Steady-State Thermal Properties of Rectangular Straw-Bales (RSB) for Building

Abstract

Straw is an inevitable product of cereal production and is available in huge quantities in the world. In order to use straw-bales as a building material, the characteristic values of the thermal performances should be determined. To not lose the benefits of the cheapness and sustainability of the material, the characteristics must be determined with simple and inexpensive means and procedures. This research aims to implement tools and methods focused at the determination of the thermal properties of straw-bales. For this study, the guidelines dictated by ASTM and ISO were followed. A measurement system consisting of a Metering Chamber (MC) was realized. The MC was placed inside a Climate Chamber (CC). During the test, a known quantity of energy is introduced inside MC. When the steady-state is reached, all the energy put into MC passes through its walls in CC, where it is absorbed by the air-conditioner. A series of thermopiles detect the temperature of the surfaces of the measurement system and of the specimen. Determining the amount of energy transmitted by the various parts of MC and by the specimen, it is possible to apply Fourier’s law to calculate the thermal conductivity of the specimen.

1. Introduction

Straw packaged in rectangular straw-bales (RSB), coming directly from harvesting in the field, is increasingly being used in buildings as an element of non-load-bearing walls in frame structures made of wood and steel and as a constituent element of bearing walls. The latter use has taken a decisive step with the publication of the Standard ICC IRC 2015 [1]: in Appendix S—Straw-bale Construction, a standard for the structural use of straw-bales.

In Europe, the possibility to use elements as components of buildings is affected by the actual knowledge of their physical characteristics, in particular values of their mechanical, thermal, acoustic, and fire resistance performances.

Currently, many studies have been performed on the use of natural building materials, since these materials have characteristics of high sustainability [2,3,4,5,6,7,8,9]. In particular, for straw, useful suggestions have been taken by the following studies [10,11,12,13].

From an economic point of view, only a small part of harvested straw finds profitable uses, such as bedding for livestock farms. In many situations, straw has a cost of disposal, both when it is buried and when it is burnt.

In addition, one of the major advantages of the use of the straw in construction is the possibility to supply the material locally.

In order to not lose these benefits, straw should be used in construction after harvesting without treatments, but in this way the RSB can be very different from each other. The different features of the straw depend mainly on the growing area, the climatic conditions of the cultivation season, the procedures of harvesting and collecting, and the species and variety of cereal.

Therefore, in order to know the characteristic values [14], it is necessary to determine these values from time to time using equipment and methods relatively simple and cheap, easy to reproduce close to the area of utilization.

This research aimed to develop procedures to determine the thermal performance of RSB in the context of Tuscany Region.

To reach this goal, Standards ASTM C1363-11 [15] and UNI EN ISO 8990:1999 [16] (“Standards”), were taken as reference.

The instructions of the Standards were adopted as guidelines, but they were not completely applied. In this study, an out-and-out Hot Box was not used, but rather a low-cost system able to characterize with engineering precision the single RSB. However, the main principles and procedures of the reference Standards were followed. Also the terminology of the Standards was adopted, even if not perfectly matching to the realized system.

2. Materials and Methods

2.1. Standard and Scientific Sources

For the construction and execution of the thermal test system, the following Standards were used: ASTM C1363-11 [15]; UNI EN ISO 8990:1999 [16]; ASTM D 4442-92 (Reapproved 2003) [17]; ASTM D 4933-99 [18].

Given the peculiarities of the specimens to be tested, the information provided by these Standards were adapted to the actually existing situation.

In addition to the above-mentioned Standards, for the construction and use of the hot box, useful suggestions were taken in the following studies [19,20,21]. Furthermore, other suggestions were taken in studies on the thermal characterization of the materials carried out in a simplified manner [22,23,24].

2.2. Equipment for the Meausurement System

The design and construction of the test system and trials were carried out in laboratories of the Department of Agricultural, Food and Forestry Systems (GESAAF).

The measurement system is composed by these sub-systems: Metering Chamber (MC); Climate Chamber (CC); heaters; MC internal ventilation; CC air conditioning system; temperature measurement; air speed measurement; energy flow measurement.

The MC has the external dimensions of 1200 × 1300 × 1800 mm³ (Figure 1 and Figure 2).

The MC (Figure 3 and Figure 4) is constituted by a central body, having the external dimensions of 1200 × 1200 × 1800 mm³. The connections between the walls of the body have a labyrinthine shape, in order to reduce the loss of heat due to discontinuities.

