Caleb,
Yeah, I suppose that makes sense. Ratings do change over time. I remember back in the 70's I bought an older home in the States in a cold weather state. I had extra insulation blown into the attic. The insulation guy thought I was crazy, he said that natural gas was a lot cheaper than insulation and that it would take me years and years to recoup my investment. Well, it did, but not as long as he thought it would, the price of natural gas, while still relatively cheap, has gone up considerably, much more than he thought it would. Meanwhile I just added an extra 4 or 5 inches in my attic here in Australia . . . for the same reason. Insulation is a one-time cost, natural gas bills come every two months or so!