APPENDIX/DIAGNOSTICS
Heat Conduction
Infrared Thermography
Infrared thermography can be used to evaluate qualitatively, and quantitatively to a limited degree,
the effectiveness of a building envelope's thermal insulation system. The technique is covered by
ISO Standard 6781-1983, and in the case of wood frame buildings ASTM C 1060. Infrared
thermography employs a thermal imaging system to evaluate the continuity of the thermal insulation
system over the building envelope and to locate and characterize any thermal defects. The imaging
system is used to provide an image of the envelope surface in which the variations in intensity over
the surface correspond to variations of the apparent radiant temperature along the surface. Under
appropriate test conditions, these variation are due to differences in the heat flow through the
surface caused by variations in the thermal resistance. A thermographic inspection involves
assessing the heat loss characteristics of the building envelope through such a thermal image. An
inspection can be conducted from both inside and outside a building as long as the building interior
is heated or cooled to a temperature significantly different from the outside. The technique can also
be applied to an envelope mock-up if one side is heated or cooled. Requirements regarding test
equipment and environmental conditions during the test are contained in the measurement
standards.
While the inspection results, thermograms of the envelope surfaces, do not lend themselves to
quantitative determinations of envelope thermal resistance, a qualitative characterization can be
made of the insulation system's performance. Various thermal defects can be identified including
insulation voids, air leakage sites, and thermal bridges. Drawings of envelope design details can be
helpful in interpreting the results of the survey.
Guarded and Calibrated Hot Box Measurements
Guarded and calibrated hot boxes are both devices used to determine the heat transmission rate
through a mock-up of a building envelope. They are the subject of ASTM Standards C 236 and C
976, respectively. In both of these techniques, the envelope mock-up is placed between two
environmentally-controlled chambers, and a temperature difference is maintained across the
specimen. The rate of heat transmission through the specimen is then measured. The R-value of
the specimen is equal to the area of the test specimen multiplied by the temperature difference
across it, divided by the heat transmission rate measured through the specimen. The two
techniques differ in how they determine the value of this heat transmission rate. Both techniques
measure this heat transmission rate under steady-state conditions, and the requirements for
determining the existence of steady-state as well as other test conditions are given in the ASTM
standards. Several commercial and research laboratories across the country possess such hot
boxes and conduct these measurements routinely.
PAGE D-2
Previous Page