The body can be closed by a movable flat rear wall and by a movable front wall. The front wall can be entirely closed, or provided with a hole for the positioning of the specimen. Even the connections of the body with movable walls have a labyrinthine shape.

The Specimen Wrapper (SW), designed to contain the specimen, leans out of the hole of the front wall. Inside MC a baffle is placed, to direct the flow of air generated by the fans and to prevent the inner surface of the specimen being directly exposed to the heat sources.

The walls of the MC and the SW are constituted by panels of Stiferite BB, having a nominal thickness of 50 mm. The panels are coupled, such as to form a wall of nominal thickness of 100 mm, by means of vinyl adhesive. Two layers of panel joints are staggered to minimize the possibility of heat loss through them. The typical emissivity of the bituminous paper surface is ε = 0.93, higher than the minimum required by [15] (ε > 0.80).

Stiferite BB is a sandwich panel made of an insulating component in polyisocyanurate foam, coated on both sides with bituminous paper. The mass per unit area of the 50 mm thick panel is 2.20 kg/m², corresponding to a density of 44.38 kg/m³. The thermal conductivity declared by the manufacturer is λ = 0.028 ${\mathrm{Wm}}^{-1}·{\mathrm{k}}^{-1}$ (at the mean temperature of 10 °C, for 20–70 mm thickness). Since the thermal insulation properties of the foam materials decay over time and the date of manufacture was not indicated, the above value of λ is considered indicative.

The Climate Chamber (CC) is a closed space, in which the MC is placed (Figure 1).

The heat is generated by a system placed inside the MC, consisting of incandescent lamps of different power. The group is enclosed in a box of Stiferite BB closed on five sides. The open side faces a baffle of the same material to reduce the heating of the walls of the MC by direct radiation (Figure 1). The lamps can be operated individually by external control.

The internal ventilation of MC is supplied by a ventilation system composed by five computer fans, two having the rated power of 2.00 W, three having the rated power of 2.28 W.

An air conditioning system is placed inside the CC.

The surface temperature measurement system is 14 thermopiles arranged on the surfaces of the internal and external walls of the MC, of the SW, and of the specimen. One thermopile is arranged around the MC, for the measurement of the temperature of the interior of the CC. Each thermopile consists of several type K thermocouples.

The thermopiles are connected to a Pico TC08 data logger.

The measurement of the air speed is performed by means of a hot-wire probe lataOHM AP471 S2 connected to the data logger lataOHM DO9847.

The energy flow relative to heat generators and fans placed inside the MC is measured by means of a power meter (Wattmeter) PCE instruments type PCE-PA6000.

2.3. RSB Geometry and Density

The RSB is a geometrically irregular item. Its size and density can vary not only as a function of the species and varieties of cereals and of the methods of cultivation and harvesting, but also from element to element of the same origin.

To allow the actual comparison of the results of thermal tests, a measurement system of the geometry of the bales was designed. In this way, only conventional values of the dimensions and of densities were determined. However, these values are effectively comparable with each other.

For the measurement of the plant dimensions, four pinwheel walls were made, in order to follow the trend of the four vertical faces of the bales (Figure 5a).

For the measurement of plant dimensions, a horizontal force of 200 N to each pair of opposite walls has been applied.

For the measurement of the height of the edges, a straight edge having a mass of 24.30 kg was made (Figure 5b).

To determine the specimen straw moisture, the Standards [17,18] were applied. A convection oven type Orlando Valentini PRG/M 250 and a scale Kern 470 having resolution d = 0.01 g was used.

2.4. Preparation of the Specimen

Since the RSB have an irregular geometric shape and do not have all the same size, when the bale is placed inside the SW, it does not completely fill the internal volume. For the trials, the empty space was filled with loose straw coming from a bale of the same batch of the one to be tested. An appropriate quantity of straw was inserted in the empty spaces, such that the whole mass of straw inside the SW has the same density of the tested bale (Tables 5 and 9).

2.5. Theoretical Observations Regarding the Process

The theoretical observations regarding the process are taken by different authors [25,26,27].

2.5.1. Basic Observation

When energy Q_aux is within the MC, its internal temperature becomes higher than the temperature of the CC. This latter temperature is kept constant by an air conditioning system. Continuing to input the same energy flow Q_aux, when both temperatures of the MC and CC remain constant, the status of steady state has been reached. In these conditions, all the energy flow Q_aux entered in the interior of the MC is transmitted, through its walls, to the CC, and it is absorbed by the air conditioning system of the CC or dispersed into the surrounding environment if its temperature is lower than the CC temperature.

