In-situ measurement of thermal resistance of building envelopes – RESBATI

The metrological tool privileged in this project is the infrared thermography in active approach. This technique consists in producing a thermal excitation artificially so as to create a variation of the temperature in the studied wall and recording a sequence of thermal images. Various techniques of thermal analysis (image processing, modeling, inverse methods) can then be applied so as to obtain characteristic quantitative information on the thermal resistance of the wall to be diagnosed. RESBATI project will try to show the advantages of the active approach: control must be able to be carried out whatever the weather conditions, for any type of building and use (occupied or not).

The robustness of the identification methods is tested thanks to a benchmark carried out between the various partners of the project. A prototype of device will be tested on instrumented test-walls thanks to the climatic rooms of partners (LNE (Cell REBECCA), CEREMA and IFSTTAR). The walls tested will be preliminary characterized by the CSTB by using the standardized method of the Guarded Hot Box. An evaluation of measurment uncertainties will be carried out by the LNE in order to determine the errors of measurement according to typologies of walls considered: Insulation brought back by interior (ITI) or outside (ITE), mono-wall (multi-alveolar brick), wooden wall framework.

The final product which will be developed at the conclusion of the project will include:
- a measuring equipment made up of various components allowing the implementation of infrared thermography in active mode (thermal camera, system of excitation, onboard computing) and additional sensors;
- an instrumentation and a methodology adapted to any type of use;
- a modeling and a robust identification of parameters;
- the display of the result of measurement and associated uncertainty.

A state of the art of the methods of diagnosis of building walls was carried out during the first phase of the project. A synthesis of this document will be written in English and submitted for publication as a Review Article at the end of 2018; a publication in the form of book of the complete state of the art will be considered at the end of the project (French and/or English version).

A benchmark of numerical methods for the estimation of parameters was carried out between four of the partners of project (CSTB, IFSTTAR, LNE and CERTES). The purpose is to perform a blind test of various methods of estimation of the thermal resistance of buildings walls in order to check their robustness. This work was the object of an intermediate report and an oral communication at the scientific workshop of the SFT (METTI group) on May 2nd, 2018. Various publications and communications in congress could be realized starting from this work over the academic year 2018-2019.

The choice of the walls to be tested and the conditions of tests led to the definition of the optimal tests to perform in test facilities of the partners concerned (IFSTTAR, Cerema, CSTB and LNE). This choice also led to define an optimal instrumentation of the walls which will allow to test the models of identification of walls thermal resistance.

Innovating solutions of onboard instrumentation were proposed to design the prototype of the measuring equipment. This work was the object of a communication to the European congress of the EGU in Vienna in April 2018. The prototype will be tested on site at the end of 2018 and one version will be made available to voluntary professionals in order to obtain an experience feedback.

A survey “Questionnaire Utilisateur” was drawn up by THEMACS Engineering company in collaboration with the other partners of the project. It was then addressed in electronic form to building professionals. The diffusion of this questionnaire was carried out by the members of the follow-up committee of the project (ADEME, ALEC St Quentin in Yvelines, French federation of the Building industry). The examination of this survey will be carried out before the summer 2018. The results of this survey will make it possible to better target the user expectations in order to prepare the preproduction of devices which will be tested in the last phase of the project (during 2019). This will also make it possible to specify the outlets for trade of the device and the situations of control to be privileged (self-checking on building site, diagnosis of the existing frame, diagnosis with reception of work).

«Problématique de l'estimation de la résistance thermique de parois courantes de bâtiments par thermographie infrarouge: apport et limitations de la réduction de modèles«, Authors: L. IBOS, Thanh-Tung HA, V. FEUILLET (CERTES), J. WAEYTENS, Z. DJATOUTI, J. DUMOULIN (IFSTTAR), S. THEBAULT, K. ZIBOUCHE, R. BOUCHIE (CSTB), V. LE SANT (LNE). Oral communication, SFT workshop « Méthodes inverses en Thermique du Bâtiment » (May 2018, Paris). This communication presents the main results of the benchmark concerning the test of robustness of identification methods allowing the estimation of the thermal resistance of building walls

“Smart and light data logger for thermal and environmental monitoring”, Authors: L. Gavérina, J. Dumoulin, J. Waeytens (IFSTTAR), J-P. Monchau (THEMACS Ingénierie), L. Ibos (CERTES). Poster presented to EGU annual conference (april 2018, Vienna), describing the structure of the prototoype developped (computer, heating sources, sensors, communication protocols...)

Thermal insulation of opaque walls remains an essential point for improving the energy efficiency in buildings. Indeed, the number of badly insulated buildings in France is still very important. In addition, current thermal regulations set high requirements in terms of thermal insulation and will continue to be more rigorous as new building will be energy-positive with the French RT2020. However, there is no systematic method for measuring the thermal insulation level of the building walls. Their thermal performance must be controlled for renovation of the building, during its construction, for its delivery or during use. The need of a method of in-situ control of walls is more relevant than ever. Such a measurement at the wall level is an interesting complement to global methods (co-heating, etc.) that concern the whole building energy balance.

The physical parameter representing the quality of the wall thermal insulation is its thermal resistance. Currently, methods for measuring this parameter exist, either in the form of laboratory or exploratory methods, or in the form of international standards or draft standards. However, each of these methods does not meet all the conditions guaranteeing a general measurement: use on any type of wall and at any time of the year, low measurement duration, ease of use, moderate cost. The RESBATI project (in-situ measurement of the thermal resistance of building walls) aims at developing an in-situ measurement device that respects these specifications. The measuring means is infrared thermography in active approach. The uncertainty and the limitations of the measurement will be identified during the project.

Infrared thermography in passive mode has demonstrated for many years its ability to reveal the presence of insulation defects in buildings. However, it is essentially a qualitative tool. The active approach of infrared thermography is not very used for building investigation and is a promising way for obtaining quantitative information such as the thermal resistance of the wall to investigate. Indeed research results have already shown that this approach could be used to obtain quantitative estimations of the thermal resistance of opaque building walls. The RESBATI project will demonstrate the potential of the active approach so that control can be performed in any season, for any type of building and any use (occupied or not) and quickly. The passive approach might nevertheless be used as a complement because it does not require the use of additional equipment ensuring the thermal load of the wall to diagnose and provides access to larger wall surfaces to analyze.

The consortium brings complementary partners together working at different levels of the building: research laboratories, technical center, national metrology laboratory, company and standards organization. The advanced knowledge and past achievements of the various partners on the subject make it possible to develop such a method with measurement uncertainty and the associated prototypes. Many facilities will be available for qualification of prototypes: climate rooms for laboratory testing, existing buildings for in-situ qualifications. Thus, a wide variety of walls (structure and isolation level) can be tested. Moreover, these buildings have different uses (residential or service buildings). In conclusion of the project, measurements will be carried out by future end-users of the device.