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The ASHRAE Handbook table on thermal resistances of airspaces notes that the values apply only to spaces of uniform thickness bounded by plane, smooth, parallel surfaces without air leakage. Additional text statements emphasize the need for caution when applying, calculated resistances to practical, nonideal situations and recommend that tests be conducted for multiple reflective airspace constructions.

This paper presents results of such tests. Data were obtained on commercially available reflective insulations carefully installed between framing members in a large vertical calibrated hot box, thus simulating a wall application. The thermal resistance of the insulated cavity was derived from measured data using a parallel path calculation. It was found that resistances calculated using ASHRAE Handbook data overstated the thermal resistance an average of 146% (almost 2 1/2 times) compared to the data obtained from the measurements. Thus, continuing caution in the use of calculated data is indicated.