Energy saved by 40% of a Barrier?

Before we look at the numbers, I should note that it may not be entirely useful to look at the 2008 energy use vs. the 2007 energy use as a measure of radiant barrier effectiveness, simply because I had only installed the barrier over (a bit less than) half the house's total area.
It may well be that installing 40% of a radiant barrier (which is my best guess for the percentage I had put in) does not give you 40% of the performance of a full radiant barrier. Why? Well, for one thing, the radiant barrier concept relies on rejecting heat before it enters the serious thermal mass of your insulation. Having a huge gap in the barrier allowed my insulation to heat up greatly during the day. Why would that be a problem? Because at night, when everything continues to radiate heat in all directions, a lot of the heat radiated by the insulation is going to bounce off the radiant barrier above it and back down into the insulation, rather than eventually working its way out of the house. Thus, a 40% radiant barrier coverage may not give you 40% of the full radiant barrier benefit.
But I have the numbers, so we might as well look at them.

Partial radiant barrier comparison:
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Total electricity usage for 2007: 8056 kWh
Total electricity usage for 2008: 6982 kWh
Energy savings: about 13%

Unfortunately for our accuracy in using these numbers, this was not a controlled experiment, but instead was affected by at least a couple of major changes in 2008. First, the careful reader will recall that the summer of 2007 in my area was the coolest and cloudiest in quite some time, probably depressing the usage numbers for that year. 2008, on the other hand, had a much more "normal" summer with a lot of sunshine and many 100+ degree days.
Second, in late July my old relatively inefficient (11 SEER) air conditioner finally kicked the bucket. Fortunately, I replaced it with a more efficient 14 SEER unit. Unfortunately, the old unit died in a way that made it run more and more frequently over a period of days, working harder and burning more energy to cool less and less.

So considering the difference in weather, the electricity savings of my partial barrier probably saved a good bit more than 13%.
But, considering the fact that the air conditioner got upgraded about halfway through the summer, the electricity savings must be at least partly due to the more efficient air conditioner, reducing the apparent savings from the barrier. Mitigating the air conditioner factor somewhat was the fact that the slow failure of the old unit burned a lot more electricity over a period of days than would normally happen.
It is impossible me to say for certain which of these factors was larger. Suffice it to say that I was impressed enough with the barrier's performance to stiffen my own resolve to spend more "quality time" in the attic over the winter to complete the job.
I will include a look at my total electricity usage for 2005-2008, with 2008 on the left, in case you can't tell :)