Understanding thermal properties and how they impact and change the environment inside a building is a crucial part of building design and construction. How thermal information on building materials is used and applied will affect the efficiency and durability of a structure in addition to the comfort of the occupants inside. Materials with low thermal conductivities that hinder the exchange of heat between the interior and exterior of the building are used as insulation. However, these materials often lack strength and therefore more dense, stronger materials are needed to create the actual structure. These materials, such as metals and stone, have higher thermal conductivities and therefore do not have the same effect on efficient thermal control within the building. The Hot Disk thermal conductivity testing system is an excellent tool to aid engineers and developers in understanding how the materials they use affect the thermal environment in the building.
A study from the United Arab Emirates provides an excellent example of using the Hot Disk thermal conductivity measurement technology to test the thermal conductivity of three of the most common building materials in use in older structures there. Suleiman (2006) determined the dry and wet thermal conductivities of hard brick, soft brick, and soft soil, which often form the building envelop in structures where insulation is absent. The objective of the study was to understand how seasonal changes and moisture content impact the thermal conductivity of these materials and generate information to help make these buildings more comfortable for the inhabitants. Suleiman was also interested in how density affected the change in conductivities between wet and dry states to apply the results to broader research in the future.
Figure 1. Example of a traditional building in the United Arab Emirates1.
The Hot Disk sensor and transient thermal conductivity measurement system was ideal for this project as the short measurement time and small temperature increase prevented non uniform distribution of water in saturated samples during the test from causing disparities in the results. The sleek sensor design of thin spiraled nickel surrounded by a thin kapton insulator (Figure 2a), and clamping the sensor tightly between two identical pieces of the sample enabled excellent thermal contact and reduced the chance of contact resistance (Figure 2b). Thermtest also offers a single sided sensor, the TPS-S (Figure 3), which would be highly applicable in projects such as this one. As the TPS-S is handheld and only needs one sample (as opposed to two pieces of the same sample) for thermal conductivity testing, it is able to test samples of unlimited size. This is attractive for clients who have expensive samples that they would like to keep intact, or for clients whose samples are incredibly difficult and time consuming to cut into two pieces.
Figure 2. (a) Illustration from Suleiman (2006) of the Hot Disk sensor used in thermal conductivity measurements, (b) set-up used by Suleiman (2006) for thermal conductivity testing on three building materials.
Figure 3. GIF showcasing the use of the TPS-S sensor, whose easy use and lack of initial sample preparation give it a wide range of applications.
The results obtained by Suleiman (2006) showcased in Table 1 revealed that the higher density material (hard brick) had a higher thermal conductivity, and that there was a large increase in the thermal conductivity in all samples after they had been washed to a saturation point of 40%. Soft soil, the lowest density material tested, had the greatest thermal conductivity increase after 40% saturation (Table 1).
Table 1. Wet and dry thermal conductivities (TC) of three common building materials and the percent difference between them obtained by Suleiman (2006) using a Hot Disk thermal conductivity instrument.
|*Difference between wet and dry TC
The conclusions drawn by Suleiman (2006) from the results of his research have important applications as far as understanding how traditional building materials in the Middle East react to moisture exposure and the impacts those reactions have on structural durability and inhabitant comfort. Although hard brick had the smallest percent change in thermal conductivity, Suleiman was not satisfied that the thermal conductivity value would provide adequate thermal control without the added presence of insulation. He concluded that structures built out of all three of these materials require added insulation in order to sufficiently protect inhabitants from moisture level changes in the outside environment. Materials with the lowest density had the strongest reaction in terms of thermal conductivity measurement changes to saturation. This can be explained by the fact that lower density materials have more air pockets in them than materials of higher density, and therefore there are more spaces for the water to permeate. Since the air, which has a thermal conductivity of 0.025 W/mK, is replaced by water with a thermal conductivity of 0.6 W/mK, the overall conductivity of the material will go up. Since higher density materials do not have the same amount of air available for such a switch, their thermal conductivities are not as strongly impacted by saturation.
This study gave a good base for future work on the relationship between density and thermal conductivity change as moisture increases in building materials. The ability of the Hot Disk Transient Plane Source thermal conductivity measurement system to quickly determine the thermal conductivities of building materials will be of continued importance as the world seeks more sustainable building designs in an effort to create a more environmentally conscious future.