2.5.2. Thermal Conductivity of the Walls of the MC

Six walls of the same material and thickness L_mc constitute the MC.

The effective area of the MC wall normal to heat flow A_mc,e,eff is given by [15]:

$${\mathrm{A}}_{\mathrm{mc},\mathrm{e},\mathrm{eff}}={\mathrm{A}}_{\mathrm{mc},\mathrm{e},\mathrm{in}}+0.54·{\mathrm{L}}_{\mathrm{mc}}·{\mathsf{\Sigma }\mathrm{e}}_{\mathrm{i}}+0.60·{\mathrm{L}}_{\mathrm{mc}}^2$$

(1)

where A_mc,e,in is the MC inside surface area, ${\mathsf{\Sigma }\mathrm{e}}_{\mathrm{i}}$ is the sum of 12 MC interior edge lengths formed where two walls meet each other.

The quantity of energy flow introduced in MC, Q_aux is given by [15]:

$${\mathrm{Q}}_{\mathrm{aux}}={\mathrm{Q}}_{\mathrm{h}}+{\mathrm{Q}}_{\mathrm{f}}$$

(2)

where Q_h is the energy flow introduced by heaters, Q_f is the energy flow introduced by fans.

Applying Fourier’s law, the thermal conductivity λ_mc,eff of the material of the walls of the MC, in the specific temperature range, can be determined:

$${\mathsf{\lambda }}_{\mathrm{mc},\mathrm{eff}}=\frac{{\mathrm{Q}}_{\mathrm{aux}}·{\mathrm{L}}_{\mathrm{mc}}}{{\mathrm{A}}_{\mathrm{mc},\mathrm{e},\mathrm{eff}}·\left({\mathrm{t}}_{\mathrm{mc},\mathrm{in}}-{\mathrm{t}}_{\mathrm{mc},\mathrm{out}}\right)}$$

(3)

where t_mc,in is mean temperature of the internal surfaces of the walls of the MC, t_mc,out is mean temperature of the external surfaces of the walls of the MC.

2.5.3. Thermal Conductivity of the Specimen

In the front wall of the MC there is an opening, from which a rectangular tube protrudes, named Specimen Wrapper (SW), suitable to contain the RSB to be tested. The four walls of the tube are made of the same material of the walls of the MC mentioned above.

The energy flow running through the specimen Q_sp is given by [15]:

$${\mathrm{Q}}_{\mathrm{sp}}={\mathrm{Q}}_{\mathrm{aux}}-{\mathrm{Q}}_{\mathrm{mc},\mathrm{o}}-{\mathrm{Q}}_{\mathrm{fl}}$$

(4)

where Q_mc,o is the energy flowing through the walls of the MC, Q_fl is the energy flowing through the walls of the SW.

In order to determine the energy flowing through the walls of the MC (Q_mc,o), it is necessary to know the effective net area of the MC wall (A_mc,o,eff) normal to heat flow, which is given by:

$${\mathrm{A}}_{\mathrm{mc},\mathrm{o},\mathrm{eff}}={\mathrm{A}}_{\mathrm{mc},\mathrm{e},\mathrm{eff}}-{\mathrm{A}}_{\mathrm{mc},\mathrm{o}}$$

(5)

where A_mc,o is the area of the opening, plus the surface, facing towards the interior of the MC of the zone highlighted in red in Figure 1.

Applying Fourier’s law and knowing the thermal conductivity λ_mc,eff of the material of the MC walls, in the specific temperature range, it is possible to determine the energy Q_mc,o flowing through the walls of the MC:

$${\mathrm{Q}}_{\mathrm{mc},\mathrm{o}}=\frac{{\mathsf{\lambda }}_{\mathrm{mc},\mathrm{eff}}·{\mathrm{A}}_{\mathrm{mc},\mathrm{o},\mathrm{eff}}·\left({\mathrm{t}}_{\mathrm{mc},\mathrm{o},\mathrm{in}}-{\mathrm{t}}_{\mathrm{mc},\mathrm{o},\mathrm{out}}\right)}{{\mathrm{L}}_{\mathrm{mc}}}$$

(6)

where t_mc,o,in is the mean temperature of the internal surfaces of the walls of the MC, t_mc,o,out is the mean temperature of the external surfaces of the walls of the MC.

