QUOTE(andrewmc @ Feb 15 2011, 02:47 AM)
so your saying to cover it halfway with cardboard instead of completely?
The idea is to pull cold air in, drag it through the heatsink fins and then push the warm air it is replacing out the back through the external fans.
By covering half of the heatsink in cardboard you are forcing the cool air to actually be sucked through the fins of the heatsink rather than over the top. However if you do cover the entire heatsink then yes you will be restricting the amount of of air that can fit though the gaps.