Variations in the U-Value Measurement of a Whole Dwelling Using Infrared Thermography under Controlled Conditions

• Published in: Buildings (Basel) 2018-03, Vol.8 (3), p.46
• Main Authors: Marshall, Alex, Francou, Johann, Fitton, Richard, Swan, William, Owen, Jacob, Benjaber, Moaad
• Format: Article
• Language: English
• Subjects: building fabric; Coefficient of variation; Computer simulation; Controlled conditions; dwellings; Energy consumption; Energy measurement; Heat flux; heat loss; Heat transfer; Heat transfer coefficients; Identification methods; in situ performance; Infrared imagery; Infrared inspection; Inspection; IR thermography; Mathematical models; Measurement methods; Measuring instruments; Performance prediction; Simulation; Thermography; U-value; Uranium; Variation
• ISSN: 2075-5309; 2075-5309
• DOI: 10.3390/buildings8030046

U-values of building elements are often determined using point measurements, where infrared imagery may be used to identify a suitable location for these measurements. Current methods identify that surface areas exhibiting a homogeneous temperature-away from regions of thermal bridging-can be used to obtain U-values. In doing so, however, the resulting U-value is assumed to represent that entire building element, contrary to the information given by the initial infrared inspection. This can be problematic when applying these measured U-values to models for predicting energy performance. Three techniques have been used to measure the U-values of external building elements of a full-scale replica of a pre-1920s U.K. home under controlled conditions: point measurements, using heat flux meters, and two variations of infrared thermography at high and low resolutions. U-values determined from each technique were used to calibrate a model of that building and predictions of the heat transfer coefficient, annual energy consumption, and fuel cost were made. Point measurements and low-resolution infrared thermography were found to represent a relatively small proportion of the overall U-value distribution. By propagating the variation of U-values found using high-resolution thermography, the predicted heat transfer coefficient (HTC) was found to vary between 183 W/K to 235 W/K (±12%). This also led to subsequent variations in the predictions for annual energy consumption for heating (between 4923 kWh and 5481 kWh, ±11%); and in the predicted cost of that energy consumption (between £227 and £281, ±24%). This variation is indicative of the sensitivity of energy simulations to sensor placement when carrying out point measurements for U-values.