A little more information on root zone heating vs. air heating in a gh
http://www.umass.edu/umext/floriculture/fact_shee
ts/greenhouse_management/jb_root_zone_heat.htm (paste back together)
and
http://www.attra.org/attra-pub/PDF/ghrootzone.pdf
and
http://www.radiant-concepts.com/appl...eenhouses.html

There was a nice article laying out a lot of the pros and cons of
root zone vs air heating in gh work in an edition of the Ball Red Book
about 10-15 years ago. I don't happen to have a copy, so I'm unable to
get you a better citation... sorry.

Another tactic you might consider for winter heating is to bring back the
old hotframe, long a northern midwestern tradition when horses and
cows were plentiful. A hotframe is built much like a coldframe, but
with a much deeper pit. The pit is bottom-filled with a lot of fresh
manure, covered with a foot or so of soil, and then cool season crops
like lettuce, carrots, etc. are planted for fresh veggies in the winter.

Yet another approach was laid out in a book I read long ago called
"Winter flowers in a sun-heated pit". I was never in a position when I
lived in Iowa to build myself a pit greenhouse, but it seems quite logical,
and you've probably got appropriate soils in N. Illinois.
You'll probably have to interlibrary loan the book.

Winter flowers in greenhouse and sun-heated pit
by Kathryn S Taylor; Edith W Gregg
Language: English Type: Book
Publisher: New York, Scribner 1969]

or

Winter flowers in the sun-heated pit, including the
lean-to greenhouse as a complement to the pit,
by Kathryn S Taylor; Edith W Gregg
Language: English Type: Book
Publisher: New York, C. Scribner's Sons, 1941.

Kay