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This paper addresses the validity of employing heat flow sensors (HFSs) on the indoor surfaces of building walls to determine thermal characteristics. It also reports on the results obtained in the field.
Some of the factors affecting HFS measurement accuracy (together with a likely percentage standard deviation attributable to that factor) are as follows: (a) the conductivities of HFS and its surroundings (3%), (b) convection mode changing over the sensor, causing a +21% bias (26%), (c) the mismatch of HFS absorptivity with the surroundings (6%), and (d) thermal contact of the HFS with the surface (1%). A propagation-of-errors analysis indicates that the resulting standard deviation of an HFS measurement would be approximately 10% of the mean of the measurements.
The R-value measurements of 20 buildings at four Army bases and installations demonstrated that the technique is repeatable—the same data time series with different start times results in the same R-value, side-by-side sensors give the same results (±5%), and different buildings of the same construction give similar results. Buildings whose construction was verified by boring into the wall gave measurements that agreed within 10% with the American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) calculations. Also, convection cells were very evident, even in insulated frame walls, necessitating a traverse of sensors from the top to the bottom of the wall.
With appropriate awareness of potential error sources, surface-mounted HFSs can render reasonable results.
heat flux, heat loss, heat transfer, thermal insulation, walls, thermal conductivity, heat flow sensors, In situ R, -value measurement, measurement error analysis, building thermal diagnosis, natural convection, radiation heat transfer, conductive heat transfer, heat flux transducers
Research civil engineer, U.S. Army Cold Regions Research and Engineering Laboratory, Hanover, NH