Implications Of Thermographic Standards On The Inspection Of Buildings

AbstractThis article reviews the recent developments in thermographic standards for the inspection of the building envelope. A general overview of the present standards and standards activities is given. The purpose of the standards is explained, their general content outlined and how they are to be used is discussed. The results of a field evaluation of the accuracy of thermographic inspectors in locating insulation voids in cavity walls are summarized. The reasons for the observed inconsistencies in the results of the inspections of the same building by differnent thermographic inspectors are given. In general, at present, it must be concluded that improvements in existing inspection techniques are needed before thermographic inspections can be considered accurate.INTRODUCTIONIn order to promote the correct application of thermography for the inspection of building envelopes several standard organizations and professional societies have in recent years formed subcommittees or working groups whose purpose is the development of standards for the thermographic inspection of buildings. The International Standards Organization (ISO) under the technical committee on thermal insulation standards (TC 163) has written the draft standard "Thermal Insulation - Qualitative Detection of Thermal Irregularities in Building Envelopes - Infrared Methods" (TC 163 DP6781.3E). This draft standard has been approved by the members of the technical committee and has been sent to ISO for formal approval. The American Society of Heating, Refigeration and Air Conditioning Engineers (ASHRAE) has developed a comprehensive standard on the use of various types of infrared sensing devices for building applications: ASHRAE Standard 101 - 82, "Application of Infrared Sensing Devices to Assessment of Building Heat Loss Characteristics". The American Society for Testing and Materials (ASTM) has at least two standards activities on thermographic applications to buildings. Subcommittees C16 on thermal insulation has written a standard practice on the use of thermography for inspecting insulation installations in cavity walls: "Practice on Thermographic Inspections of Insulation Installations in Envelope Cavities in Wood Frame Buildings". Subcommittee E6 on infiltration, in conjunction with subcomittee C16, is starting work on methods for locating air infiltration leakage sites in buildings in which thermography will be considered. The development of these standards has been reported in previous articles in the Thermosense series [1-4],Each of these standards complements the other. The ISO standard is a standard which is to serve as a guide for the development of specific standards on thermography in different countries. For this reason it is quite general in nature. The ASHRAE standard discusses the specifications of many types of infrared sensing devices for buillding applications: spot radiometers, line scanners, infrared imaging systems and aerial infrared sensing systems. It is the first standard in the building applications area to introduce the minimum resolvable temperature difference (MRTD) as the parameter most suitable for specifying the suitability of a specific thermal sensing system for detectiong and identifying thermal deficiencies. The ASTM standards activities are directed at specific applications which are judged to be important applications of thermography to buildings. The standard practice for the inspection of cavity walls was developed for those who wish to use thermography as a quality control tool for insulation installations.CONTENTS OF THERMOGRAPHIC STANDARDSThe purpose of thermographic standards is to specify instrumentation requirements, test conditions, inspection procedures and data interpretation methods for guaranteeing the detection of building thermal defects with a known degree of accuracy and certainty. Each standard has sections describing the application of the standard, restrictions on the use of the standard, specifications for the infrared sensing systems to be used, test requirements and conditions, test procedures, data to be recorded and its format, the evaluation and interpretation of the thermograhic data and the reporting of the data. Though each standard treats each of these topics, they all do not do so in the same manner. ASHRAE and ASTM standards recognize the MRTD of an infrared imaging system as the appropriate parameter for