Which case do you have?
Does the case have any case fans? If so, where are they located.
What cpu heatsink are you using?
When a power supply is located at the top rear of the case the psu fan should be on the bottom of the psu. It will help to cool the cpu. That was part of the original atx specifications developed by Intel. Intel was making new cpu's that were running hotter so they specified the power supply should be placed near the cpu. The psu could help with cooling the cpu.
In modern pc cases the power supply is often moved to the bottom rear of the case and mutiple case fans are installed near the cpu. The multiple case fans do a better job of exhausting hot air..