ISO 20393:2007 specifies equipment and procedures for determining the long-term water absorption of test specimens by diffusion. It is intended for use with thermal-insulation products. It is designed to simulate the absorption of water by products subjected for a long period of time not only to high relative humidity, approximating to 100 %, on both sides but also to a water vapour pressure gradient, as experienced by e.g. inverted roof or unprotected ground insulation.

The test is not applicable to all types of thermal-insulation product. The product standard should normally state whether this test is applicable to a particular product.