I agree re: attics being a large factor for infiltration. I attribute the air sealing efforts made to the ceiling plane in my own home as one of several key factors why my a/c system performs well during extremes in outdoor climate. I am now vacationing in Northern California, where the weather is so wonderful we have all the windows in the house open. If I could import this weather to DFW for the summer I'd do it, but then how could I marvel at how well building thermal boundary improvements perform? :D