Influence of environmental parameters on the thermographic analysis of the building envelope

Abstract

Typically, thermographic measurements are used to analyze the building envelope in a qualitative way (e.g. detection of thermal bridges, missing insulation, etc.). Besides, thermography can also be used to obtain surface temperatures, which can give an indication of the thermal performance of the building envelope. In this paper the influence of the most important environmental parameters on the thermographic analysis of different wall types is examined using a multivariable parameter study. This results in guidelines that are in good accordance with the existing guidelines. Furthermore some remarks were made concerning the ambiguities in the existing guidelines. INTRODUCTIONEurope has high ambitions concerning energy efficiency and the reduction of greenhouse gas emissions. By 2050, one of the goals is to reduce the CO2-emissions by more than 80 %, which is impossible with the current policy. In order to reach this goal, the percentage of renovated buildings has to increase from 1% to around 2,5% per year by 2020 [1]. For new buildings, one of the key factors to satisfy the need for energy efficiency is a high performing building envelope, which can be achieved by a high insulation level and an excellent airtightness. In this context, infrared thermography (IR) is a valuable non-destructive tool which can be used both for evaluating the execution quality of the building envelope or for determining where renovation of the building envelope is necessary. For a proper assessment of the wall surface temperature the building envelope has to be as close as possible to steady-state conditions. Therefore an adequate knowledge of the influence of environmental parameters (solar irradiation, wind velocity, sky radiation, etc.) and building parameters (thermal mass of the structure) on the dynamic behavior of the building envelope is needed. Due to the large number of parameters that influence this dynamic behavior together with the limitations of the existing thermographic cameras, additional research is necessary to make quantitative surveys more reliable. In what follows an overview of the currently existing guidelines is given. Starting from the existing guidelines, simulations were conducted to investigate the dynamic behaviour of the wall surface temperature for different combinations of environmental parameters. Finally, feedback on the currently existing standards is given taking into account the results obtained by the simulations.

Review of in situ methods for assessing the thermal transmittance of walls

“…In situ measurements can give more representative values [25,38,40,[45][46][47]51,61,71], but the use of such methods is affected by many factors, with environmental factors being the most important. In situ measurement methods require [45,46,48,51,53,65,94,120,143,145] (i) a high thermal gradient (𝑇 𝑖𝑛 − 𝑇 𝑜𝑢𝑡 >10 °C), (ii) a wind speed of 0-1 m/s, (iii) zero rainfall, and (iv) no solar radiation or other radiation sources to affect the wall of interest. Other factors, such as metrological errors and data analysis, are also assessed in this review.…”

“…Wind speeds <1m/s allow steady conditions to be guaranteed. Vijver et al [145] found that wind combined with a clear sky and solar irradiation strongly affect QIRT test results. Tests should therefore ideally be performed 2 h before dawn [46] and when (i) the difference between the internal and external air temperatures is 7-16 °C, (ii) the wind speed is 0.1-1 m/s, (iii) no rain is falling, and (iv) the wall has not been exposed to direct radiation (to avoid thermal inertia effects).…”

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

“…Implementation of quantitative IRT encompasses aspects relating to operating conditions (outdoor air temperature), thermophysical properties (the kappa value, also called the heat capacity per unit of area) and technical conditions (test duration and data acquisition interval) [50,51,52,36,39,29,25,53,34,54,55]. In previous studies conducted in laboratories or experimental rooms, the first two aspects might be considered a source of discrepancy in the determination of the measured U-value, regardless of the technique that has been undertaken (simulation, climatic chamber or HFM) [29,31,33,55].…”

Automatic thermographic scanning with the creation of 3D panoramic views of buildings

“…In both cases the differences between indoor and outdoor temperature and the thermal properties of the walls under inspection (e.g. thermal conductivity, thermal mass, and surface emissivity) should be known or estimated 10 . Thermography could be applied in passive or active configuration, depending whether the analyzed element is stimulated by an external thermal source 11 .…”