Precise U-value measurement of installed windows

Abstract

As part of the global goal to increase buildings energy efficiency, this work focus on the ability to measure U-values of windows in an effective way. To be more precise the investigation focuses on 5 different sized windows, while the window type remain the same. This is done by measuring the window glazing from different points using Rapid U-value meter. The instrument works by adding thermal insulation on a part of the window. Due to the insulation the temperature under it starts to decrease. This is compensated by a system with temperature sensors and electrical heaters to keep the temperatures identical under and beside the instrument. The required power to keep the temperature under the insulation at same level with the undisturbed surface gives the heat flux. The measurements is taken mainly during night time to limit the exposure to solar radiation. Based on the measurement on the glazing, a method to approximately calculate the U-value for whole window including frame is investigated and the accuracy estimated. The method is tested on 7 other windows not part of the initial testing, making the total amount of windows tested to 12. The results, based on totally 796 measurements, are promising with relatively good accuracy. The results are discussed and it is concluded that the method can already be used in field work, however a better accuracy can be achieved by securing the validity of calibration of the U-value meter and measuring the U-value for a longer time.