In order to determine the energy flowing through the walls of the SW (Q_fl), it is important to know the effective net area of the SW wall (A_fl,eff), normal to heat flow. It is given by the relation [15]:

$${\mathrm{A}}_{\mathrm{fl},\mathrm{eff}}={\mathrm{A}}_{\mathrm{flin}}+0.54·{\mathrm{L}}_{\mathrm{mc}}·{\mathsf{\Sigma }\mathrm{e}}_{\mathrm{i}}+0.60·{\mathrm{L}}_{\mathrm{mc}}^2$$

(7)

where A_fl,in is the internal area of the SW, minus the surface, facing towards the interior of the SW, of the zone highlighted in Figure 1; ${\mathsf{\Sigma }\mathrm{e}}_{\mathrm{i}}$ is the sum of 4 SW interior edge lengths formed where two walls meet each other, minus the lengths of the edges in correspondence of the zone highlighted in red in Figure 1.

Applying the Fourier law and knowing the thermal conductivity λ_mc,eff of the material of the SW, in the specific temperature range, previously determined, it is possible to determine the energy flow (Q_fl) flowing through the walls of the SW:

$${\mathrm{Q}}_{\mathrm{fl}}=\frac{{\mathsf{\lambda }}_{\mathrm{mc},\mathrm{eff}}·{\mathrm{A}}_{\mathrm{fl},\mathrm{eff}}·\left({\mathrm{t}}_{\mathrm{fl},\mathrm{in}}-{\mathrm{t}}_{\mathrm{fl},\mathrm{out}}\right)}{{\mathrm{L}}_{\mathrm{mc}}}$$

(8)

where t_fl,in is the mean temperature of the internal surfaces of the walls of the SW and t_fl,out is the mean temperature of the external surfaces of the walls of the SW.

Applying (4), the value of the energy flow passing through the specimen, Q_sp, can be obtained.

Applying the Fourier law, the value of the thermal conductivity λ_sp of the specimen relative to the actual test conditions can be obtained:

$${\mathsf{\lambda }}_{\mathrm{sp}}=\frac{{\mathrm{Q}}_{\mathrm{sp}}·{\mathrm{L}}_{\mathrm{sp}}}{{\mathrm{A}}_{\mathrm{mc},\mathrm{o}}·\left({\mathrm{t}}_{\mathrm{sp},1}-{\mathrm{t}}_{\mathrm{sp},2}\right)}$$

(9)

where L_sp is the length of the specimen in the prevailing direction of the energy flow, t_sp,1 is the mean temperature of the hot surface of the specimen, and t_sp,2 is the mean temperature of the cold surface of the specimen.

2.6. Main Simplifications

In addition to those suggested by the [15], the most significant simplifications introduced are the following:

a. Fourier’s law

The Fourier’s law in its one-dimensional ($x$) form is used (see

Table 1. Symbology.

Energy, Heat	J	(kg⋅m2⋅s−2)
Time	T	s
Temperature	t	°C, K
Heat Flow density, time rate of heat flow through a unit area	q	W⋅m−2
Heat Flow, time rate of net heat flow through the area A	Q	W (J⋅s−1)
Apparent thermal conductivity	λ	Wm−1·K−1

):

$${\mathrm{q}}_{\mathrm{x}}=\frac{{\mathrm{dJ}}_{\mathrm{x}}}{\mathrm{dT}}=\mathsf{\lambda }·\frac{\mathrm{dt}}{\mathrm{dx}}$$

(10)

For two points at constant temperature, at a distance Δx (see Table 1):

$$\mathrm{Q}=\mathsf{\lambda }·\frac{\mathrm{A}·\Delta \mathrm{t}}{\Delta \mathrm{x}}$$

(11)

b. Geometry and density of straw-bales and of the specimen

For this purpose, the system described in Section 2.3 was designed and operated.

c. Energy flows

The energy flow that occurs between the MC and CC through the area marked in red in Figure 1 is very complex. In this zone, the heat exchange between the MC and CC is not direct, but occurs through the adjacent zones of the walls of MC. The definition of the way in which this heat exchange occurs requires a dedicated study, which will be performed in a second step of the research. In this work, considering that the surface of the zone in question is relatively small compared to the total, the effects of this singularity are neglected.

d. Air flows

The specimen shall be sealed relating to exchanges of air with the MC and the CC. For this purpose, two surfaces in contact with MC and with CC were closed by means of a film of Linear Low-Density Polyethylene (LLDPE), having a thickness of 23 µm and thermal conductivity of 0.33 Wm⁻¹·K⁻¹.

