About 12 years ago at work I had a housing project with some research funds attached as part of a 'green' initiative - we were able to do a lot of thermal modelling - we tried out everything - insulation, orientation, window sizes, different glass types, different construction techniques, materials, trombe walls, different plantings etc.
The big thing that came out of it for me was the basics were most important - orientation and insulation.
Yep, also the double brick cavity holds the heat in more, the insulation works well in summer if you want to run aircon on all the time but that's not what I want to do.
The problem with double brick is the thermal mass, once the inner skin of brickwork heats up it radiates the heat back into the house. I think this is why it feels as though the insulation is heating up the house - the heat being radiated by the inner brick skin is being kept in by the insulation.
Without knowing your house you need to look at the western walls and increase the insulation or shade to that wall (particularly if it has a lot of glazing). Same goes for eastern and northern walls to lesser extents. Southern walls are largely irrelevant.
I've proven this theory wrong in workshops many times in Qld summers.. If you place an industrial fan by the window on the western side windows of a large shed on an afternoon of a hot summers day it actually heats the shed up because you drawing hot air from outside. If you don't believe me try it with a thermometer.
The western side is the hot side. Don't draw air from the hot side, it is only ever going to make it hotter.