It is the amount of water vapour, by weight, under a specific air condition, that penetrates one square metre of the sample being tested within 24 hours, expressed in g/sq.m. 24 hr.

• 它是在一定的温度和湿度条件下，在于24小时内，透过1平方米试样的水蒸气的重量，以克每平方米（24小时）表示。