I personaly built a bare board shop from the ground up. This was 65K sq ft and I did put in a true clean room with air locks, HEPA filters and the whole deal. It proved to be worth the time and effort as the yields were great compared to shops which I worked in that did not have this clean environment. It is tough to do, and worth it if you do not get close to 100% yield at the imaging point. The effort is not worth it if you do not have problems with these processes.