Journal of Building Physics

Three-dimensional analyses of the pore structure of building materials are becoming progressively more important in recent years. The main goal is to obtain more accurate interpretations and simulations of their properties and performances. Computed tomography has proven to be an excellent and versatile tool to perform these analyses non-destructively. The reconstruction of the pore structure is of high importance for establishing accurate models, as it plays a crucial role in determining important characteristics of building materials such as their hygrothermal, mechanical, or acoustic properties. These models allow us to better understand the results of corresponding laboratory tests and in the near future might replace these time-consuming experiments. In this article, the strengths and weaknesses of computed tomography as a data acquisition technique are examined and discussed.

Cellular porous materials are frequently applied in the construction industry, both for structural and insulation purposes. The progressively stringent energy regulations mandate the development of better performing insulation materials. Recently, novel porous materials with nanopores or reduced gas pressures have been shown to possess even lower thermal conductivities because of the Knudsen effect inside their pores. Further understanding of the relation between the pore structure and the effective thermal conductivity is needed to quantify the potential improvement and design new optimized materials. This article presents the extension of a 3D numerical framework simulating the heat transfer at the pore scale. A novel methodology to model the reduced gas-phase conductivity in nanopores or at low gas pressures is presented, accounting for the 3D pore geometry while remaining computationally efficient. Validation with experimental and numerical results from the literature indicates the accuracy of the methodology over the full range of pore sizes and gas pressures. Combined with an analytical model to account for thermal radiation, the framework is applied to predict the thermal conductivity of a nanocellular poly(methyl methacrylate) foam experimentally characterized in the literature. The simulation results show excellent agreement with less than 5% difference with the experimental results, validating the model’s performance. Furthermore, results also indicate the potential improvements when decreasing the pore size from the micrometre to the nanometre range, mounting up to 40% reduction for such high-porosity low-matrix-conductivity materials. Future application of the model could assist the design of advanced materials, properly accounting for the effect of reduced pore sizes and gas pressures.

The airtightness of eight apartment buildings containing six to 11 units each on three or four floors was tested with and without guard-zone pressure, that is, with and without consideration of internal leakages. The layouts of these buildings varied: two of them had no central stairwell; in two other buildings, only some of the apartments were connected to the central stairwell; and the third type had all apartments connected to a central stairwell. Airtightness tests were performed with and without guard-zone pressure conditions. During these tests, two to eight BlowerDoor systems were used simultaneously to create guard-zone pressure conditions. In this report, the authors evaluate the test results of three buildings of different layout types. Furthermore, a reference model for the natural air permeability of all construction materials used in the interior and exterior envelopes of each apartment was created for two buildings in accordance with the German Industrial Standards (DIN). We present the results of this assessment and put them in context with the airtightness tests with and without guard-zone pressure. The results indicate that the air leakage contribution of internal partitions is significant, namely 32% and 27%. As this affects sound transmission, fire protection, odor transfer, and the quality of ventilation, it is essential to assess the airtightness of not only the exterior but also the interior envelope of each apartment.

This work deals with the influence of envelope thickness and solar absorption on the time lag and the decrement factor. For this, a test cell of 1 m³ of volume is built with a material commonly used in construction in Senegal, the compressed earth brick stabilized with cement. The ambient-air temperature inside and outside of test cell and solar direct normal irradiance is measured. The test cell is modeled using EnergyPlus software. The comparison of experimental and theoretical ambient-air temperature puts out a great linear showing the reliability of the model. The time lag and the decrement factor are calculated using the air-sol equivalent temperature of the test cell and the inside ambient-air temperature. The time lag and decrement factor of the compressed stabilized earth brick envelope are respectively 0.22 and 6.6 h showing the good thermal inertia of those bricks. A parametric study is performed to determine the effect of envelope thickness and solar absorptivity on the time lag and decrement factor. The results show that the decrement factor decreases with envelope thickness while the time lag increases linearly and that an envelope thickness of 32 cm has a decrement factor of around zero with a maximum time lag of about 12 h for this type of material. The envelope’s solar absorption has a moderate effect on the decrement factor and time lag.

The present trend in building research is to improve sustainability in building construction and operation. The development of new renewable technologies is essential to improve the sustainability and to reduce emissions. The incorporation of phase change materials in buildings is an effective way to reduce the room temperature fluctuations and cooling loads/heating loads. Although several works have been carried out in this field, a novel phase change material clay hollow-brick composite has been used in this work. This article discusses the research on investigating the thermal performance of phase change material integration in building walls. Two identical test rooms (3 m × 3 m × 3.65 m) were constructed to study the effect of phase change material integration in buildings. The experimental buildings were constructed for the warm and humid weather conditions of Chennai city, India. Phase change material integration in the building wall is beneficial for reduction of room temperature and provides passive cooling of the building. The temperature drop in a phase change material room compared with a non-phase change material room varies from 6°C to 2°C, during various months of the year. DESIGNBUILDER simulation was carried out for phase change material and non-phase change material buildings during the months of January, March, May, and July. The simulated room temperature variation follows the same pattern in these months.