In my situation, I have to agree with Fozzer. My CPU increases by about 15-20F with the case open.
Hi Skittles...

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Case manufacturers go to great lengths to design their cases to provide the most efficient air flow through the case.
Anything which interrupts the normal air flow through the case ie: lid removal, cable bundles, ribbon cables, cobwebs, dead rodents, etc, will adversely affect the temperature of the components.
There are a number of articles on the WEB describing methods of ensuring efficient air cooling within computer cases.
Cheers all...

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Paul....off in the garden to bury the latest mouse...

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LOL...!
Win 8.1 64-bit. DX11. Advent Tower. Intel i7-3770 3.9 GHz 8-core. 8 GB System RAM. AMD Radeon HD 7700 1GB RAM. DVD ROM. 2 Terra Byte SATA Hard Drive. Philips 17" LCD Monitor. Saitek Cyborg X Fly-5 Joystick. ...and a Briggs and Stratton Petrol Lawn Mower.