methods have been categorized according to measurement principles in resistance, voltage, capacitance, thermal-based, and innovative (e.g., neutron probes, nuclear magnetic resonance, medical ECG electrodes, and fibre optic sensor) methods [
80]. Resistance-based moisture methods are widely used, thanks to the variation of the electrical resistance of the materials under different moisture contents [
80]. Particularly, this method has been successfully used mainly in timber construction [
79,
80,
83], and, most recently, in solid brick walls [
80]. No standards procedures are defined because several factors affect the electrical resistance, such as the timber species, the speed of growth, the origin, and the storage [
80,
83]. Otherwise, calibration factors exist for different timber species [
80]. However, this method has proven to be stable for slow and long-term moisture measurements, with examples of sensors working for a minimum of 20 years [
83]. The results obtained in shorter monitoring periods are not accurate.
Herein, a new approach is suggested: to assess in situ the hygrothermal performance of historic walls (aiming at testing future indoor insulation solutions), a new metering hot box is proposed in combination with T-RH sensors (and eventually added thermocouples if desired), through a low cost and simplified data acquisition system [
34]. To the best of the authors’ knowledge, the new box suggested within the next sections is the first of its kind, totally addressed to historic buildings in situ measurement. Moreover, the developed experiment allows long-term monitoring, against ‘punctual’ measurement in laboratories or short-time HFM measurements as proposed in [
69], not addressed to historic material. Commonly, most studies of this kind and in this field involve the thermal behaviour of walls solely. Alike [
56,
84,
85], the hygrothermal performance assessment is also intended.