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Abstract

The use of a hot box apparatus allows for thermal performance testing of a specimen in both static and dynamic regimes. In the static (steady-state) regime, the thermal conductivity or R-value of a specimen can be obtained. In comparison, the specific heat capacity and thermal diffusivity data can be obtained during the transient regime. Additionally, if heat flux pads are employed in the hot box apparatus, it is possible to monitor the energy that passes through a specimen during thermal cycling. In the case of wall systems, it is of interest to compare the thermal performance of one system against another for the purpose of selecting suitable energy efficient structures. However, the nature of this comparison is not as simple as it would seem. In this study, the thermal performance of several wall systems were compared to evaluate their energy efficiencies relative to one another. Under steady-state conditions, it is valid to simply compare the R-values against one another assuming equivalent measurement conditions. However, we are actually interested in the real-world performance of these systems, and in this case, the comparison of R-values is not sufficient to fully characterize differences in actual thermal performance. For this reason, a 24-h cyclic thermal loading was employed that mimicked a day/night cycle. By measuring the energy transmitted through the wall, the energy required to maintain thermal comfort could be determined. This study aims to characterize the real-world thermal performance of several systems by using this method.

Wall ties or masonry veneer anchors are a common part of brick veneer wall assembly construction. They serve to anchor the brick veneer to the backup wall, whether that is a wood-stud, steel-stud, or concrete block wall. They provide structural support and keep the brick veneer from moving. However, since wall ties are made from steel, they have a significantly higher thermal conductivity than the surrounding materials in the building envelope, and this difference may cause thermal bridging. The overall impacts of several common types of wall ties in residential and commercial construction were tested using a small-scale hot box apparatus under steady-state conditions. Each test panel was first tested without any wall ties and then subsequently tested with wall ties present. This procedure allowed for direct measurement of the impact of the wall tie while holding all other factors constant. In the case of a typical residential wall without continuous insulation, both types of wall ties tested were found to have no measurable impact on the overall thermal performance. In the commercial walls that contained continuous insulation, the walls ties were found to have a minor impact on the overall thermal performance. Wall systems with significant thermal mass, such as brick veneer, have better performance under dynamic thermal loading, which is not reflected in steady-state measurements. This paper focused on steady-state worst-case results, and future work will address dynamic performance.

Typical brick veneer wall assemblies contain an air space to provide drainage inside the wall. This air cavity results in heat transfer through the wall via conduction, convection, and radiation, with radiation typically being the dominant heat transfer mechanism across the air space. Due to the significance of radiative heat transfer, any modification to the surface properties of the wall facing the air space can result in significant energy savings if the radiation heat transfer can be reduced. Radiation heat transfer can be minimized by installing a radiant barrier in the brick veneer wall. A radiant barrier is a thin metallic foil whose surface has an average emissivity of around 0.05. In this research study, first several commercially available radiant barriers were characterized using a heat flow meter apparatus. The thermal emissivity was estimated indirectly by measuring the thermal resistance of a known air space. Second, small-scale hot box testing was carried out to determine if the radiant barriers performed as expected in a more complex wall assembly. Third, full-scale hot box testing was carried out using one of the commercial radiant barriers. Finally, finite element models were verified against experimental results to allow for predicting the performance of radiant barriers in other wall systems. It was found that installing a radiant barrier in a typical residential brick veneer wall resulted in an increase in the R-value of the system and a significant increase in the dynamic thermal performance of the wall.