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

Abstract

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.

Comparison of the Experimental Measurement Methods for Building Envelope Thermal Transmittance

“…Marshall et al [43] studied the variations of U-factors measured by indoor IRT Method for a whole residential building and the corresponding difference in the estimated building energy performance. The authors explored the application of low-resolution IRT and high-resolution IRT and pointed out that while being more informative, the variation of U-factors obtained from the IRT method is larger than that resulting from the HFM Method.…”

U-value time series analyses: Evaluating the feasibility of in-situ short-lasting IRT tests for heavy multi-leaf walls

“…Focusing on technical conditions, some researchers underlined the difficulties in using quantitative IRT tests with a short sampling duration [58,37,38,39,59,55,60,61]. Kisilewicz et al [37] stated that data recorded by an IR camera should be collected for a sufficiently long period (an integer multiple of 24 hours) to determine the thermal resistance reliably and, avoid the fluctuations in temperature and heat flows.…”

Measuring and modelling retrofit fabric performance in solid wall conjoined dwellings

“…Recent work has also emphasised the limitations of point measurements in determining whole element U-values, particular to solid wall construction. Marshall et al employed three different techniques to measure in situ U-values in controlled conditions and found there to be a significant difference between the plane element U-values derived using point measurements when compared to those derived with the aid of high-resolution thermal imaging [88]. Akin to the work presented in this paper, the range of U-values estimated by Marshall et al were used to calibrate energy models that predicted HTC values between 183 -285 W/K [88].…”