The experimental characterization of the overall thermal transmittance of homogeneous, moderately-and non-homogeneous walls, windows, and construction elements with innovative materials is very important to predict their thermal performance. It is also important to evaluate if the standard calculation methods to estimate the U-value of new and existing walls can be applied to more complex configurations, since the correct estimation of this value is a critical requirement when performing building energy simulations or energy audit. This paper provides a survey on the main methods to measure the thermal transmittance and thermal behaviour of construction elements, considering laboratory conditions and in-situ non-destructive measurements. Five methods are described: the heat flow meter (HFM); the guarded hot plate (GHP); the hot box (HB), considering the guarded HB (GHB) and the calibrated HB (CHB); and the infrared thermography (IRT). Then, previous studies dedicated to the assessment of the thermal performance of different heavy-and light-weight walls are discussed. Particular attention is devoted to the measurement of the U-value of nonhomogeneous walls, including the effect of thermal bridging caused by steel framing or mortar joints, and the presence of PCMs or new insulation materials in the configuration of the walls. hot box; calibrated hot box; infrared thermography. Highlights: -Review on the main methods to measure the U-value of non-homogeneous walls. -Methods: heat flow meter, guarded hot plate, guarded and calibrated hot box, infrared thermography. -Standards framework and discussion of the main advantages and drawbacks of each method. -Description of methodologies and working principles of laboratory and in-situ measurements. -Measurement of the thermal transmittance of different heavy-and light-weight walls.

Given the great influence of the thermal transmittance of the building envelope on the overall thermal performance and energy efficiency of the building, it is essential to accurately determine the U-value of the main building envelope elements. Due to the great heterogeneity of the thermal conductivity of the elements presented in a lightweight steelframed (LSF) wall, and to the geometric complexity of some steel framed structures, a reliable estimation of the thermal transmittance of LSF elements is even more challenging. Indeed, thermal bridging originated by steel studs must be considered in the assessment of the thermal transmittance of LSF walls. In this work, the thermal transmittance (U-value) of three LSF walls with different configurations will be investigated based on four different approaches: experimental laboratorial measurements based on the Heat Flow Meter (HFM) method; 3D finite element method (FEM) simulations using ANSYS CFX ® software; 2D FEM-based simulations using THERM software; analytical estimations based on the ISO 6946 procedure for building components with inhomogeneous layers. Several verification procedures were performed to ensure the reliability of the results. It was found that a secondary wood stud can mitigate the thermal bridging effect of the steel frame and improve the LSF thermal performance, which is more noticeable when there is no thermal insulation. Furthermore, a good agreement was found between the results of the 2D FEM and the analytical ISO 6946 approaches for the LSF wall with only vertical steel studs.

Ordinary Portland Cement is the most widely used binder in the construction sector; however, a very high carbon footprint is associated with its production process. Consequently, more sustainable alternative construction materials are being investigated, namely, one-part alkali activated materials (AAMs). In this work, waste-based one-part AAMs binders were developed using only a blast furnace slag, as the solid precursor, and sodium metasilicate, as the solid activator. For the first time, mortars in which the commercial sand was replaced by two exhausted sands from biomass boilers (CA and CT) were developed. Firstly, the characterization of the slag and sands (aggregates) was performed. After, the AAMs fresh and hardened state properties were evaluated, being the characterization complemented by FTIR and microstructural analysis. The binder and the mortars prepared with commercial sand presented high compressive strength values after 28 days of curing-56 MPa and 79 MPa, respectively. The mortars developed with exhausted sands exhibit outstanding compressive strength values, 86 and 70 MPa for CT and CA, respectively, and the other material’s properties were not affected. Consequently, this work proved that high compressive strength waste-based one-part AAMs mortars can be produced and that it is feasible to use another waste as aggregate in the mortar’s formulations: the exhausted sands from biomass boilers.

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