The connections of the detachable walls with adhesive tape were sealed.

e. Effective areas

The Equations (1) and (7) are derived by applying to this particular case the relation (A3.2) of the ASTM [15], which is given for a chamber having five walls, that is eight edges. In this case, there are respectively 12 and 4 edges: relations are calculated using the corresponding values of the two cases.

3. Results and Discussion

3.1. Thermal Conductivity of the Walls of the MC

To implement this test, the MC was entirely closed. The MC walls were all identical in terms of material and thickness. Thermopiles and thermocouples were arranged as shown in

Table 2. Thermopiles and thermocouples arrangement.

Equipment		Thermopiles		Thermocouples
		Internal	External	Internal	External
MC	Left	1	2	4	4
	Right	1	2	4	4
	Top	3	4	6	6
	Bottom	5	6	5	5
	Front	7	8	6	6
	Back	9	10	6	6
CC		17			6

.

Considering that the diffusion of heat on the left and right walls is symmetrical, there was only one thermopile for their internal surfaces and one for the external ones.

The test was carried out from 16 March 2015 11:47 to 22 March 2015 13:27. For the calculation, the data related to the time interval 21 March 2015 12:27 to 21 March 2015 23:17 were used. The data were collected with an interval of 10 min. The results are summarized in

Table 3. Determination of the Thermal Conductivity of the walls of the MC.

Climate Chamber Temperature	tcc	5.05	°C
Air speed, near the hot side of the specimen	assp	0.31	m/s
Metering Chamber inside surface area	Amc,in	8.92	m2
Metering Chamber wall thickness	Lmc	0.103	m
Sum of all (total of 12) Metering Chamber interior edge lengths formed where two walls meet	Σei	14.80	m
Metering Chamber effective area normal to heat flow	Amc,eff	9.75	m2
Metering Chamber inside wall surface temperature	tmc,in	31.22	°C
Metering Chamber outside wall surface temperature	tmc,out	7.32	°C
Net heat added by the heaters	Qh	56.61	W
Net heat added by the fans	Qf	10.31	W
Net heat removed by the cooling coil	Qc	0.00	W
Net heat flow due to the fan, heater, and cooling coil	Qaux	66.92	W
Metering Chamber wall thermal conductivity	λmc,eff	0.030	W⋅m−1⋅K−1

.

3.2. Thermal Conductivity of the Specimen SB01

To implement this test, the MC was closed with the holed front wall bearing the SW. The MC and SW walls are all identical, in terms of material and thickness. Thermopiles and thermocouples were arranged as shown in

Table 4. Thermopiles and thermocouples arrangement.

Equipment		Thermopiles		Thermocouples
		Internal	External	Internal	External
MC	Left	1	2	4	4
	Right	1	2	4	4
	Top	3	4	6	6
	Bottom	5	6	5	5
	Front	7	8	6	6
	Back	9	10	6	6
SW	Path 1	11	12	8	8
	Path 2	13	14	8	8
Specimen	Hot side	15		9
	Cold side		16		9
CC			17		6

.

Considering that the diffusion of heat on the left and right walls of MC is symmetrical, there was only one thermopile for their internal surfaces and one for the external ones. The geometrical dimensions and density of the RSB were conventionally determined as illustrated in Section 2.3 (Figure 6).

The specimen was prepared as described in Section 2.4. (See also

Table 5. Dimensions and density of the RSB and of the specimen.

RSB (Code)	RSB Volume (m3)	RSB Mass (kg)	RSB Density (kg/m3)	Cereal Species	Cereal Origin	SW Internal Volume (m3)	Specimen Mass (kg)	Mass Added to Fill SW (kg)
SB01	0.178	11.70	65.69	Triticosecale	Sorano (GR)	0.237	15.60	3.90

).

The moisture of the specimen was determined as described in Section 2.3 and results are shown in

Table 6. Specimen straw moisture.

RSB (Code)	Sample (ID)	Unladen Mass (g)	Starting Straw Mass (g)	Starting Mass (g)	Controll Mass (g)	Final Straw Mass (g)	Water Content (g)	Moisture Content (%)
SB01	M01	96.39	58.07	154.46	148.01	51.62	6.45	12.50
	M02	91.43	50.18	141.61	136.18	44.75	5.43	12.13
	M03	95.11	63.10	158.21	151.34	56.23	6.87	12.22
	M04	90.37	67.97	158.34	150.77	60.40	7.57	12.53

.

