I live in Michigan in a two story farm house built in 1890. It currently has
a rock foundation. We plan to move the house 200 feet back on our property
and build an addition. I am debating between a foundation of block, poured
or
polystyrene construction (Reddi-Wall). I like the Reddi-Wall product from
what I can tell by their advertising. What I would like to know is what are
some of the potential bad things that could happen with a polystyrene
foundation? This way I can have some proper pros and cons to make my
decision. So far I have not been able to get anyone to give me any
information except the company itself, which of course is all positive. Can
someone help me?

Dearescamilla,
If it were my home, considering expense, I would use the same rock foundation the house is now sitting upon. The house has held up for 112 years on this foundation. Given the technology advances since the home was built, the bonding materials now are 500% better than those of 112 years past. By doing this you will also preserve the integrity of "this old house". I built the house I reside in and put the Bluestone rock foundation in myself. Rock dosen't shrink or move if installed properly and the astetic results are beautiful.
Len