This International Standard specifies the equipment and procedures for determining the long term water absorption of test specimens by diffusion. It is applicable to thermal insulating products. It is intended to simulate the water absorption of products subjected to high relative humidities, approximating to 100 %, on both sides and subjected to a water vapour pressure gradient for a long period of time e.g. inverted roof or unprotected ground insulation. The test is not applicable for all types of thermal insulating products. The product standard should state for which of its products, if any, this test is applicable. NOTE For unprotected ground insulation the temperature of 50 °C may be replaced by a lower temperature, when more data is available.