## 2 Straw Construction
#### 3.1.1 Straw as a Building Material  
It is firstly important to distinguish straw from hay. Hay is the product of the moving of grass (green, moist, and nutritious) for animals. Straw is the dry part of the plant, which is cut when ripening has been completed. This process guarantees a humidity percentage lower than 12%, which is optimal for construction (Figures 2 and 3). It is possible to distinguish, as a function of the dimensions and the placement of straw bales, two bale positions for construction [7]:  
* placed on their edges (sides), face up, with a thickness of 37 cm and a height of 47 cm;
* flat, with the edges (sides) visible, with a thickness of 47 cm and a height of 37 cm. Table 1 shows data taken from the literature [8; 9; 10] referring to the thermal characteristics of straw, straw bales, and straw walls. It can be observed that the thermal conductivity of straw is generally  
Figure 3: Dimensions and placement of straw bales: on their edges (sides), face up; flat, visible sides. Figure 2: A straw hygrometer used to control humidity during construction. The moisture content of each straw bale is recorded. higher than a standard insulating material, such as expanded polystyrene (EPS) or rock wool, with a conductivity of about 0.035-0.04 W/m/K. Straw can be produced from different plants, including wheat, rye, barley, and rice. Each and every cereal gives a different final product in terms of thermal resistance and durability. The growth of the plants in the fields is much more important than the variety; for example, the presence of too many weeds could seriously compromise the durability of a construction material. From an environmental point of view, the ideal scenario would be to collect straw from a field close to the construction site, thereby ensuring a short-chain production cycle. Straw bales should be stored in a dry place and protected from bad weather conditions. However, it is also possible to deposit bales outside, arranging them in a pyramid shape, and to cover them with a waterproof cover. Pallets can then be used as a base to lift them and avoid contact with damp earth. Particular attention should be paid during the construction phases; if the construction site protection systems are not sufficient to shield the straw from rain, further quality controls should be conducted to verify the degree of dryness before proceeding with subsequent coating layers (Figure 4). Straw quality alone is not enough to guarantee that a building will have good performance and meet requirements similarly to standard buildings. In fact, it is necessary to monitor all the straw bale supply phases of the process and the implementation of the construction system. This is the only way of ensuring good thermohygrometric and energy-saving performance in straw buildings. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Straw Bale Density**} & \multicolumn{3}{c}{**Thermal Conductivity \(\lambda\)**} \\  & \multicolumn{3}{c}{**of Straw [Wm-1K-1]**} & \multicolumn{3}{c}{**Thermal Transmittance U**} \\ \cline{2-4}  & **Flat** & **on Edge** & \\ \hline
62 & 0.082 & - & - \\
75 & 0.057 & 0.052 & - \\
81 & - & 0.057 & - \\
90 & 0.05–0.06 \({}^{*}\) & 0.05–0.06 \({}^{*}\) & - \\
100 & - & 0.038 & - \\
150 & 0.06 & 0.048 & - \\
200 & 0.06 & & 0.665 \\
250 & 0.07 & & 0.762 \\
300 & 0.075 & & 0.809 \\
350 & 0.08 & & 0.855 \\ \hline \hline \end{tabular}  
* Direction of bales not specified.  
\end{table} 
Table 1: Thermal characteristics of straw [8; 9; 10]. Figure 4: Example of storage inside a building. Once the construction of the straw house has been successfully completed, it is advisable to avoid prolonged exposure to humidity, as previously mentioned. The presence of water in the construction elements may be related to poor sealing of the roof covering and the plaster, the capillary rise of water from the ground, leakage from plumbing, or condensation. It is equally important to prevent water from entering the structure and to let the humidity produced in the rooms out.