Volume 15, Issue 3 (May 1987)
Design and Calibration of a Guard Added to an Existing Hot Box
The 6.10 by 4.27 m (20 by 14 ft) vertical heat flow hot box at the Owens-Corning Fiberglas Technical Center has recently been used to make measurements on a variety of roof/ceiling insulation systems used in the steel bar joist and metal deck construction typical in commercial buildings. Since these systems varied in geometry from one with roof insulation only to one combining roof insulation, a plenum, and an insulated hung ceiling, the sample support needed to be nearly 1.22 m (4 ft) tall. This meant that the extraneous energy flows through this support would be a substantial fraction of the sample energy flow for well-insulated samples. Furthermore, these relatively large flows combined with the variety of system geometries would have required that calibrations be developed for multiple geometries.
The solution to this problem was to provide a thermal guard around the sample support to minimize the losses through it. This guard consisted of a channel, in which the sample was replicated and exposed to the same indoor temperature, maintained by an unmetered energy source. This resulted in approximately the same vertical temperature profile along the outside of the sample support wall as on the metering chamber side. This system was successful in keeping the energy flows through the sample support below 5% of the sample energy flow. A calibration procedure was then used to quantify these small losses, which were used in the calculation of sample thermal resistance.