Hi Mark,
I also have a heated reptile room for my cycluras and other iguanas (Ctenosaura,Fijis,..) and the temperature goes down at night to 60-68F, every winter. And no problems since 25 years! If you want to keep iguanas in your building, then you can be sure that these temperatures are not a problem for them.
I would use the sun, to heat up a large tank and use the warm water to heat up the building at night! I do the same here in cold europe.
rgds
Juergen

