I recently installed Sisalation under the roof of my RoR Obs open half to prevent dew condensing inside and ripping on to equipment.
I plan to never insulation the walls, as they are E, S and W facing and conduct heat out of the observatory very effectively to assist in cool-down times.
My thread has more info:
http://www.iceinspace.com.au/forum/s...d.php?t=137980
The Anticon I used was about $115 and will insulate better than the foam insulation, but arguably might not look quite as nice because it's thicker so bulges. I spent about $50 in other parts - longer tech screws and tape.
Roger.