The DualSun Spring panels have an insulation layer on the backside of the panel composed of 30mm of high temperature rock wool.
This material provides excellent thermal performance and durability. The rock wool used is non-flammable, water resistant, and designed to resist high temperatures. The material is classified as an A1 non-flammable material. 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.
In particular, here are the "rain penetration" test results performed during Solar Keymark certification at the TUV laboratory.
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.