Thank you very much for the questions and I am very sorry for the issues!
Our developers are aware of the problem with the Insulation materials already. In this case, the same layer parameters are used twice (in Color and Bump also). I will ask for a workaround in this case.
When Redshift finishes the render, it will apply an extra finishing step to brighten the scene. This can be configured by checking the Detailed Settings > Photographic Exposure. This option works somewhat similar to a camera, so you can see something like shutter speed, aperture and ISO. If you find the scene to be too bright, you can adjust these values, or disable it completely.
I will let you know once we learn more regarding the Insulation materials.
Thank you very much!
Technical Support Engineer