The test was carried out from 23 March 2015 16:38 to 27 March 2015 10:18. For the calculation the data related to the time interval 26 March 2015 16:08 to 26 March 2015 21:38 were used. The data were collected with a time interval of 10 min. The test results are summarized in

Table 7. Calculation of the thermal conductivity of the specimen SB01.

Climate Chamber Temperature	tcc	5.830	°C
Air speed, near the hot side of the specimen	assp	0.315	m⋅s−1
Metering Chamber inside surface area	Amc,in	8.920	m2
Metering Chamber wall thickness	Lmc	0.103	m
Sum of 12 Metering Chamber interior edge lengths formed where two walls meet	Σei	14.800	m
Metering Chamber effective area normal to heat flow	Amc,eff	9.750	m2
Metering Chamber inside wall surface temperature	tmc,in	31.83	°C
Metering Chamber outside wall surface temperature	tmc,out	8.277	°C
Net heat added by the heaters	Qh	58.26	W
Net heat added by the fans	Qf	10.604	W
Net heat removed by the cooling coil	Qc	0.00	W
Net heat flow due to the fan, heater, and cooling coil	Qaux	67.430	W
Metering Chamber wall thermal conductivity	λmc,eff	0.030	W·m−1·K−1
Metering Chamber opening area	Amc,o	0.792	m2
Metering Chamber net area	Amc,eff,n	8.958	m2
Metering Chamber wall loss	Qmc	61.452	W
Specimen Wrapper inside surface area	Asw,in	1.307	m2
Specimen Wrapper wall thickness	Lsw	0.103	m
Sum of all (total of 4) Specimen Wrapper interior edge lengths formed where two walls meet	Σesw,i	1.720	m
Specimen Wrapper effective area perpendicular to the heat flow passing through it	Asw,eff	1.409	m2
Length of the heat flow path (test specimen)	Lsp	0.530	m
Specimen Wrapper wall thermal conductivity	λsw,eff	0.030	W·m−1·K−1
Specimen Wrapper inside wall surface temperature	tsw,in	16.457	°C
Specimen Wrapper outside wall surface temperature	tsw,out	7.457	°C
Specimen Wrapper wall heat loss	Qsw	3.694	W
Specimen heat flow	Qsp	2.283	W
Specimen hot surface temperature	tsp,1	31.388	°C
Specimen cold surface temperature	tsp,2	6.698	°C
Apparent thermal conductivity of the specimen	λsp	0.062	W·m−1·K−1

.

3.3. Thermal Conductivity of the Specimen SB02

To implement this test, the thermopiles and thermocouples were arranged as shown in Table 4.

Considering that the diffusion of heat on the left and right walls of MC is symmetrical, there was only one thermopile for their internal surfaces and one for the external ones.

The geometrical dimensions and density of the RSB were conventionally determined as illustrated in Figure 7.

The specimen was prepared as described in Section 2.4 (see also

Table 8. Dimensions and density of the straw-bale and of the specimen SB02.

RSB (Code)	RSB Volume (m3)	RSB Mass (kg)	RSB Density (kg/m3)	Cereal Species	Cereal Origin	SW Internal Volume (m3)	Specimen Mass (kg)	Mass Added to Fill SW (kg)
SB02	0.205	17.20	84.11	Triticum turgidum durum	Vicarello (PI)	0.237	19.97	2.77

).

The moisture of the specimen was determined as described in Section 2.3 and results are shown in

Table 9. Specimen straw moisture.

RSB (Code)	Sample (ID)	Unladen Mass (g)	Starting Straw Mass (g)	Starting Mass (g)	Controll Mass (g)	Final Straw Mass (g)	Water Content (g)	Moisture Content (%)
SB02	M01	96.39	86.59	182.98	174.33	77.79	8.80	11.31
	M02	91.43	77.10	168.53	160.60	68.99	8.11	11.76
	M03	95.11	41.20	136.31	132.10	36.99	4.21	11.38
	M04	90.37	54.73	145.10	139.11	48.94	5.79	11.83

.

The test was carried out from 8 April 2015 10:30 to 10 April 2015 11:10. For the calculation the data related to the time interval 9 April 2015 21:10 to 10 April 2015 08:10 were used. The data were collected with a time interval of 10 min. The test results are summarized in

Table 10. Calculation of the thermal conductivity of the specimen SB02.

