Insulator vs thermal mass?

I wrote a "building green" expert and asked the following question:

I am physically building a passive solar Irish cottage using AAC block. In researching AAC block I discovered conflicting opinions as to whether it was a thermal mass or an insulator. I knew it couldn't be both and I eventually concluded that it was up to me to decide which property to exploit. I decided to proceed as if the AAC block was an insulator. My question is this: "do you see a problem with putting foil/foam/foil between the AAC block and my interior sheet rock to increase the R value of the wall while taking advantage of the reflective properties of the foil?" It's a relatively cheap "add on" and would increase my wall's R value from approx R-16 to R-30. I wasn't sure about moisture problems, etc. Thanks for your answer.

This was his reply:

I think that what you propose is a good idea. I wouldn't worry about the foil inducing moisture problems, since it will be on the inside and should not sweat, and the AAC wall is not a breathable wall either, so it should be fine.
I think that you are right that AAC is primarily an insulator. Your sheetrock interior will provide some thermal mass, but you might think about adding other mass elements into your interior design.All we did today was clean-up/pick-up and begin getting ready to build the south wall... YEAH!