On the majority of cases with either user installed or factory installed top fans they are typically used as exhausts. The most cited reason for this is because hot air rises so it makes sense to use it as an exhaust. While that may be true in natural convection once there is airflow in the case the effect of convection is minimal.
Silverstone is one of the few exceptions to this and they have repeatedly introduced cases which use an AP181 as a top intake. This arrangement has garnered good reviews and results even in less space constrained cases like the FT-01. In fact in their tech talk post a few years ago they specifically call out the arrangement of having both an exhaust at the rear and the top as being "poor".
http://www.silverstonetek.com/techtalk_cont.php?area=en&tid=wh_airflow
So why the discrepancy between Silverstone and other manufacturers and even enthusiasts? I don't see anything inherently different between most of Silverstone cases and other manufacturers that would explain why increased effectiveness with a top intake. It seems advantageous to use the top fan as an intake which can draw in cool air directly near the CPU area, and it also helps maintain positive pressure.
Silverstone is one of the few exceptions to this and they have repeatedly introduced cases which use an AP181 as a top intake. This arrangement has garnered good reviews and results even in less space constrained cases like the FT-01. In fact in their tech talk post a few years ago they specifically call out the arrangement of having both an exhaust at the rear and the top as being "poor".
http://www.silverstonetek.com/techtalk_cont.php?area=en&tid=wh_airflow
So why the discrepancy between Silverstone and other manufacturers and even enthusiasts? I don't see anything inherently different between most of Silverstone cases and other manufacturers that would explain why increased effectiveness with a top intake. It seems advantageous to use the top fan as an intake which can draw in cool air directly near the CPU area, and it also helps maintain positive pressure.