Describes apparatus used to measure air leakage through walls, the types of walls and the test procedure. Gives results of tests on plain walls and shows the effect of adding plaster and paint. Concludes that infiltration rates of plain walls vary greatly. Of the three factors, affecting infiltration rates, workmanship is the most important, the composition of mortar next and the type of brick the least important. Finds that gypsum plaster stops almost all infiltration and that the application of paint reduces leakage. Suggests that infiltration loss through brick in a dwelling would range from 6 to 25 percent of the loss through windows and doors.