Hi, I started to convert my loft for a layout, when one July day I tried to enter, and the heat was staggering. I then looked into insulating the rafters, but could have caused air circulation issues leading to condensation. In short it all got a bit impractical... and expensive. In the end, I made a layout in the garage that can be hauled up to allow access to the garden when needed. Here is a link to another forum where I posted its progress
http://www.classicogauge.net/forum/viewtopic.php?f=32&t=1136 Here is a link showing it running.
Good luck, the planning and building are some of the best bits