I like that for apparent ROI. Halved the monthly bill. If it really saved 600kwh/mo for 3 months of the year at 0.12/kwh that is about $216/season. (that is rough nominal cost of electricity where I am)

well we are basically at the end of the cooling season too. highs in the mid 60-70 range and its in the 50's at night.. so i haven't really been running it at all. i will be insulating the fan soon tho and i will post up on how that goes.. other then that i think a whole house fan is TOTALLY worth the investment.. i still plan on installing another fan at some point but all founds are headed towards the roof at the moment..

Well I got it all sealed up for the winter I didn't take many pictures.. Basically I went up in the attic and used that real duct tape and taped all the seams up. Then I pulled down the grill and put up a piece of the 2" foam and some r-19 insulation in the large hole and put the grill up to hold it all in.. I figure that's probably double the insulation I have in the rest of the attic at the moment..

Nice write-up. I have a cheap whole house fan right now that can displace all the air in our house in a matter of minutes. I throw fiberglass insulation strips on it in the wintertime, but I know that is nowhere near good insulation.

We are moving soon, so instead of fixing the current situation, I have been thinking about how to improve it at the next house. I came up with using a centrifugal fan to minimize the footprint of the attic hole. Plus they seem to be more efficient and quieter than propeller style fans. Has anyone tried using one of these?

Centrifugal fans often aren't as efficient, but they can produce a higher pressure differential.