What insulation material is used for the Dualsun SPRING panel?

The Dualsun SPRING4 and SPRING3 in their insulated version have an insulation layer on the backside of the panel composed of 17mm (SPRING4) and 21 mm (SPRING3à of expanded polystyrene (EPS), pressed against the heat exchanger by means of a retaining jumper.

This material provides excellent thermal performance and durability. The EPS used by Dualsun is designed to withstand high temperatures.  Tested in extreme conditions specific to solar panel testing, between -40°C and +85°C at humidity levels up to 80%, the insulation meets all normative requirements.

Here are the results of the rain penetration test carried out during the Solar Keymark certification at the TÜVlaboratory.

The "rain penetration" test consists of spraying the panel constantly for 4 hours and then doing a very precise weight measurement before and after to detect any increase of the panel's mass.  The Dualsun panel succeeded this test, which signifies that the insulation does not absorb any humidity.

To go further : What is the difference between the non-insulated and insulated versions of the SPRING panels?