This note describes a test method for determination of air flow resistance of ex terior membranes and sheathings. The test specimen is placed between two chambers with different air pressures and the volumetric air flow rate through it at a steady state is determined. The relevant experimental quantities can presently be measured with precision better than 0.5% and with an accuracy of 2 to 3 % , using commercial instruments. However, the instrumental precision does not mean much, due to the uncertainty introduced by material variability normally occurring in commercial products. This aspect of the test method is studied and a practical test procedure is suggested.
