Noninvasive Residential Building Envelope $R\text{-Value}$ Measurement Method Based on Interfacial Thermal Resistance

The building envelope plays an important role for the contributions to the whole building thermal load, especially for residential houses without significant internal heat gain; hence, it is a critical component for influencing a building’s thermal performance. However, the envelope $R\text{-value}$ is usually acquired from the original design value, regardless of the thermal insulation degrading that has occurred during the whole long lifecycle. The main reason lies in that, with traditional methods, the in situ envelope $R\text{-value}$ is extremely inconvenient to measure. In this study, a simple noninvasive approach was proposed to measure the envelope $R\text{-value}$ using the interfacial thermal resistance from the thermal boundary layer between the envelope exterior surface and the ambient air, and the envelope surface temperature measured through the infrared thermal imaging. A field measurement procedure was developed and implemented for a typical detached residential house, along with a widely accepted method using film heat-flux sensors as a validation test. The final results show a good approximation between these two tests, which shows that the proposed method could extend the functions of thermography from only qualitative diagnosis to quantitative measurement on envelope thermal performance.