Just killing some time here in the library at school, doing some research for the solar space heater.
The tank enclosure will be similar to this:
Which was taken from this article at Mother Earth News. This article is full of great information regarding Integral Passive Solar Water Heaters.
I also found a page with some good information on the efficiency of old cast iron radiators vs. newer, pressed steel radiators:
It left me thinking that if there are differences in overall performance then ....

Do It Yourself Solar