Almost Bernd, it used to get warm in there,,,,,

It's a water holding tank for my solar panels, about 600 ltrs. Lined with 100mm of Kingspan insulation and a butyl liner.
Used to get pretty hot in there last summer, trouble was condensation, I just couldn't stop it when the temp rose above as low as 30 deg C, higher and it became a real problem.
I've now fitted a 300lt SS unvented tank so no probs.
And yes the solar does work quite well. Summer water can get above 100 deg C if you are not careful. This time of year when the sun barely gets above horizon it raises my tank by about 10 deg when the sun sines. It'll soon start improving as we pass the solace.