Climate Chamber Temperature	tcc	5.815	°C
Air speed, near the hot side of the specimen	assp	0.315	m⋅s−1
Metering Chamber inside surface area	Amc,in	8.920	m2
Metering Chamber wall thickness	Lmc	0.103	m
Sum of all (total of 12) Metering Chamber interior edge lengths formed where two walls meet	Σei	14.800	m
Metering Chamber effective area normal to heat flow	Amc,eff	9.750	m2
Metering Chamber inside wall surface temperature	tmc,in	31.727	°C
Metering Chamber outside wall surface temperature	tmc,out	8.272	°C
Net heat added by the heaters	Qh	56.826	W
Net heat added by the fans	Qf	10.604	W
Net heat removed by the cooling coil	Qc	0.000	W
Net heat flow due to the fan, heater, and cooling coil	Qaux	67.430	W
Metering Chamber wall thermal conductivity	λmc,eff	0.030	W⋅m−1⋅K−1
Metering Chamber opening area	Amc,o	0.792	m2
Metering Chamber net area	Amc,eff,n	8.958	m2
Metering Chamber wall loss	Qmc	61.194	W
Specimen Wrapper inside surface area	Asw,in	1.307	m2
Specimen Wrapper wall thickness	Lsw	0.103	m
Sum of all (total of 4) Specimen Wrapper interior edge lengths formed where two walls meet	Σesw,i	1.720	m
Specimen Wrapper effective area perpendicular to the heat flow passing through it	Asw,eff	1.409	m2
Length of the heat flow path (test specimen)	Lsp	0.530	m
Specimen Wrapper wall thermal conductivity	λsw,eff	0.030	W⋅m−1⋅K−1
Specimen Wrapper inside wall surface temperature	tsw,in	16.159	°C
Specimen Wrapper outside wall surface temperature	tsw,out	7.106	°C
Specimen Wrapper wall heat loss	Qsw	3.716	W
Specimen heat flow	Qsp	2.520	W
Specimen hot surface temperature	tsp,1	31.040	°C
Specimen cold surface temperature	tsp,2	6.994	°C
Apparent Thermal Conductivity of the Specimen	λsp	0.070	W⋅m−1⋅K−1

.

The value of the thermal conductivity of the walls of the MC (λ = 0.030 ${\mathrm{Wm}}^{-1}·{\mathrm{k}}^{-1}$) is congruent with the value of the thermal conductivity declared by the manufacturer (λ = 0.028 ${\mathrm{Wm}}^{-1}·{\mathrm{k}}^{-1})$ of the base material for the reason mentioned in the Section 2.2.

The values of the thermal conductivity of the specimens (SB01 and SB02) experimentally determined are consistent with values found in the scientific literature [28,29]. Therefore, it is reasonable to consider that the system leads to results of sufficient accuracy for the purpose of the research.

However, the results are extremely sensitive to the values instrumentally detected, in particular to the value of the energy flow fed into the MC and the values of the internal temperatures of the MC and the SW. In consequence of this criticality, the instrumental measurements of these quantities must be made with great accuracy and this involves the use of instrumentation and procedures too expensive and complicated with respect to the intentions of this research.

Moreover, it is important to remember that the influence of the area marked in red in Figure 1 was not studied in this research. For these reasons, the research must be developed, especially with the intent to reduce the influence of the criticalities above reported in significant quantities.

4. Conclusions

The aim of the research is the identification of simple and economic equipment and procedures that can be replicated in proximity to the area of use of RSB. These methods should be suitable for the determination of the values of thermal conductivity of RSB actually available on site for building, although not of absolute precision.

The ways to reach this goal may be mainly two:

(a)

to modify the current system in order to reduce the sensitivity of the system to the values of energy flow and temperatures instrumentally detected;

(b)

to design and implement a system that, instead of trying to determine the absolute value of the thermal conductivity of the specimens, determines the relative value compared to specimens of materials having a known value of thermal conductivity.

Currently, it can be assumed that method (b) can be realized with tools and procedures simpler and cheaper than these required by method (a). The implementation of a transportable version, from time to time, on the zones of effective use of straw-bales, can even be hypothesized.

Future developments will be aimed to continue tests in this direction, with the purpose to reduce the uncertainty margins of results, mainly due to some of the adopted simplifications.